Exterior trim can be applied in many different ways, depending on the design of the house and the type of siding. For wood shingles or clap­boards, use trim boards that are thicker than the siding, and apply trim to the sheathing before putting up the siding. For flat shiplap and board- and-batten sidings, apply trim boards over the siding. In general, try to use the same materials and installation methods that were used on the house originally.

Solid-wood exterior trim should be a rot – resistant species such as redwood, cedar, or hard pine and sufficiently dry to avoid shrinkage, cup­ping, and checking. For those reasons, avoid sugar pine, knotty pine, hemlock, fir, and the like. If you’ll be painting the trim, you may find it cost effective to use finger-jointed trim stock fabricated from shorter lengths of high-grade wood. Such stock is widely available and can be durable if you keep it sealed with paint. For best results, specify vertical-grain heartwood grade because it

Eaves Trim___________

Fascia, soffit, and frieze boards are collectively called the eaves trim.

resists decay, holds paint well, and is the most stable dimensionally. Caution: If this trim is allowed to absorb moisture, its finger joints may separate.

BACK-PRIMING AND PAINTING

Apply primer to all faces and edges of exterior wood (and engineered wood) siding and trim, including the back faces. Back-priming is critically important because wood will cup (edges warping up) when the sun dries out the exposed front face, while the back unexposed face retains mois­ture. The greater the moisture differential between front and back faces, the more likely the cupping.

While cutting trim or siding, keep a can of primer and a cheap brush nearby to seal the ends after every cut; unprimed end grain can absorb a lot of moisture. (It’s especially easy to forget to prime cut edges when you’re using preprimed trim.) Ideally, apply at least two top coats of acrylic latex paint after priming to seal trim and siding. If you want stained or clear-finished trim or siding, use cedar or heart redwood.

ATTACHING TRIM

As a rule, for best attachment, secure exterior trim to framing. In those rare instances where you have only sheathing to nail to, angle the nail so that it will be less likely to pull out.

Choosing attachers. Pick a nail meant for exte­riors. If you’ll be using a transparent finish, mak­ing nail heads visible, stainless-steel nails are the premier choice; though expensive, they won’t

rust. Aluminum nails won’t stain but are some­what more brittle and more likely to bend. Galvanized nails are the most popular because they’re economical, stain minimally, and grip well. Many nail types (including stainless) also come in colors matched to different wood types—cedar, redwood, and so on. Ring-shank nails hold best.

For stained exteriors, some contractors prefer galvanized finish or casing nails because their heads are smaller and less visible. Box nails are a good compromise. Their larger heads hold better than finish nails, yet their shanks are smaller than those of common nails, making box nails less likely to split wood. There are also “splitless” siding nails that come with preblunted points to minimize splits. (The blunt point smashes through wood fibers, rather than wedging them apart.)

Where trim is exposed—say, at cap trim atop a half wall—and you want maximum grip, use stainless-steel trim-head screws instead of nails.

drive nails quickly and accurately, reducing splits and eliminating errant hammer blows that mar trim. After setting the trim with finish nails, you can always can go back and hand nail with headed nails to secure the trim further. Or you can use headed siding nails in the nailer.

Nailing schedules. To face-nail nominal 1-in. trim (actual thickness, % in.), use 8d box nails spaced every 16 in. Nail both edges of the trim board to prevent cupping, placing nails no closer than h in. to the edge. If the trim goes over sid­ing, say, at corners, use 8d to 10d box nails. To draw board edges to each other, use 6d nails spaced every 12 in., and drive them in a slight angle. If you’ll be painting the trim, also caulk this joint or glue it using an exterior urethane glue, such as Gorilla Glue®.

rior Trim tips

About nail heads. Taking the time to line up nail heads makes the job look neater. For example, when nailing up jamb casing, use a combination square to align nail pairs. If you’re putting up a long piece of trim that runs perpendicular to studs, snap chalklines onto the building paper beforehand so that you’ll know stud positions for nailing. If the trim will be painted, take the time to set the nail heads slightly below the surface, using a flathead punch. Then use exterior wood filler to fill the holes. If you don’t set the heads slightly, they may later protrude as the wood shrinks, compromising the paint membrane and admitting water. On larger jobs, carpenters are usually expected to set nail heads. Painters fill and paint them.

EAVES TRIM

Because eaves trim is often complex and can impact framing, roofing, ventilation, and the house’s aesthetic integrity, draw a cross-section of it as early as possible.

There is no single correct way to construct the eaves, but the boxed eaves on the facing page are a good place to start. First, a fascia board that overhangs a soffit by % in. to in. enables you to hide rafter irregularities—rafters are rarely per­fectly straight or cut equally long. Second, that overhang accommodates a rabbeted fascia-soffit joint, which protects the outer soffit edge, even if the wood shrinks slightly. Third, if you rabbet out the back edge of a frieze board or build it out using blocks, the frieze will conceal the top edge of the siding. A built-out frieze also creates an inconspicuous space to install an eave vent.

Ventilation channels at eaves allow air to flow up under the roof and exit at ridge or gable-end vents. This airflow is beneficial because it lowers
attic temperatures and helps remove excess mois­ture from the house, thus mitigating mold, ice dams, and a host of other problems. To keep insects out, soffits need screening. In a wide sof­fit, there’s plenty of room for screened vents in the middle. In a narrower soffit, you may need to leave a M-inch space at the front of the soffit or at its back, hidden behind a built-out frieze board.

If the house has exposed rafter tails rather than soffits, cut down the blocking between rafters so air can flow over the top. Again, staple fine mesh screen or corrugated vent strips behind ventilation passages to keep insects out.

Flashing paper is available in several forms, the most convenient being fiberglass-reinforced rolls either 6 in. or 9 in. wide. (These strips are some­times called felt splines.) There’s also a plethora

of peel-and-stick flexible flashings; but because they’re expensive, they tend to be used mainly as cap flashing for the tops of doors and windows. Foil-faced flexible flashings like Polyken® Foil- astic, for example, adhere aggressively and seal head joints so water can’t back up behind them.

Uncased windows are commonly used in reno­vation when it’s desirable to install casing that matches an existing style. Since precased units offer a limited range of casing styles, many builders install uncased windows and buy appro­priate casing from a local mill.

Flash the rough sill before installing an uncased window. Tuck the paper into the kerf at the bottom of the sill (if there is a kerf) or fold the paper into the RO, caulk its edge, and set the sill onto it. This provides an unbreachable seal, and apron casing or siding will cover the edge of the paper later.

To seal side jambs, consider the double­caulking method shown above. Caulk each jamb edge, embed a flashing-paper spline in the caulk, and then run another bead of caulk atop the spline before installing the jamb casing. This caulk-and – spline sandwich stops infiltration effectively. Finally, caulk the edge of the head jamb before installing the head casing, but do not insert a spline between the head jamb and the casing— that could direct water behind the head casing.

After the window is cased, add the (metal) cap flashing over the head casing. Before installing flashing paper over metal flashing, careful
builders first caulk the upper flange of the flash­ing. This will prevent water that gets under the siding from wicking back up under the paper when it hits the window head. This is especially important with stucco siding, which is water per­meable and often collects water between the stucco and the building paper. Chapter 6 offers more specifics about cap flashing.

Should jamb flashing go over or under the building paper covering the sheathing? It doesn’t really matter because the strip flashing is extra protection. But under the windowsill, leave the lower edge of the flashing spline unstapled so it can overlay the building paper or housewrap below.

Windows and doors with integral nailing flanges require slightly different detailing. If the flanges are metal, apply fiberglass-reinforced flashing paper along the bottom and sides of the RO first. Then apply a bead of exterior caulk around the perimeter of the paper—close to the edges of the opening—and set the unit over the flashing paper. Position and shim the unit; then nail or screw through the flanges, into the sheathing. Install cap flashing (some units have an integral cap flashing) over the head of the unit, caulk its upper flange; then install flexible flashing over that cap flange. Building paper above will overlay the flexible flashing. Note: Most flanged windows don’t have casing per se. Unlike wood-cased windows, flanged units have jambs that protrude above the surface of the

sheathing, so siding (or casing) is butted to the sides of these protruding jambs.

Exterior Trim

In appearance, exterior trim is sometimes ornate and complex, but its basic function is to cover critical building seams.

Installation techniques for housewrap and build­ing paper are much the same; in this chapter, both materials are often referred to generically as building paper.

Before starting, survey the sheathing and knock down protruding nail heads and wood sliv­ers that could puncture the paper and admit water.

Installation is two-person job, especially if you’re working with 9-ft.-wide rolls of Tyvek. As one person unrolls the material, the other aligns and staples it. Start at the bottom of the wall, overhanging the bottom edge of the sheathing by about 1 in. Position the building paper, tack an upper corner in place, roll out about 8 ft., and raise or lower the roll till the edge of the building paper is roughly parallel to the bottom edge of the sheathing. When it’s in position, staple the paper every 16 in. along the edges and every 24 in. in the field; use!4-in. or Мб-in. staples. Thereafter, roll out 3 ft. to 4 ft. at a time, stapling as you go. Trim the 1 in. overhang later.

As you roll out the building paper, make sure it lies flat to the sheathing. Otherwise, puckers can elevate the siding or trim applied over them, compromising weather tightness. Typically, the person holding the roll maintains a slight tension on the building paper to prevent puckers. Also, take care to avoid tearing the membrane while handling and stapling it—more of a problem with paper than with plastic housewrap, which stretches.

Overlapping and taping seams. Overlap verti­cal end seams by 4 in. to 6 in.; overlap horizontal seams 6 in. to 8 in. As you work up the wall, applying courses of building paper, upper courses should always overlay those below so any water that gets behind the siding is directed out and down, away from the sheathing. Building paper should also overlay any flashing, especially cap flashing over doors and windows.

Seal building-paper seams, tears, and punc­tures with seam tape; smooth down the tape to make sure it adheres well. If you’re installing plastic housewrap, use a seam tape recommended by the manufacturer. If you tear building paper, just staple a patch of the same material over it and apply caulking over the top edge. Last, install siding as soon as possible after installing the building paper because wind can lift and stretch it, especially plastic housewraps.

Corners and rough openings. Take special pains with corners. Although a 4-in. overlap is often adequate for corners, you’ll be safer to over­lap outside corners by 1 ft. In addition, many builders reinforce corners with a self-adhering flexible flashing to keep water out should corner boards or siding gap. To prevent the material’s bunching in inside corners—a common problem with building paper—use a straight furring strip to press the paper into the corner before stapling it. That done, tape the corner seams immediately— before wind has a chance to lift the paper.

Builders installing plastic housewrap usually roll it right over rough openings (ROs) for win­dows and doors, cutting an Xin the middle of the opening and folding the housewrap into the sides of the framing. Well, that method can’t hurt. But it’s doubtful that it truly waterproofs the opening. If you’re installing metal windows, maybe a layer of housewrap will isolate the moisture that con­denses on cold metal. But if flashing paper is correctly installed about the perimeter of the window or door, water shouldn’t enter the RO in the first place.