Understanding Roofs

A pitched roof sheds water much the same way a duck’s feathers do. Courses of roofing material—most often some variation of the shingle—lie one atop the other and overlap, like a bird’s feathers. Shingles on a roof are layered at least two deep, with exposed portions slightly smaller than half the total area of the shingle. At the top, a ridge vent or an extra layer of shingles covers the ridge.

Valleys, the places where two slopes meet, direct runoff into gutters, which in turn direct the water into downspouts. For additional protection against leakage, metal or composition flashings are placed under the shingles at the roof’s most vulnerable points. Typically these include valleys, dormers, vents, and chimneys, or anywhere the roof’s surface is penetrated. Valleys are notorious problem areas; the flatter the roof, the greater the potential for leaks. Metal valley flashing must be wide enough and the pieces must overlap correctly. If no flashing is used, roofing must be woven together seamlessly—a job that’s definitely for pros. A valley that does not have metal flashing is vulnerable to damage; avoid walking on it.

Any area with flashing— especially around a chimney—is liable to leak. Chimney flashing must be installed correctly and sealed tightly against the vertical surface that it abuts. Vents, whether they are plumbing pipes or exhaust fans, usually come with integral flashing. Typically, the lower half of the flashing is exposed, and the upper half is covered with roofing. Beneath the roofing material lies a house’s most complex structure, framing (typically gable or hip). The framing ties together the wall structures and supports not only the weight of the shingles but also other loads, such as snow and ice in colder climates. Rafters, rising from the top plate of the wall to the ridge board, define the roof’s pitch. Collar ties in the attic help keep the rafters from spreading; headers box in any openings.

Deck sheathing, usually plywood, goes on top of the rafters to give the structure rigidity. A layer of roofing felt seals the sheathing against moisture. Rafter ends are trimmed at the eaves with a fascia board (to which a gutter is fastened) and along the rake with rake boards. Once the trim is protected with drip cap, the shingles can go on.

Understanding Exterior Walls

Regardless of your home’s exterior skin (siding or brick), its skeleton probably looks like the one shown here. The sill plate rests on the masonry foundation. Rim joists, collectively called a box sill, rest on the outside perimeter of the sill plate. Floor joists (which also act as ceiling joists for the basement, if there is a basement) attach to the rim joists. In some cases a short framed wall rises from the foundation, and the joists rest on this wall. If your basement ceiling is open, periodically check the rim joists, the sill plate, and the sill seal. Attend to any moisture immediately because rot here could cause structural damage. It is also here that termites and other wood-boring insects often enter the house.

Exterior wall framing is made of 2x4 or 2x6 lumber (2x6s are common in newer homes in cold climates because they accommodate thicker insulation). A sole plate rests on the joists. Vertical studs are usually spaced 16 inches on center (sometimes they are 24 inches on center). The top plate is usually doubled to ensure that it can support second-story joists or the roof. Over a door or window, a header carries the load and must be as strong as the rest of the wall.

Insulation between the studs conserves heat . Wood or composition board sheathing adds insulation and strength. Then a layer of building wrap, either asphalt-saturated building paper or plastic, seals the sheathing. It is essential that the building wrap be installed correctly so that moisture is sealed out rather than trapped inside the wall.

Outside, siding faces the elements and gives your home its visual character. Shown here is horizontal lap siding, named because the boards overlap each other. Other types of siding may be vertical or horizontal, but all overlap in some way in order to seal the exterior . Vinyl and aluminum siding often give the appearance of wood, but with differing maintenance requirements. Siding made of pressed board needs to be kept well protected or it will disintegrate quickly.

Regardless of its composition, siding deserves a careful semiannual inspection. Scan its surface systematically, using binoculars for closeups of high places if necessary. Look for cracks, splits, peeling paint, and evidence of rot or insect damage. Any breaks in your home’s skin— no matter how small—will eventually admit water into wall cavities. If you neglect the repairs explained in this section , moisture could wreck insulation, framing, or even interior wall surfaces.

Protecting your Roof

Note that the first five categories in this chart work well on the pitched roofs. But they will not protect a flat or only slightly sloping surface from standing water. Such surfaces require a watertight membrane system and often a contractor’s help. Asphalt shingles and roll roofing are the least expensive options; slate and clay tiles are the most expensive. Most do-it-yourselfers can apply asphalt shingle and wood shingles and shakes. Other types require advanced or pro skills.

Roof Material Comparison




Life Span

Asphalt shingles

Most popular by far, these are made of roofing felt saturated with asphalt and coated with mineral granules; newer types have a fiberglass base for better weather- and fire-resistance

Little at first, but over the years, shingles curl, crack, and lose surface granules; most cement themselves down in the hot sun; repairs are fairly easy

15 to 30 years under temperate weather conditions; better-quality shingles carry 2 5-year guarantees

Wood shingles and shakes

Shingles have a uniform, machine-sawn appearance; shakes, a rustic, hand-split look; both have a poor fire rating unless specially treated; both are expensive

Unsealed types sometimes rot, warp, and split—and soon weather to a soft gray; like asphalt shingles, they are not difficult to repair or replace

20 years or more for shingles; up to 50 years for shakes if maintained properly

Slate or clay tiles

Both are heavy, expensive, and absolutely fireproof; tiles are more common in the Southwest, and slate in the East, where the quarries are located

An occasional cracked or chipped tile can be tricky to repair; slate is somewhat easier to repair

75 years or more, provided repairs are made before underlayment is damaged

Roll roofing or selvage

Same material as asphalt shingles, but comes in the form of wide strips that are lapped horizontally across the roofs surface

Lightweight, single-layer installations fail frequently— but repairs are easy

5 to 15 years; ask if company will come back for patching

Metal roofing

Older types include terne —a tin/steel alloy—and copper; modern styles include corrugated or ribbed aluminum and galvanized-steel panels. Aluminum, steel, and terne often are painted

All must be flashed and fastened with the same type of metal or electrolytic action will cause deterioration; may need periodic painting.

35 years for aluminum and steel; copper and terne are even more durable

Built-up and “rubber” roofing

Used on flat or low-pitched roofs, fabricated on the job by laminating layers of felt with asphalt or coal tar, then topped with gravel; “rubber” or modified-bitumen roll roofing is installed by torching it down

Leaks due to a poor job are fairly common; fortunately, repairs are not very difficult

5 to 20 years; generally, the more layers, the longer life you can expect it to have; a rubber roof lasts 30 years or more


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