Floor Framing


Floor DIY How-To Articles


This article describes the traditional method of building a wood-frame floor, as shown in Fig. 1. In most wood-frame houses, the basement or crawl space under the house is spanned by joists that run in parallel from one side of the house to the other, their ends resting on the foundation walls. If the span is greater than, say, 15 ft, two sets of joists are installed, one end of each joist resting on the foundation wall and the other end being supported by a large girder that runs from wall to wall down the center of the house crosswise to the direction of the joists. The girder thus supports about one-half the total floor loads. The center of the girder is usually sup ported by one or more posts, depending on the span. The joists are covered over by sub flooring, which consists of boards or plywood panels. The subfloor ties the joists together into a single structural unit and it also provides a base for the finish flooring.

Floor framing: (1) Nailing bridging to joists, (2) nailing board subfloor to joists, (3) nailing header to joists, (4) toe-nailing header to sill (U.S. Forest Service).


Before a builder can obtain a building permit, he must submit plans, or written specifications attached to the plans, that show the joist size and spacing, and the species and grade of the lumber to be used. What sorts of things must the builder take into account when laying out the joists?

The most obvious factors are the overall size of the house and the applied floor loads, which will determine the sizes and lengths of both the girder and the joists.

The next most obvious factor is the cost of the lumber, which increases with the cross-sectional area of the joists and especially with their length. The builder, therefore, tries to lay out the floor framing to minimize the lengths of the joists. The reasoning is as follows. The shorter the joists, the less weight each joist will have to support. The less weight each joist has to support, the smaller it need be. And the smaller it need be, the less it will cost. The cost of the joists will decrease even more as their length decreases.

But still other factors must be taken into account that may complicate the joist layout. If the house is to be more than one story high, the builder must take into account the location (or locations) of the second-floor bathroom (or bathrooms), since space must be found underneath the flooring in which to run the plumbing that will lead to and from the bathroom fixtures. If it is at all possible, this plumbing should run parallel with the joists; otherwise the installation of the joists may become quite complicated.

If the house is to be heated by a warm-air heating system and/or cooled by a central air-conditioning system, the builder must also take into account the location of the ductwork and its outlets, since the cheapest and most efficient duct installation is one in which the ducts run the shortest possible distances and have the fewest possible bends. If there seems to be an insoluble conflict between the locations of the plumbing, the ducts, and the joists, it must be resolved by the architect, builder, and plumber before construction begins.

The builder must also be sure the lumber is dry. If the lumber is green, it will shrink as it dries out, sometimes by a considerable amount. The shrinkage lengthwise is trivial and may be ignored. Green lumber does, however, shrink considerably across its grain. A 12-in.-deep joist may shrink as much as ½ in., for example. When one length of green lumber rests upon another, as a joist may rest upon a girder, the total amount of shrinkage will double, assuming both pieces of lumber are the same size and equally green, which will cause the floors to sag perhaps an inch or more as the lumber dries out.


Most girders are supported at their centers by a post (refer to Figs. 2 and 3). Posts made of wood or steel are usually installed in a house having a full basement. Masonry or concrete piers are usually built when there is a crawl space under the house, although a masonry pier can also be built in a full basement, of course.

Whatever the material, the post must be supported by a large footing if it is to be prevented from settling into the soil. For one- and two-story houses, most building codes require that footings be at least 12 in. sq and 6 in. deep, and for three-story houses, the footings must be at least 16 in. sq and 6 in. deep.

Wood Post

Wood posts (Fig. 2) are usually made from a solid timber. The timber supporting first-floor girders is usually 6 in. sq. The footing on which the end of the post rests should be raised 2 to 3 in. above the basement floor to prevent any dampness in the floor from rising through the footing and into the wood, which would cause the wood to rot. In addition, the top of the footing should be made waterproof by pouring a layer of hot asphalt over it before the post is placed in position.

Figure 2 shows how the post is held securely to the footing by a pin. The top of the post is sometimes capped by a steel plate to prevent crushing the wood fibers of the girder where the girder rests upon the post.

Installation of a wood post to support a girder: (top) connection to girder; (bottom) installation of base (U.S. Forest Service).

Installation of a steel post to support a steel girder: (top) connection to beam; (bottom) base plate supported by a footing and embedded in a concrete floor. The base plate may also be mounted on and anchored to a concrete pedestal as shown in Fig. 2. (U.S. Forest Service).

Steel Post

For most houses, a steel post need be only 3 or 4 in. diam. and it may look rather spindly under a girder but, rest assured, it can safely support all the loads likely to be imposed on it. Steel plates are welded to both ends of the post. The top plate is secured to the girder by bolts (if the girder is made of steel) or lag screws (if the girder is made of wood).

The bottom plate is secured to the footing either by bolts or by being embedded within the footing, assuming the footing is made of concrete.

Steel posts having a screw thread in one end are available. After the post has been set in position and the girder has been installed on top of it, the post can be lengthened by turning the screw until the post is bearing firmly against the girder. Thus, an exact fit can be obtained without metal or wood wedges being required between the girder and the post. Thereafter, if the house should begin to settle because the girder shrinks down slightly, the settlement can be compensated for immediately by lengthening the post an amount equal to the shrinkage.

Masonry Piers

Piers made from brick are usually 12 to 16 in. sq, depending on the loads they are expected to carry. Piers made of solid concrete blocks are usually 16 in. sq regardless of the loads they will carry, as this is the size of concrete blocks. The piers are built on concrete or brick footings that extend 4 to 8 in. away from each side of the piers. The actual size of the footings will depend on the type of soil under the house. A clay soil, for example, will require a larger bearing surface than a gravel soil.

The top of the pier can consist either of a steel plate set into mortar or of a solid masonry cap 4 in. thick, which is also set in mortar. The height of the piers must be such as to maintain a minimum distance of 12 in. between the soil and the bottom of the girder to minimize the possibility of termite infestation.

Concrete Post

A pier made of reinforced concrete is usually 10 in. sq. Both the pier and its footing are poured at the same time and in the same form. The dimensions of the footing should be the same as for masonry piers.


Wood Girder

A wood girder may consist of one solid timber, be built up out of nominal 2- or 3-in-thick planks, or be made of laminated construction. Although a solid timber is usually stronger than a built-up girder, depending on the species and grade of the lumber, a timber is more likely to shrink after it has been installed because it will have a much higher initial moisture content. It is, therefore, also much more likely to develop splits. In addition, a solid timber that is long enough to span the basement of a house is quite expensive, and it would also probably require a crane to set it in place. Most girders made of solid timbers usually consist, therefore, of several short timbers that abut each other at center posts.

Built-up girders (see Fig. 4) are usually cheaper than solid timbers because the lumber out of which they are made usually costs less than solid timbers. Lumber that is only 2 to 3 in. thick is also more likely to be thoroughly seasoned than a solid timber, which means the wood will shrink much less. By spiking together a number of short lengths of lumber, a built-up girder of almost any size and length can be constructed, within limitations, of course. One cannot have an excessive number of joints, for example, and the joints must be located as much as possible over posts.

Installation of a built-up wood girder (U.S. Forest Service).

Steel Girder

In a comparison of wood and steel girders, if they have the same strength and stiffness, the steel girder will be both smaller and heavier than the wood girder. Steel girders are used because they can span wider spaces with less trouble than wood girders and because, whatever else may happen, the girder won’t shrink down with the passing of time. Steel girders and steel posts are usually, though not necessarily, installed together and fastened by bolts as shown in Fig. 3.

Girder Installation

The ends of both wood and steel girders may be supported in either of two ways on the foundation walls. The girders can rest in niches formed in the walls during their construction, or they can rest on top of the foundation walls. When a girder rests in niches, as shown in Fig. 4, the niches must be at least 4 in. deep if they are to provide sufficient bearing surface. In addition, a steel plate is usually set in the niche to give the girder a solid support, with a ½ in. space left all around the girder to allow air to circulate. It is poor practice to seal the ends of a girder in a wall by pouring concrete or mortar around it under the belief that the construction is thereby made more secure. It is far more likely that the ends of a wood girder will rot under such circumstances, in which case it may suddenly collapse one day.


Joists are usually made of 2-in.-thick lumber and are anywhere from 6 to 14 in, deep, depending on the expected floor loads, the spacing between joists, the total span, and the kind and grade of lumber used. These factors are summarized in joist tables, such as Table 1, which are used by the builder as a guide to the selection of the joists. It should be remembered, however, that joist tables always give the minimum required joist sizes; it is always possible, if thought necessary, to increase the size of the joists to carry heavier than usual loads.

Joists with Wood Girder

There are three ways in which wood joists can meet at and be connected to a central girder: (1) the joists can rest on top of the girder; (2) they may be installed level with the top or bottom of the girder and rest on ledgers that are nailed to the bottom of the girder; or (3) they may be level with the top or bottom of the girder and supported by joist hangers or framing anchors resting on or attached to the girder. All three methods are shown in Fig. 5. The illustrations should make these construction methods clear.

The simplest method of installing joists is to place them on top of the girder, allowing abutting joists to overlap so they can be spiked together. Simple as it is, this method of installation may also lead to the greatest problem with wood shrinkage.

The total amount of shrinkage at the girder will be the shrinkage through the girder itself plus the shrinkage through the joists it is supporting. If green lumber is used, this total shrinkage can amount to as much as 1 in. Furthermore, since the total depth of the lumber at the foundation wall will be about half the depth of the lumber at the girder, the amount of shrinkage at the foundation walls will be about half of what it is at the girder. As the wood dries out over a period of months, the center of the house will settle by as much as h in., which is sufficient to cause the plaster to crack and the doors to jam as the house frame twists.

For this reason, it is preferable that the joists and girder be placed at the same height. The choice of construction methods to accomplish this will then be among ledgers, joist hangers, or framing anchors.

A ledger is nothing more than a 2 X 2 or 2 X 4 in. length of wood that is securely nailed to the bottom edge of the girder.

The joists rest on the ledger and are also spiked to the girder or to each other for additional support, as shown in Fig. 5. A girder is usually deeper than the joists, which means that even with a ledger strip nailed to the bottom of the girder the tops of the joists will usually be level with the top of the girder. When this method of installing joists is used, the amount of wood shrinkage at both the girder and foundation walls will be approximately equal.

It is possible, however, for the joists either to be deeper than the girder or for the joists to be the same depth of the girder. When this is the case, and when ledger strips are used, the joists can be fastened to the girder by notching them as shown in Fig. 5 so that abutting joists will abut or overlap each other on top of the girder. When joists are notched above a girder, however, a gap of at least 1/2 in. must be allowed for between the girder and the joists; otherwise, as the joists dry out, the notched ends may shrink down upon the girder and the stresses that result will cause the joists to split at the notch cut outs.

Another alternative is to leave the ends of the joists square and to nail scabs across the top of the girder to join the joists, as shown in Fig. 5.

Joists may either rest on a girder, they may rest on ledgers nailed to the girder, or they may be supported by metal joist hangers.

The use of joist hangers is also illustrated in Fig. 5; the illustration should be self-explanatory. If joist hangers are used, one must be careful that they are spiked into the sides of the girder and not merely hung over the top of the girder. If a joist hanger rests on top of a girder, it will allow the same total amount of shrinkage through green wood as if the joists were laid on top of the girder. This may not be obvious at first but if one remembers that all the wood in a house shrinks down toward the foundations, one can see that the joist hanger will drop down in., say, as the girder shrinks, and that the joist will shrink down another 1/2 in.

Joists with Steel Girder

Everything that has been said above about the installation of wood girders will apply also to steel girders, except that the builder won’t have to worry about shrinkage in a steel girder (refer to Fig. 6). There may still be a small amount of differential shrinkage between the two ends of each joist when a steel girder is used, and the builder must take suitable precautions to prevent such differential shrinkage.

Joists supported by steel girders usually rest on wood ledgers to equalize the amount of wood shrinkage at both ends of the joists.

Very often in wood-frame construction, for example, the ends of the joists at the foundation walls will rest upon a 2 to 4-in.-thick wood sill that is bolted to the top of the foundation walls (see Fig. 7) with the other ends of the joists resting on a steel girder installed in the center of the house. The result may be a slight amount of shrinkage at the foundation walls. The solution is simple. As shown in Fig. 6, bolt to the top or bottom flange of the girder a length of wood that is as thick as the sill and rest the joists on this length of wood. A wood support is usually necessary, in any case, to give the joists a material to which they can be nailed for added support.

The assembly of joists to the foundation wall of a platform-framed house (U.S. Forest Service).

If the steel girder and the joists are to be at the same height, ledger boards can be bolted to the bottom flange of the girder as shown in Fig. 6. A ½ in. gap must be allowed for between the notched ends of the joists and the top of the girder to avoid splitting the wood.


The way in which the joists are secured at the foundation walls will of course depend on the type of wood framing that is used in the original construction. (For details see WOOD-FRAME CONSTRUCTION.)

In platform-frame construction the joists rest on top of a sill that runs around the perimeter of the building, the sill being fastened directly to the top of the foundation walls (see Fig. 7). A header joist is spiked into this sill (the two pieces of lumber thus forming what is called a box sill) and the joists are then attached to the header joist by spikes driven through the header joist and into the ends of the joists.

In balloon-frame construction, the joists also rest upon a sill but they are spiked into the wall studs rather than to a header joist (see Fig. 8).

Fig. 8. The assembly of joists at the foundation wall of a balloon-framed house (U.S. Forest Service).

Masonry Walls

Neither of the above methods of supporting the ends of the joists will work in a foundation wall made of brick or concrete blocks. In a masonry wall, the builder may either set the ends of the joists in niches, as is done for the girder, or he may rest the joists in joist hangers built into the masonry. If the joists are supported in niches, they should rest on bearing surfaces that are 4 in. deep.

In a masonry house, the second-floor joists that rest in niches have one construction peculiarity. The ends of the joists must be cut at a “self-releasing” angle, as shown in Fig. 9. The angle allows the joists to fall clear of the walls if they should ever collapse during a fire. If the ends of the joists were left square, there is a chance that they would catch against the masonry as they fell, thus causing the entire wall to collapse.

It is also usual in brick-masonry construction, and especially in parts of the country where earthquakes or hurricane-force winds occur, to tie the walls and joists together at each floor level with steel straps or anchors embedded in the wall. For a description of this construction see CONCRETE-BLOCK CONSTRUCTION

Fig. 9. The ends of second-floor joists in a masonry wall are beveled to enable them to fall clear of the wall in case they collapse during a fire.

Floor Openings

In two-story dwellings, or in a one-story dwelling with a full basement, stairs must obviously lead from one level to another. And it is equally obvious that, if the joists are cut through to make a stairwell opening, they will collapse. The construction, therefore, must be reinforced at the stairwell in such a way as to maintain its integrity.

When a floor opening is less than 4 ft wide, short lengths of joist called headers span the opening and tie the cut joists together (refer to Fig. 10). When the floor opening is more than 4 ft wide, these headers must be doubled. When the headers are doubled, the joists running parallel to the sides of this opening must be doubled also. These added parallel joists are called trimmers.

When the floor opening is 6 ft or less in width, it is sufficient if the doubled headers are attached to the trimmers by spikes that are driven through the trimmers into the ends of the headers. But when the opening is more than 6 ft in width, then the headers must, in addition, be attached to the trimmers by joist hangers or framing anchors.

The joists that have been cut through in order to make the opening are called tail joists, tail beams, or header joists— different sections of the Country have their own nomenclature. These tail joists (as we shall call them) are spiked into the headers as shown in Fig. 10. If the tail joists are longer than 12 ft, they must also be supported by framing anchors or rest on 2 x 2 in. ledgers nailed to the bottoms of the headers.

The installation of reinforcement in the framing around a floor opening.

Chimney Openings

The method of framing a floor opening through which a chimney passes is the same as for a stairwell opening. The only difference between the two is that the gap between the brick work and the floor framing at the chimney opening must be closed off to prevent smoke or hot gases from traveling from floor to floor, if a fire should break out in the house. The gap is there in the first place to prevent any part of the brickwork, which can get very hot, from touching and possibly igniting the wood. No part of the wood framing should, therefore, be closer than 2 in. to the chimney, if the walls of the chimney are at least 8 in. thick.

Bathroom Joists

Bathroom joists must support much greater loads than the joists in other parts of a house. For one thing, the bathroom floor may consist of ceramic tiles bedded in from 1 to 3 in. of concrete; this floor can weigh as much as 30 lb per sq ft.

For another thing, bathroom fixtures are usually made of porcelain-coated cast iron and they are quite heavy. The fixtures may add perhaps another 10 to 20 lb per sq ft to the floor. The joists must also support the weight of a bathtub filled with water. Water weighs 62.4 lb per cu ft and an ordinary sized bathtub filled with water can easily weigh 1250 lb. The weight of this water, spread over the floor area covered by the bathtub (10 sq ft, say) represents an additional, if temporary, load of 125 lb per sq ft. Therefore, it should be obvious that the floor framing must be reinforced if it is to support these loads.

This reinforcement can be provided in different ways. If the bathroom floor is otherwise unsupported, doubled headers located between doubled joists can be installed to help support these loads. The principles of construction are pretty much the same as described above for the construction of floor openings. It is often possible to locate a second-floor bathroom over a first-floor partition. When this is the case, the partition can help support the weight of the floor, which will reduce the need for doubled joists. Better still, if the second-floor bathroom can be located above a first-floor hallway so that the partitions on either side of the hallway will help support the weight on the joists, no doubled framing may be required at all, There is, however, another problem with second-floor bathroom floors that additional framing cannot cope with, and this is the problem of having to cut into the joists to make room for the plumbing, The closet bend located under the water closet, which transports solid and liquid wastes from the water closet to the soil stack, can prove an especially difficult problem (for a description of plumbing systems see PLUMBING). The closet bend can be as much as 4 in. in diam., and it requires space under the flooring to make the sweeping turns that take it from under the water closet to the soil stack. The plumber and carpenter must decide in advance on the best location for the water closet and on the best way of running the closet bend.

The carpenter can then allow for the weight of this fixture and also for the space required for the closet bend when he frames out the floor. What he usually does is treat the space required for the closet bend as another floor opening and he frames this space in the same way as he would a stairwell opening.

Partition Loads

There are two kinds of partitions: Load-bearing and non-load bearing. Load-bearing partitions support a load in addition to their own weight. Non-load-bearing partitions support only their own weight.

Joists supporting non-load-bearing partitions need not be especially strengthened in order to support the partitions, if the partitions run crosswise to the joists. The bottom plate of the partition is simply nailed to the rough flooring (see WALL FRAMING). If, however, the wall should run in a direction parallel to the joists, care should be taken that the partition is located over a joist.

For load-bearing partitions, if a partition runs parallel with the joists, the joists under the partition may either be doubled or spaced apart under the partition as shown in Fig. 11. If the joists are doubled, the carpenter must make sure that the partition is located directly above the joists. The joists must, in turn, be supported all the way to the footings by other partitions, beams, or walls.

A load-bearing partition running parallel to the joists must be properly supported by doubled joists. Sometimes, 2 x 4 in. blocks are nailed to the joists under the partition to give added support.

When a load-bearing partition runs across the joists, every other joist under the partition is usually doubled. If the load being carried by the partition is unusually heavy, as when it is supporting the weight of a bathroom floor, for example, every joist can be doubled to help carry the load. Again, the joists must, in turn, be supported all the way to the footings in some way.

Installation Techniques

A few additional remarks on joist installation before we leave the subject. Joists should be installed carefully so that their top surfaces provide a level surface for the finish flooring. For the best construction, the carpenter should sight along the edges of each joist before installing it to see if it has warped in such a way that one edge has a crown to it. If it has, the carpenter must install that joist crown-side-up. After the subfloor and finished flooring have been nailed in place and the floor has been in service for awhile, the joist will straighten itself out.

Since there may be slight differences in the depth of the joists, the carpenter must also bring the joists up to the same level surface before nailing them to their supporting members. He does this by placing shims under the low joists.


Once the joists have been installed, bridging is nailed between them. Bridging consists of wood or metal bracing that runs from joist to joist in a crisscross pattern, as shown in Fig. 12; or it can consist of solid blocking.

Bridging installed between joists to stiffen the construction.

The main function of bridging is to stiffen long spans of unsupported joists since the joists would otherwise tend to bow sideways under a load. If a plastered ceiling were to be installed under such unsupported floor, the plaster, not being able to resist this sideways pressure, would crack. A lack of bridging is also responsible sometimes for floor squeaks that develop because the nails holding the subflooring to the joists work loose as the joists flex back and forth.

It had long been believed that bridging helped to transfer a load concentrated upon one joist to the rest of the floor construction. The ability of bridging to spread loads in this way has been put in doubt as the result of experiments conducted by several building research organizations. It is now the general opinion that having a subfloor that is solidly nailed to the Joists is of far greater value in helping to spread floor loads evenly.

Nevertheless, bridging is still required by some local building codes. The consensus at present seems to be that a line of bridging may be installed for every 8 ft of unsupported span—it depends on how solid the builder wants the overall floor construction to be; but a line of bridging should be installed if the joists are unusually deep and if the unsupported span is greater than 8 ft. An unusually deep joist means a joist in which the nominal depth us more than six times the nominal thickness. Thus, for example, a joist that is 2 x 14 in. in size requires bridging to give it an additional amount of stiffness, assuming the joist is also more than 8 ft long. The most common type of bridging consists of strips of wood nailed crisscross between joists. For joists 2 x 10 in, or less in size, the wood is 1 x 4 in. in size. For joists larger than 2 x 10 in., the bridging may be either 2 X 2, 2 X 3, or 2 X 4 in. in size, depending on the depth of the joists.

The ends of wood bridging must always be cut accurately at such an angle as to provide solid bearing against the joists. When the carpenter first installs the bridging, he will nail only the upper ends of the bridging to the joists. The bottom ends are allowed to hang free until after the subfloor and finish floor have been completely installed, at which time it can be assumed that the joists will have settled into their final positions relative to each other. The bottom ends of the bridging are then nailed in place.

Metal bridging can also be used. If it is, the bridging must have a V-shaped cross section to give it stiffness. Bridging made of flat metal strapping is useless and should never be used.


The subfloor is the final portion of the floor framing to be built. A subfloor has several functions: (1) it provides a working platform for the workmen; (2) it gives a base for the finish flooring; and most important, (3) it transforms the joists from a collection of individual beams into a single, rigid structure. This is especially true when the boards are laid diagonally to the joists rather than at right angles to them. A diagonally laid subfloor not only makes a single structural entity of the floor framing, but the boards that tie the corners of the framing together also help to stiffen and strengthen the entire structure of the house; subflooring that is laid at right angles to the joists cannot do this as well. In addition, a diagonally laid subfloor allows the finish flooring to be laid parallel to or across the joists in any room to suit the taste of the owner.

The lumber used for subflooring consists either of matched boards (which are boards that have their long edges, and sometimes their ends also, tongue-and-grooved or ship-lapped) or common boards (which are boards having straight edges); or the subfloor can be made of plywood.

Board Subflooring

In the best-quality construction, nominal 1-in.-thick lumber, 6 to 8 in. wide, and with tongue-and-grooved or ship-lapped edges is used. For ordinary construction, the boards are nominally 3/4 in. thick.

To lay the boards diagonally, the carpenter starts in one corner of the house with a board that has been cut into an equilateral triangle. This is called the starter board. The carpenter then gradually works his way across the joists toward the opposite corner of the house. Each board must end over a joist, unless end-matched boards are used. Often, however, a gap of about 1/8 in. is left between the ends of abutting boards to allow for any expansion caused by wetness (and a surprising amount of water is used in a house during its construction). A gap of 14 in. must also be left wherever a board meets a partition or wall. If the boards are 6 or 8 in. wide, two nails are required at each joist and at both ends of each board. If the boards are more than 8 in. wide, three nails are required at each joist and at each end of the boards.

Plywood Subflooring

Plywood subflooring has the great advantage over wood-board subflooring in that it takes much less time to install—the cost of installing plywood can be as much as 50 percent less. One carpenter can usually install the plywood subflooring in a house in one day.

Plywood panels can be used in two different ways when they are used as subflooring. First, the panels can be installed merely as a substitute for wood boards, with the builder still intending to install a finish wood floor over the subfloor (refer to Fig. 13). For a description of this type of installation see FLOORING, WOOD. Wood flooring has considerable structural value of its own, which adds to the overall strength of the construction. The subflooring, therefore, need not be too thick, whether it consists of wood boards or plywood panels.

Second, instead of wood flooring the builder may intend to install a resilient flooring material, such as asphalt tiles, vinyl tiles, or linoleum, or he may intend to install a carpeting material over the subfloor. Apart from their obvious physical differences, all these materials have in common the fact they have no structural value whatsoever. The subflooring, therefore, must provide the strength that would otherwise have been provided by the wood flooring. In this case, therefore, the plywood subfloor must be much stronger and stiffer than if a wood floor were to be laid down over it (refer to Fig. 14 The plywoods used for both types of installation are manufactured in different qualities and thicknesses to suit different requirements. If, for example, the subflooring is going to be exposed to the weather for a considerable period of time, and the climate is a rainy one, then the plywood must be manufactured with a waterproof glue that will resist separation of the plies. That is, the plywood must be an exterior plywood. If the climate is dry, or the subfloor will be exposed to the weather for only a brief period of time, then an Interior panel made either with an interior or an exterior glue can be used (see PLYWOOD).

The thickness of the panels will depend in part on the spacing between the joists. Obviously, the wider the joist spacing, the thicker (and, thus, stiffer) the panels must be. The stiffness of any particular panel will also be dependent in part on the wood used in its manufacture, some woods being stronger and more rigid than others. This is reflected in the tables of Figs. 13 and 14, which show that plywoods in Group 1 can be thinner than plywoods in Group 4 used for the same service, the reason being that the plywoods in Group 1 are made from woods that are inherently stronger than the woods used for Group 4 plywoods. (For a description of the grading of plywood panels see PLYWOOD.)

In sum, the builder has a choice among plywoods, and the particular plywood he selects will depend on the costs of the different panels balanced against the purpose to which he wants to put them.

Plywood panels are usually installed with their long sides laid crosswise to the direction of the joists. This construction is stronger since more joists are tied together by a single panel.

The long sides of the panels should also be staggered so that adjacent panels do not begin or end on the same joist. Nor should the panels be butted tightly against each other, whether they are square-edged or tongue-and-grooved. Instead, a gap of about 1/16 in. should be left between panels along their 4-ft edges, and an 1/8 in. gap should be left along their 8-ft edges.

The gaps are necessary because, despite popular belief to the contrary, plywood panels do expand slightly when they become wet.

Sometimes a builder will install a wood-board subfloor over which he intends to install a resilient flooring material or perhaps a carpeting material. When this is the case, he must then install lightweight plywood panels over the entire subfloor, as subflooring made of boards is neither flat enough nor smooth enough to prevent irregularities and gaps between the boards from showing through the resilient flooring or carpeting.

These lightweight plywood panels are called underlayment (refer to Fig. 15). Like other types of plywood paneling, under-layment is made in Exterior and Interior grades, from different woods, and with exterior and interior glues, to suit the particular application.

The underlayment should be installed just before the finish flooring, which means the house will have been enclosed by this time and there is no danger of rain soaking into or damaging the plywood.

The installation of plywood subflooring

Use 6d common nails for ½-in plywood, 8d for thicknesses from 5/8 to 7/8 in., and 10d for 1 1/8 and 1 1/4-in, thicknesses. Space nails at 6 in. along panel edges for all thicknesses. Along intermediate supports, space nails at 10 in., except when plywood spans 48 in., space nails at 6 in.

Installation of a combined subfloor and underlayment when the finish flooring will consist of tiles or carpeting.

To minimize the effects of framing shrinkage, ring-shank or spiral-thread nails should be used. Use 6d deformed-shank nails for thicknesses through in. Use 8d for panels 7/8 in. and thicker. Space nails at 6 in. along panel edges and at 10 in. along intermediate supports. Unless joists are of thoroughly seasoned material and have remained dry during construction, countersink nail heads 1/16 in. below surface of the underlayment just prior to laying finish floors to avoid nail popping. Do not fill holes. If resilient flooring is to be applied, thoroughly sand joints.

The T&G joint is designed so that the upper plies of the panel will be spaced to avert ridging if the panel picks up moisture and expands. Joints should not be tightly butted, but left open slightly. Normally a space of about in. (the width of a 6d box nail) between the upper plies of the panels will be enough for the T&G joint. A space of 1/16 in. is recommended for panel end butt joints.

If wet conditions are anticipated, additional spacing of up to 1/16 in. at both sides and ends is advisable.



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Thursday, 2013-05-09 6:12