How to Build Additions: Installing Sheathing and Siding

Sheathing is a solid membrane of panelized material, such as plywood or fiber board, that stiffens the wall framing and provides a nailing surface for certain types of siding, such as shingles or boards.

Permanent diagonal bracing of metal straps or let-in 1 by 4 boards is sometimes used instead of sheathing to brace the walls when siding can be applied directly to the studs. But in high-quality construction, walls are commonly sheathed with solid material. In some cases, as with plywood siding, the same material provides both sheathing and siding, which saves money.

Installing plywood sheathing: Cutouts for foundation vents, Blocking where plywood seams will fall

Installing Sheathing

Plywood sheathing must be at least 3/8 inch thick and is nailed at least every 6 inches around the edges and every

12 inches in the field with 6-penny (6d) common or box nails. Check your plans for additional nailing or blocking requirements for special conditions such as earthquakes, high winds, or a steep hillside. Thicker plywood (at least ½-inch) is necessary if you plan to nail shingles or stucco lath to the sheathing.

Seams must fall over studs or framing. Apply plywood horizontally for greatest strength. Line bottom sheets up along bottom of mudsill to lap over floor framing and tie foundation sills to studs. Install blocking behind top horizontal seam. If there will be no soffit and plywood sheathing is also the siding, notch out tops of panels for roof rafters and butt up to roof sheathing. When they are avail able, it's more convenient to install full-length 9-foot panels vertically from mudsill to top plate or roof sheathing. Always use large sheets as much as possible. Don’t patch several scraps together to save cutting up a full sheet.

Cut plywood flush with inside edges of rough openings for windows and doors. Make cuts before applying panel by taking careful measurements and transferring them accurately; or install the full panel and cut out the opening later with a reciprocating saw.

Apply fiberboard sheathing vertically and nail with 1½-inch or 2-inch galvanized roofing nails spaced 3 inches around edges and 6 inches in the field. In some cases plywood is used instead of fiberboard for corner panels to provide extra shear strength.

If siding is applied directly to studs, you must first install permanent diagonal bracing on the exterior side of the studs. Metal straps, which are stronger and easier to install than 1 by 4 let-in bracing, must cross each wall at 30 to 45 degrees, must connect the top plate and soleplate (or mudsill, if possible), and must be nailed at each stud with a 16d common nail.

Installing Windows

Although it's sometimes possible to find recycled windows to match the old ones perfectly, it's better to choose new windows that match as closely as possible. Modern pre-hung units are more durable, more energy efficient, and easier to install.

Rough Openings

Most types of windows, whether wooden or metal, are installed after the sheathing, if there is any, and before the siding. Be sure the framed opening dimensions match the rough opening specifications, not the nominal size of the window.

Always measure both diagonals of the rough opening to see if it's square. Variations of up to ¼-inch are acceptable, but if the window is out of square by more than that, adjust the framing. The most critical member is the rough sill, which must be level. Shims can compensate for a slight deflection.

Most pre-hung wooden windows include exterior casing, making trim work easier later on. Siding material is butted up to the casing edge. Metal windows have thin nailing flanges that are covered by siding.


Staple window flashing across bottom of rough opening and along both sides, so side pieces overlap bottom and all inside edges are flush with opening. Flashing paper comes in rolls 6 inches wide. Do not apply flashing over the top until after window is in position.

Caulk back of wooden window casings or nailing flange of metal windows. Then, with a helper, set window into opening. The person inside levels the window frame with shims to match other windows. The person outside keeps window from falling out.

Drill holes through casing or nailing flange (if not predrilled), and nail with 12d HDG casing nails if casing is wood or with galvanized roofing nails if it's metal.

From inside, place shims between side jambs and studs. With wood windows, if required by the manufacturer, drive nails through jambs and shims into studs, toenailing from edge of jambs rather than face-nailing. Be sure sill is shimmed and blocked securely. Pack insulation loosely between jambs and studs.

If window is wood, install drip cap and metal flashing above head casing outside. Staple final piece of paper flashing to overlap side pieces and cover metal flashing or flange. Wall is now ready for siding.

Installing a metal-framed window: Top strip of flashing stapled over flange; Flashing paper

Choosing Siding

Materials for siding are usually selected during the planning phase and specified in the plans. For most room additions it's best to match siding material to the house, but sidings on older homes may be hard to match. Begin your search early so you can make changes, if necessary, before the material is needed.

To match an obsolete or unusual pattern, try recycling yards that specialize in building materials or a lumberyard that does custom milling.

If you are unable to duplicate your house siding, be careful in choosing a different material for the addition. Your addition shouldn't look “stuck on,” but should harmonize so that it looks like part of the original house.

There are ways to minimize discrepancies. You can use a similar pattern, paint it to match, and depend on the trim, roofing, and window styles to maintain continuity. Or you can plant large bushes in front of the seam, where the discrepancy is most obvious. You might consider re siding the entire house with the same material as the addition. Or, if the proportions of the new addition allow it, you could even choose a siding completely different from the house to provide a contrast. Siding for the top half could be plywood or similar plain material. Such mixing of materials can be effective, but it demands skillful and sensitive treatment.

Installing Siding

Siding for a room addition is applied the same as for any new construction, except that care must be taken in joining it to the old siding. If the walls intersect at a corner, most new sidings are simply butted up against the old. Careful application of building paper, caulking behind the seam, and tight installation of trim board prevent leaks.


If the siding is stucco, a strip of old stucco must be broken out where the new joins the old, leaving 3 inches of lath (wire mesh) exposed so the new stucco lath can be bent around the corner to lap over the old wire. Al though you will probably have any stucco work done by professionals, you can save some of the cost by doing the demolition yourself, under your installer’s supervision. Be sure to expose at least 3 inches of wire at every seam. To avoid cracks in the rest of the stucco, start with a series of hammer blows along the cutting line before you try to break up and re move any of the stucco in the 3-inch-wide section. Practice first in an inconspicuous place.

Wood Shingles

If siding for the addition is wood shingles to match the existing walls, the addition will present a stark contrast to the weathered house shingles. This is only temporary, however, unless the old shingles have been stained or painted. Simply leave the new shingles alone; in a year or two they will weather enough to match the old. If you want the addition to match the house sooner, apply paint or stain to both the new and the old shingles, following manufacturer’s recommendations. Some paints cause the shingles to deteriorate by locking in moisture.

The new shingles should be the same size as the old. The most common lengths are 16, 18, and 24 inches. If you aren't sure what size is on your house, and you aren't able to measure a full shingle, you can gauge their size by adding ½-inch to the exposure and doubling this number. E.g., a 7½-inch expo sure would indicate a 16-inch shingle. Use the same shingle exposure for the new walls. Start the first course at the same level as one of the existing courses. If the house has settled slightly or the original shingles were not applied perfectly level, your first course may not align with the house shingles after wrapping around the old, cover the seam with batten, or hide it with tall plants.

Vertical wood siding is easy to join o the old. To ensure a weathertight seam, remove enough old boards to lap the new paper underlayment over the existing paper about 4 inches. Then replace the old boards and continue installing new boards the same way. If the vertical siding is tongue-and-groove it can be blind- nailed. Otherwise the boards must be facenailed. Follow the same pattern as the original siding, making sure that horizontal furring strips are nailed across the studs at the nailing line if the addition does not have solid sheathing to nail into.

Plywood Siding

It should be easy to match the existing plywood siding because patterns and thicknesses are fairly standard. You may be able to cut costs by using a different type of veneer if you plan to paint the siding. If you are staining the new siding, however, use the same species of veneer as the old. Otherwise it will be difficult to match the stain color.

Where the new panels join the house you can cover the seam with a batten, or use trim boards at a corner. Most corners are trimmed with two overlapping 1 by 4s, but the joint will appear more symmetrical if you use a 1 by 3 on one side and overlap it with a 1 by 4 on the other. Inside corners can be done the same way, or with a single piece of trim such as a 2 by 2 or some cove molding.

Installing Exterior Trim

Exterior trim is most commonly in stalled around doors, windows, corners, and roof overhangs. It may be fashioned from dimension lumber or exterior moldings, which are usually redwood or other durable species. Do not use finger-jointed moldings outdoors.

39 Shingles, siding, and exterior trim: Ridge shingles overlap shingles; Shingles overlap fascia, Shingles overlap each other, Gutter carries roof water directly to drain via downspouts, Fascia overlaps soffit, Siding overlaps drip cap, Drip cap overlaps casing, Soffit, Casing overlaps window assembly, Water table overlaps foundation. Drainpipe carries water away from house.

Installing trim is fairly straightforward—usually a matter of precise measuring and fitting. However, you must take some extra precautions to ensure a good, weathertight seal:

+ Prime the back of all trim pieces.

+ When joining pieces of trim end- to-end—especially vertical members such as battens or corners—cut ends on a bevel instead of square. Install so that face of upper board overlaps lower board slightly. That way, when the boards shrink, the joint will not open up and reveal a gap.

+ When covering a horizontal seam of plywood or other material with trim, make sure seam has a tightly sealed rabbet joint or proper flashing; otherwise trim will trap water in seam thereby making a bad joint worse.

+ Pre-drill nail-holes at end of each board to prevent splitting.

+ Apply a bead of caulk to back before installing each board, and set all nail heads.

+ When installing soffit beneath roof overhang, make sure each space you are enclosing has ventilation from attic; or install soffit vents.

+ When trimming windows, install metal flashing or a drip cap over head casing if the window does not have integral flashing, such as a nailing flange. Flashing should be tucked under siding and overlap casing.

Next: Access to the Room Addition

Prev: Installing Skylights

Top of Page  All Related Articles  Home