Sagging Floors: Their Causes and Remedies

A few minor dips in floors, particularly in older homes, are not uncommon. Over a period of years, some settling is bound to result from gradually applied stresses and fatigue. But if floors sag or are noticeably springy, the problem could range in scope from modest to serious.

Consider the gravest possibility first. If there are cracks in the plaster or other wall materials in or near the house’s normal stress points — in corners, over windows and doors, or where walls meet — more than simple structural repairs may be necessary.

Damage beyond the floors may be a sign of a deteriorating foundation or excessive settling. For example, horizontal movement and the cracking of foundations sometimes occurs in houses located on sloping ground or near cut slopes, often as a result of inadequate diversion of surface or roof water.

Sticking doors or windows or leaky plumbing may also be signs of foundation problems.

Many older homes may have uneven floors as the result of settling that occurred shortly after the house was built, even though the house was well built and may have been stable for many years. Before deciding to correct a problem of such long standing, consult an engineer or builder familiar with older homes. The results may not be worth the risks involved.

Solving major problems may involve the complex and expensive job of raising the house to permit structural and foundation repairs—clearly a job for a contractor with special skills and equipment. Happily, trouble in the foundation is one of the least common causes of sagging floors.

Locating the cause

The first thing you must do is isolate the cause of sagging floors and determine where weak nesses exist. On the following pages, you’ll find a logical series of checks you can make. The problem may be caused by a single defect, like a settled footing or a weak splice in a beam, or it may be the result of a combination of two or more weaknesses.

Working from above, you’ll first have to locate low spots; then from underneath the floor, you’ll have to make a detailed examination of joists, girders, posts, footings, foundation sills, and wall studs where they rest on sills. In most older homes, girders and posts, like joists, are made of wood and are vulnerable to deterioration. In newer homes, steel girders and sup port posts are common. These are very unlikely to break down, but may direct your attention to problems in the foundation or footings.

In any case, it’s important that you make a thorough inspection of the supporting structure beneath a sagging floor. If you stop, say, at the joists just because you think you’ve found the cause of the problem there, defects in the girders, posts, or footings may go undetected — and come back to haunt you later.

If you’re unable to pinpoint the cause of a sagging floor, you’ll need the advice and assistance of a professional.

Fig 87-0: Supporting structure for floor in typical frame house consists of joists, girders, and posts resting on concrete piers and footings. Bridging adds strength to joists.

Find the low spot. Take a straightedge at least 8 feet long and check systematically for the low spot in the floor. A straight edge can be a length of rigid pipe; a piece of lumber at least 4 inches wide, selected f or straightness; or the uncut edge of a strip of 3/4 inch plywood.

Fig. 88-0

Once you find the low area, mark the spot. Then take a tape measure and “map” the spot in relation to two or more reference points (corners, ducts, pipes) that you can use to locate it on the underside of the floor from the basement or crawl space.

Checking joists and girders. Here’s what to look for. Joists should be checked for sway, sagging, or warping. Use an awl to poke into the wood, particularly where joists rest on foundation sills or adjoin other wood members, and look for rot or in sect damage. Extreme softness in the wood is evidence of rot. If termites have been feasting on your home, you’ll discover their tunnels in the wood or on the foundation, and you’ll need immediate help from an exterminator.

Discolored wood, usually dark, on joists or in the subfloor can indicate a current or recent plumbing leak or exterior water leaks. Moisture can also gather where there is inadequate ventilation below floor level. Moisture attracts subterranean termites and promotes fungus growth, which ultimately results in rot.

If the joists are straight and appear to be sound, examine the girders. Check carefully where the girders rest in pockets in the foundation walls or on wood sills. And if the girders are made up of varying lengths of wood, check the points at which they’re joined—weaknesses commonly occur where splices have been made other than directly over supporting posts.

Checking posts, piers, and footings. If the low spot in a sagging floor is over or close to a post and the joists and beams are in good shape, the post or its pier or footing may be in bad condition.

Check wood posts for signs of deterioration, especially where they sit on a pier or a footing; moisture here can cause decay. Inspect wood posts thoroughly for termite damage too.

If there’s no apparent decay or insect damage, or if your posts are made of steel, then the footing has cracked or settled, al lowing the whole structure above to sink. The best solution is to re place the footing and, if necessary, the post (see “Is the footing adequate?”).

Checking the concrete slab. If your home is built on a concrete slab and the slab has cracked substantially or tilted, it usually means the slab was poured on insufficiently compacted fill or on unstable land. Get professional advice.

Hairline or narrow cracks in exposed concrete slab floors are usually not serious and are probably unrelated to structural problems. Once discovered, they can be patched easily with one of several commercial products made for the purpose and avail able from a hardware dealer or home improvement center. If you’re unsure how serious a crack is, check with a professional.

Upper floor sags

Multistory homes may have problems in the upper floors identical to those on the first floor because they’re directly related.

For example, a load-bearing wall that has settled or weakened may cause both the first and second floors to sag because both are supported by the same structure. In this case, leveling the first floor may level the second floor as well (see “You must level the floor,” facing page).

If an upper floor sags between the exterior wall and an interior load-bearing wall, correcting the problem can be complex. Have a structural engineer or capable contractor examine the structure. Because the underside of the floor is probably the finished ceiling of the room below, repairs can be messy and costly—not a job for the homeowner. If there are no serious structural complications, it may be easiest to live with the slope in the floor.

Should you tackle the job yourself?

Once your preliminary investigations have turned up the specific weaknesses that have allowed your floor to sag, the next step is to decide if you want to tackle the necessary repairs yourself. Reading through the repair instructions in the next few pages will help you understand what’s involved.

But before you decide to take on the job, especially if you’re uncertain about what might be involved, consult a structural engineer familiar with local building codes. The engineer can help you determine which re pairs are practical for you to do yourself, and can provide specifications and a list of materials that will be needed. A building inspector should also be contacted early in the planning phase so that building permits can be acquired if necessary.

If the general structure supporting the floor is in good condition and the sagging is minor—say, less than ¼ inch over an 8-foot span—you may choose to live with the imperfection. You can correct minor dips by driving wooden shims between the joists and subfloor, much as shims are used to eliminate squeaks or to stabilize springy floors (see “Getting rid of the squeaks”).

The most common structural repairs that the homeowner may elect to do are doubling or rein forcing joists, replacing posts or footings, and reinforcing girders. Reinforcing a joist, how ever, can be difficult if it involves working around or disturbing wiring, plumbing, or duct work.

Replacing beams and joists or repairing sills and studs are jobs best left to professionals; the work can be difficult, and it re quires skill and capable assistants. Only the most talented do-it-yourselfer should attempt major structural repairs—and then only in straightforward situations when professional ad vice is available.

You must level the floor. Correcting any sagging floor means first bringing the floor back to level. In most cases, this will involve some heavy work with heavy pieces of equipment. Structural timbers will be needed as well as one or more jacks.

Fig 89-0: Basic jacks—either house jocks or adjustable jock posts are used to raise floor in a frame house. Adjustable jack posts can be left in place as permanent support posts.

Understanding the basic jacks. House jacks are required for the basic business of raising a structure while repairs are made. You can also raise the structure with jack posts and leave them in place as permanent support for a newly leveled floor.

For houses with basements, either a basic house jack with a timber extension or an adjustable jack post may be used. If your house has crawl space only, you’ll need a house jack set on a pyramid or cribbing built of structural timbers, usually 6 by 6s. The pyramid provides a solid foundation for the jack and raises it high enough to reach the girders or joists.

The adjustable jack post is fitted top and bottom with heavy metal plates that can be secured temporarily to a wooden pad and a beam with duplex nails — double-headed nails that are easy to remove.

The jacks should be “screw-type” jacks — not hydraulic, which are difficult to adjust in the fine gradations required for raising a house slowly enough to avoid damaging it.

Any rental company that supplies jacks should have structural timbers available or be able to direct you to a source. Railroad ties or other used timbers are often available from house movers or demolition companies.

How to double a joist

The easiest way to strengthen a weakened or decaying joist is to “double” it by securing a new joist to the old. The new joist should be the same thickness and width as the old one, but need not be long enough to sit on the foundation sill or on a girder.

Before you double a joist, you should have an exterminator treat any decay or insect dam age on or around the old joist.

Doubling of joists should be done only in moisture-free areas. Dampness seeping between the two joists can cause rot that will be impossible to detect on their surfaces.

Fig 89-1

Placing the jack. To begin, re move any bridging between the sagging joists (see drawing). Set a house jack on the top timber of the cribbing, or set a jack post on a wooden pad made out of a 4 by 8 or wider timber. You can also use a house jack, set on a wooden pad, with a 4 by 4 timber extension. Use the last two methods only where a concrete slab or footing is available as a base (see “Understanding the basic jacks”).

Place a 4 by 6 or 4 by 8 beam, long enough to span the area of the floor to be raised (a minimum of 4 feet), perpendicular to the joists. Set 2 by 6 blocks on top of the beam so they will be directly under each joist when the beam is raised into place. This will make it easier to slide the reinforcing joist into place once the floor above is level.

Extra hands will be required to raise the beam and hold it in place while the jack is made plumb and enough pressure is applied to hold the beam snugly against the joists (see drawing at right).

If you’re using an adjustable jack post, nail the bottom plate to the wooden pad and the top plate to the beam.

Raising the floor. Grease the threads of the jack and start raising the floor 1/16 inch every 24 hours. Because the threads vary among different types of jacks, you’ll have to determine how much of a turn is required for the jack you’re using.

Depending on how high the floor needs to be raised to eliminate the sag, the process can take several days. To allow for some settling, the floor should be raised to ¼ inch above level.

Resist the temptation to accelerate the process. The house will need time to make subtle adjustments to the new stresses created as the floor is raised. If you try to hurry it, you’ll risk cracked plaster and structural damage.

Installing the joist. Coat the surfaces to be joined with a waterproof wood glue; then lift the new joist into place and secure it with nails or bolts.

If nailing is your preference, use 16-penny common nails staggered top and bottom about every foot, and clinch the protruding points by bending them flush with the surface.

With the limited space between joists to swing a hammer, 1/2-inch bolts may be easier to in stall and will work as well as nails. Pre-drill bolt holes in the new joist about every 18 inches before lifting it into place. Secure it with a few nails or C-clamps, then use the holes in the new joist as guides to drill through the old joist. Insert the bolts and tighten.

Once the new joists have been added, lower the jack at the same rate of 1/16 inch per day until it can be removed.

Install new bridging between the joists, and the job is finished. You can choose between diagonal bridging or solid block bridging.

Diagonal bridging can be cut from 1 by 3s or 1 by 4s. One end of each piece is nailed to the up per edge of one joist and the other end to the bottom edge of the adjacent joist.

Solid block bridging can be cut from lumber the same size as the joists to fit at right angles between joists; the pieces are then nailed in place. Solid block bridging is usually staggered to make it possible to nail through the joists and secure the bridging from both ends.

Fig 90-0

How to replace a post and footing

Replacing a post is one of the less complicated structural repairs. A homeowner can tackle the job with one or two helpers, some professional counsel, and the necessary local building permits. Before buying a new post, check with your building department to determine what size you’ll need.

The easiest post to install is one made of wood the same size (not length) as the post you’re re placing. Wood pressure-treated for termite and rot resistance is best. Using a saw, you can cut a wood post to the exact length required.

A steel post, on the other hand, must be made by a steel fabricator to the exact length required; while you can shim it if it’s too short, you’ll have no way of cutting it if it’s too long. A steel post comes with predrilled flanges top and bottom.

Another alternative is to use an adjustable jack post, but you should check first with the local building department.

Raising the structure. To make it possible to remove an old post, use adjustable jack posts or house jacks to level the floor and take the weight off the post. Place two jacks under the girder, 3 feet from each side of the post (see drawing below), and make sure they’re plumb. Raise the floor 1/16 inch per day until it’s 1/4 inch above level. Patience is required, as the structure may be damaged if an attempt is made to hurry this step.

Removing the old post. While raising the structure, remove any visible nails or bolts that fasten the post to the beam. By the time the floor is level, you should be able to remove the post. If the post doesn’t slide away, it may be held by a pin at the bottom or there may still be a load on it.

If you’re sure the weight has been lifted free and the post still proves stubborn, take a saw and cut through the post near the top at a slight angle. The lower portion should come away easily. Then remove the top portion if it’s still attached.

Fig. 91-0

Is the footing adequate? If the post beneath a sagging floor is in good condition, then the footing has probably cracked or settled. Even when you’ve identified a deteriorated post as the apparent cause of sagging, check the footing on which it sits. Though not cracked or settled, it may be too small for the job it’s being asked to do. Ask a building inspector for the correct size — footings 2 feet square and 2 feet thick are common.

Removing an old footing — and the concrete slab that may be covering it — can be heavy work. There’s no easy way of doing it. You can rent an electric jackhammer to do the job, or you can break up the concrete into chunks with a sledge. Or you may want to hire a professional.

Replacing a footing. To prepare for a new footing, dig a hole the required size and dampen the soil. If your building inspector requires the top of the new footing to project above the adjacent slab level (many do), you’ll need to build a form to contain the concrete to be poured above the slab. No form is needed below the level of the slab as the earth walls of the hole provide a natural form.

To determine the amount of concrete you’ll need for the footing, calculate the volume of the footing in cubic feet; don’t forget to add in the amount you’ll need to repair the slab around the footing. Add the volumes together if you have more than one footing to pour. Adjust the recipe that follows for the amount of concrete you’ll need.

To make 10 cubic feet of 1:2:3 concrete, you’ll need 1 sacks of cement, 4 1/2 cubic feet of sand, 9 cubic feet of ¾-inch gravel, and 9 gallons of water.

The amount of water is based on 5 gallons per 94-pound sack of cement, and sand of average wetness. If your sand is very wet, use about 4¼ gallons of water per sack of cement. For barely damp sand, increase the water to 5½ gallons per sack.

For small quantities of concrete, you can purchase 90-pound sacks containing a dry mix of cement, sand, and gravel. Each sack will make about 3 cubic foot of finished concrete.

When the concrete needed is about 1/3 yard, as it is for a 2-foot cube, it can be mixed easily by hand. A wheelbarrow is convenient for mixing; each batch can be poured directly into the hole (see fig). Vibrating or stirring the concrete in place will help eliminate air pockets.

Fig. 91-1

Use a straight board to scrape off any excess concrete above the level of the form. If you’re going to install a wood post on the new footing, a stock metal post base should be set in the concrete while it’s still wet. You can also use a pre-cast pier (with a wood nailing insert) set on the wet concrete (see fig. above). If you’re going to in stall a steel post, it will have a flange with predrilled holes for anchor bolts. The bolts should be set in the concrete before it sets.

When the base or pier is in place, cover the fresh concrete with a sheet of polyethylene and keep the concrete moist for two weeks until it cures. Let it cure thoroughly before installing a new post on it.

Installing a new post. Once your new footing has cured, or you’ve made sure that an old footing is not defective, you’re ready to in stall the new post. Wood posts and steel posts are installed in slightly different ways; both techniques are described in the following paragraphs.

But first, check to see if the beam above is spliced directly over the post you’ll be installing. If it is, you’ll want to reinforce the beam before setting the post.

Fig. 92-0

Unless you’re using a wood post (in which case the prefabricated post cap you’ll use to in stall the post will do the job of reinforcing the beam), you’ll have to cut two pieces of ¾-inch ply wood as wide as the beam and 4 feet long. Center the pieces over the splice, one on either side, and fasten them to the beam with 8-penny common nails or ¼-inch lag bolts.

Now you can set the post in place. If it’s a wood post, put it on the post base or pier and nail it to the base or insert, but do not drive the nails home. Then fit the post cap, plumb the post, and hold the cap in place with nails, bi do not set them (see drawing at right). If the post is steel, place it on the anchor bolts and plumb it. Using the top flange as a guide, drill pilot holes and start the lag bolts, but don’t tighten them.

When everything is set, be gin lowering the jacks 1/16 inch per day until the girder is settled firmly on top of the post. Resist any temptation to hurry this process. Finish securing the post by nailing through the post cap and metal post base or insert if you’re installing a wood post, or by tightening the bolts on a steel post.

Continue to lower the jacks until the new post has assumed the full weight of the girder and the jacks can be removed.

How to reinforce a girder

If your sagging floor is caused by a wood girder that has rotted or is infested with termites, you’ll need professional assistance to replace it.

Should the girder appear to be sound — no discoloring or evidence of rot or insect damage — it may be possible to level the floor without replacing the girder. A girder that sags between supports can be jacked up to level and an additional post added as a permanent support (see “Understanding the basic jacks”). If the sag is over an existing post, the footing may have settled, and footing and post should be replaced.

Another alternative to re placing a sagging girder is to jack it up to level the floor and then reinforce it with steel plates or channels — their flat sides turned to the wood — on both sides of the beam. The steel should be bolted in place through the girder. An engineer should be consulted to size the steel and determine bolt spacing. This approach is also practical for strengthening a sound girder in order to add usable space by re moving a post.

Fig. 92-1

With proper counsel and help from friends, a talented handyperson can handle the reinforcing of a girder, but it is hard work. If it’s necessary to relocate plumbing, ducts, or wiring, the job should certainly be turned over to a professional.

Prev: Repairing/Refinishing -- Introduction

Next: Squeaking Floors — Annoying But Fixable

top of page   home

Friday, 2008-12-12 23:59