More than 80% of homes in the U.S. have asphalt-shingle roofs. Asphalt shingles are popular for several reasons: for the money, they offer good value to homeowners; there's a wide, good-better-best range of materials to choose from; and of all prevalent roofing materials, asphalt shingles are easiest to install.
But you still have to do it right. In this article, we'll cover the essentials of roofing and reroofing with asphalt shingles, including techniques and details for tear-off, layout, underlayment, flashing, and shingle installation.
The fundamental material choice is between strip-type and laminated shingles, both of which are built on fiberglass mats. (We know of no U.S. manufacturer that still makes organic-mat shingles.) Visually, most strip shingles have a flat, three-tab design (no-cutout designs are also available), while heavier laminated "architectural" shingles exhibit variations in thickness, shading, and butt-edge design that produce more random or irregular patterns similar to wood singles or shakes. Length of warranty is widely considered a measure of quality, with heavier shingles carrying longer warranties. Most warranties cover prorated material cost, but not tear-off or replacement labor (some manufacturers offer more generous terms to their certified installers).
Tear-Off
Consider local recycling of both the existing asphalt materials and any metal flashing and fasteners. To avoid damage from falling debris, cover shrubs and garden beds with tarps, and tack up tarps under roof overhangs to protect siding and other exterior surfaces. After tear-off (and at the end of each day during reroofing), use a magnet to "sweep" nearby pavement, lawn, and landscape beds to pick up stray nails.
Tear off asphalt shingles from a seated position (see photo). Start at the peak and work down toward the eaves using a shovel-like, serrated stripping tool such as the Shingle Eater to get under shingles and pry out fasteners. Workers should wear safety harnesses secured to a stable anchor point. Three workers can typically tear off about 4 squares of roofing per hour.
Strip only as much area as you can cover with underlayment the same day. Replace unsound sheathing, re-nail loose sheathing, and drive flush any protruding nail heads.
Fall Protection
In March 2013, OSHA issued a directive (STD 03-11-002) that rescinded and replaced its Interim Fall Protection Compliance Guidelines with a standard that requires residential construction workers who are working 6 feet or more off the ground (or within 6 feet of a roof edge or opening) to be prevented or protected from falling by means of a net, a harness, or a guardrail.
As a practical matter, harnesses and guardrails are most feasible for residential projects. Harnesses, which arrest a fall before the worker hits the ground, are less expensive and are easier to set up. Ideally, they are anchored to framing at the highest point on the roof to minimize the need for repositioning (see "Fall Protection for Roof Work," Dec/13).
Guardrails prevent a fall by creating a physical barrier. When used instead of harnesses, they must be installed at both eaves and rake edges, and around any roof openings that are 6 feet or more above the floor below. Manufactured guardrail systems, such as those from Acro Building Systems, are probably the easiest and least expensive option. Acro's Steep Pitch Guardrail (for eaves) and Open-Edge Guardrail (for rakes) include steel base plates that are fastened through keyhole slots so they can be interleaved with shingles, then removed without creating leaks. The posts have integral brackets sized to receive OSHA-compliant curbs and rails.
Underlayment
A variety of materials, including asphalt felt, synthetic sheeting, and self-adhering membranes, may be used under asphalt shingles, either separately or in combination.
Asphalt felt is the most economical underlayment material. Those who still use #15 or #30 felt value its vapor permeability, which they claim enables moisture to dissipate through evaporation (see "Whatever Happened to 30# Felt?" Jan/13). The downside is that paper-based felt is prone to tearing and puckering when left exposed and subject to wet/dry cycling. Fiberglass-reinforced asphalt felt is said to be more tear-resistant and less slippery.
Synthetic underlayment is made from polyethylene or polypropylene, is stronger and lighter than asphalt felt, and comes in wider, longer rolls that cover the roof faster (see "Do Synthetic Underlayments Make for Better Roofs?" Jan/13). Synthetics don't soften or wrinkle when exposed to moisture and are said to offer better footing than felt, even when wet. Most synthetics can be left exposed to sunlight for up to a year. A couple of brands—GAF's Deck-Armor and VaproShield's SlopeShield—are vapor permeable and are recommended for unventilated roofs. Impermeable or not, all synthetic underlayments are at least twice as expensive as felt.
Self-adhering membranes have a continuous adhesive backing that is self-healing, which means the material seals around fasteners that puncture it. However, any of the three basic formulations—rubberized asphalt, butyl rubber, or acrylic—may be incompatible with some other building materials, including caulks. sealants, and underlayment (see "Working With Flexible Flashing," Apr/14). Because of their high cost (about $100 per square), peel-and-stick membranes are usually applied in "problem areas," such as eaves and valleys, and in combination with metal flashing at chimneys, vents, and other roof penetrations (see "Dormer Flashing Details" and "Chimney Flashing Details" illustrations).
Drip Edge
Prefabricated metal drip edge is designed to protect the absorbent edges of sheathing and to drain water away from fascia and rake trim. For a custom look, bend drip edge flashing from copper, aluminum, or other corrosion-resistant metal that matches or complements other roof flashing or trim color.
There is some disagreement about whether drip edge should be applied under or on top of self-adhering membranes. While we recommend following manufacturer instructions to preserve product warranties, best practice at the eaves is to install drip edge directly on the sheathing, apply any self-adhering membrane on top, then overlap that with a compatible underlayment. Along the rake, apply underlayment first, then fasten the drip edge on top, making sure it overlaps the drip edge at the eaves.
When a single length of drip edge isn't long enough, overlap end joints at least 1 1/2 inches. At the rake, pieces higher on the roof should overlay pieces lower on the roof to promote drainage. Fasten drip edge flashing with compatible 1 1/2-inch nails spaced 8 to 10 inches apart.
Shingle Layout
Most roof dimensions are not an even multiple of standard shingle sizes, so fractional shingles will be needed to fill out courses. Tabbed shingles will look better if the fractional pieces are the same size at each rake. (This is less critical for laminated shingles because the finished appearance is meant to be more random.) Determining where to begin requires some preliminary measurements (see "Shingle Layout" illustrations).
Horizontal alignment. First find the midpoint along the eaves and, using the 3-4-5 triangle method, snap a line to the ridge at a right angle. (If dimensions at the ridge differ from that of the eaves by no more than an inch, you may be able to cheat the drip edge flashing at the rakes to split the difference.) Next, measure along the eaves from the centerline to each rake, then divide by the length of a shingle (typically 36 to 40 inches). Divide the remainder in half, and if the result is less than 3 inches, arrange for all fractional shingles to run along one side of the roof. Otherwise, arrange for fractional pieces at each rake to be approximately the same size. This may require using a different offset and working the first course from the centerline out to the rakes (see "Shingle Layout" illustrations). Ensure that starter strip seams are offset from seams in the first finish course. Once the first course is in place, install subsequent courses beginning at the rake.
Vertical alignment. Although exposure marks are printed on many shingle brands, best practice is to snap a horizontal line every three to five courses to ensure that your shingle line isn't wandering. Also measure from the eaves to the ridge to see if you need to make up for a bowed or sagging ridge line that will be accentuated by the cap shingles (see "Shingle Layout" illustrations). You can shrink or stretch shingle exposure 1/8 inch per course, so calculate the number of courses you'll need to make up for any discrepancy. (For example, you can make up 1 1/2 inches by adjusting the exposure 1/8 inch in 12 successive courses.) As with many remodeling details, best appearance may be achieved when materials are installed out of square to match existing visual lines.
Shingling Around a Dormer
Install courses normally on the larger adjacent roof area, snapping lines frequently to ensure proper exposure (see "Shingle Layout" illustrations). When shingle courses clear the dormer ridge, snap a horizontal line to the rake and measure down to align the remaining courses on the other side of the dormer.
To improve speed, production roofers snap a line above the dormer ridge first, then install shingles above and below simultaneously. This requires one infill course.
Nailing
Use hot-dipped galvanized nails; stainless steel nails add cost, but are ideal for coastal areas. For most installations, four nails per shingle are required; in high-wind zones, fasten with six nails per shingle (see "Six-Nail Method" illustration).
To prevent blow-offs, drive nails straight and flush. Pneumatic nailers improve speed but can affect quality unless used with care. Seal an over-driven nail head with roof cement and drive a new nail next to it. For a mis-driven, angled nail, drive the head flush, then repair it as you would an over-driven nail.
