CLC: Basic Roofing Systems

by Kacey Larsen
NARI recertification credits

Roofing systems are not created equally. There are two main classes: those designed to shed water, and those designed to be waterproof. Our focus is on those designed to shed water.

Water Penetration

Gravity carries water down the roof, and anything that penetrates a joint finds another shingle beneath and flows back to the surface. Mechanical fastening of overlapping units is easy, and the holes for fastening can be protected by the overlap. Because such units are usually of reasonably small size, there is no problem with differential movement, which is easily accommodated at joints without affecting the water-shedding qualities of the system. Moisture escaping from inside the building can find its way to the outside atmosphere because the system is not a sealed membrane.

During rainstorms with high wind conditions, water might penetrate some distance into this space, but it is not a cause for concern. Gravity draws water down the roof and out of the headlap space. Prevent penetration to the interior by making the headlap large enough and the slope steep enough, so the force of gravity drawing water out of the space and down the roof exceeds the effects of all the other forces tending to carry water inward.

Asphalt Shingles

Asphalt roofing comes in a wide variety of styles, making it possible to satisfy almost every roofing requirement. Their fire resistance features, combined with low cost and ease of application, make asphalt shingles versatile and economical roofing products.

Asphalt shingles are made on organic felt bases or fiberglass mat bases. Some advantages of fiberglass mat bases include better weather resistance, longer life and a Class A fire rating. Both types are voluntarily submitted to Underwriters Laboratories, which tests the shingles in accordance with established fire resistance requirements. Roofs are particularly vulnerable to fire from external sources, such as sparks or branches carried from nearby fires. Asphalt shingles with a self-sealing feature, which is activated by the sun, have been designed and tested to withstand gale force winds.

Steep and Slope

The common principles and problems of steep roofing presented here deal mainly with asphalt roofing because it is the most popular roofing used today. These principles and problems include the following subjects: decking, fastening, slopes, flashing and ventilation.

Decking is an extremely important part of the planning stage for roofing. For steep roofing, the decks most commonly accepted by manufacturers and codes are plywood, oriented strand board, and 6-inch-wide tongue-and-groove boards. In general, roof insulation presents fastening problems and, in almost all cases, is not acceptable as a suitable deck by most roofing manufacturers.

Fastening goes hand-in-hand with decking. With plywood, panels and tongue-and-groove boards, the standard roofing nail or staple is acceptable. The nails must be long enough to penetrate at least ¾ inch into the deck. This is particularly important when applying a second layer of shingles over an existing roof. Staples are not currently recommended for fastening a second layer of asphalt shingles or in some high-wind or hurricane areas.

In steep roofing, there are different requirements for different slope conditions, and each type of roofing has its own requirements. With asphalt roofing, there are three basic slope conditions:

  • The standard slope is from 4 in 12 up to 21 in 12. This slope requires the use of one layer of underlayment felt and standard fastening.
  • The mansard or steep slope condition is from 60 degrees or 21 in 12 up to near vertical. One layer of underlayment felt, six fasteners and tab cement are required.
  • The low slope condition is below 4 in 12 but not less than 2 in 12. This condition requires two layers of 36-inch-wide underlayment felt laid up in shingle fashion, exposed 17 inches. Some codes require this felt be cemented to itself—in effect providing a waterproof membrane below the shingles. The shingles used must be of the self-sealing type or have all the tabs hand cemented.


Safety must be your primary concern when working on a roof. Ensure workers are in full compliance with OSHA’s Fall Protection Rules. In addition, workers need to consider the following hazards:

  • Stability—Remodelers working on older homes must be on the lookout for rotted roof members.
  • Weather—A sudden storm can be dangerous and provide little time to escape.
  • Falls—Roof edge and skylight awareness (and the recognition of any other roof openings) goes a long way in preventing accidents.

The lead carpenter is the remodeler’s primary eyes and ears, and the first-line safety monitor on the jobsite. QR

 Take the NARI Recertification quiz for this article here.

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