May, 2002
Inspection News and Views from the American Society of Home Inspectors

Inspecting Today's Predominant Residential Installed Roofing Material: Asphalt Composition Shingles


I  find myself inspecting asphalt composition shingles more often  than any other installed roofing material. During the past 25 years as this type of shingle gained in popularity, the most frequently used type changed from a primarily organic (cellulose) reinforced, Class C fire-rated shingle to an inorganic (fiberglass) reinforced, Class A fire-rated shingle. Flexibility is one of the differences between the two. Fiberglass or inorganic shingles are brittle and easily damaged in cold weather. If organic shingles are still used, it's probably because they are more flexible than fiberglass in colder weather. Even though the make-up has changed, the term “asphalt composition” is used to identify both organic and fiberglass shingles.

To inspect roof covering and flashing as required by the ASHI Standards of Practice, a home inspector must be able to identify a properly installed roof system, and be able to recognize one that is not. Often this will require knowing something about an asphalt composition shingled roof. This is true even with a new roof. I routinely find mistakes in newly installed shingles, both on new construction and re-roofed houses. Because of my roofing background, my knowledge of basic asphalt composition shingle installation may exceed the scope of what is needed for a typical home inspection, but I believe home inspectors will find it useful to know what shingle manufacturers recommend and/or what is common practice.


Because the deck performance can have a strong effect on shingle performance, most shingle manufacturers require certain minimum decking standards. The deck must be structurally stable, solid, smooth and secured adequately to rafters or trusses. Sheathing spacing must be maintained to allow for expansion, typically 1/8". The sheathing must be able to hold the nails, resist wind force and meet weight requirements, such as snow loads. Typically, shingle manufactures require the following minimum thickness on sheathings:

• Veneer type plywood 3/8"

• Non-veneer type (OSB) oriented-strand board 7/16"

• Wood boards must be 1" nominal thickness and not more than 6" wide. The 6" restriction is because wide boards (greater than 6") continuously expand and contract as the humidity and moisture conditions change.

Construction in the past frequently used various widths up to 1"  x 10" but due to expansion and contraction shingle buckles are common. Boards wider than 6" need to have wider spaces and will move more than the shingles can allow without tearing or buckling. When re-roofing, the contractor should cut the boards that are wider than 6" and re-nail, but this is rarely done.

Some roofers do not understand why the buckles occur or refuse to accept the reason. Occasionally you may see other special types of sheathing installed, but not in my geographical area.


Basically there are two types of shingle underlayment: water-resistant and waterproof. Shingle manufacturers recommend the use of underlayment except on roof-over installations. When a roof-over is being done, installing underlayment between the old and new shingles interferes with the “nesting” or proper placement of new shingles over old, and shingle manufacturers advise against it.

Water-resistant shingle underlayment - Resistant is the key word. It is often referred to as roofing felt, tarpaper or asphalt felt. The most common type is 15# asphalt felt. Less common is the heavier version 30# asphalt felt. There are also special premium felts available that have heavier asphalt with fiberglass re-enforcement, which provides more strength and resistance to wrinkles. The original water-resistant underlayments were used for “drying in the roof,” or to keep the deck boards dry until the shingles could be installed. It was also useful as a separation between the asphalt shingles and the pine resin in the boards. Pine resin can cause the asphalt to break down and prematurely fail. Some roofers say the layer also helps conceal the minor imperfections in the decking or the “picture frame look” that you will commonly see on sheathed homes. Although not a major concern, felt underlayment that becomes wet will wrinkle. When severe, it may show through to shingles. Roofing felt paper (tarpaper) is a temporary water deterrent at best. Sun and moisture degrade the material quickly, and then the nails penetrate the material. If the wind blows off a few shingles, it serves as a backup – making the difference between a few drips and a waterfall.

Waterproof shingle underlayment - Commonly referred as “ice and water shield” or by its many brand names, waterproof membranes are made of asphalt and polymers with a fiberglass re-enforcement, which creates rubbery, sticky asphalt. The material seals around nails as they are driven through, which allows it to remain waterproof. Waterproof underlayments were designed to seal the roof and to prevent water from getting inside the building due to ice dams and/or wind driven rain. It is an excellent product for use in critical areas, such as low slopes, eaves, valleys, skylights, or at any flashed area. Unlike the standard felt underlayment, it is unaffected by moisture so it does not wrinkle.

Drip edge flashing

Though not used throughout the United States, drip edge flashing is often installed to prevent wind driven rain from getting under the underlayment and damaging the sheathing along the edges. As with all roofing principles, the upper pieces should always overlap the lower ones. On water-resistant (tarpaper) installation, the underlayment should be under the drip edge flashing along the rake, and over it along the bottom. This prevents the wind driven rain from getting under the underlayment and allows the rain to run off at the bottom. You will see it installed both ways, but under at rake is the correct way per manufacturer’s specifications (see Figure 1). Keep in mind the purpose of the flashing. On the waterproof type either over or under is acceptable along the rake, but it should still be over at the bottom. The material is sticky, and it seals itself to the flashing, which is why it can go over instead of under when used with waterproof underlayment.

Figure 1


Common mistakes found during an inspection

Misaligned shingles –
The mistake I see most often is shingles out of alignment, a sure sign of amateur workmanship. Looking at a 3-tab shingle roof, the cutouts (also known as slots or keyways) should be straight up and down, and the butted joints should be horizontally lined up. Excessive variance on either is a red flag to check carefully for other signs of poor workmanship. But if everything else is correct, a roof with misaligned shingles will probably shed water.

The starter course – The absence of or the incorrect installation of the required starter course shingles is another common mistake, especially on re-roofed houses. A start-er course strip must be installed along the eaves to prevent water from penetrating the sheathing under the cutouts of the bottom course of shingles. Shingle starters are typically standard (3 tab) shingles. They are used routinely on both standard and dimensional type installations.

The most common method of installing starter shingles is a whole shingle positioned with the tabs up – offset 6". Sometimes whole shingles are used with tabs down, still offset 6". Some roofing contractors install starters both ways, stating they've been doing it that way for 25 years and never had a problem.

Most shingles manufactures recommend a different method, (see Figure 1). The bottom tabs are always cut off. By cutting off the bottom tabs, the self-sealing adhesive can be placed along the bottom of the overlying first course to help prevent blow off. The use of a whole shingle makes that impossible, and also creates a slight hump under the bottom of the second course that could play a role in damming. By removing the bottom 5" (the tabs), the starter shingle is 7", which is perfect for new installations.

On re-roof (shingle over shingle), the shingles must nest into the old lay of the shingles, (bridging will create an irregular appearance on the finished roof). This requires a 5" starter instead of a 7" one. This is accomplished by cutting off the bottom 5" tabs and cutting off an additional 2" off the top of the shingle. (See Figure 2). This allows for the starter course to nest into the old shingles. To prevent the ends from lining up, the starter course must have a 6" offset from the overlay top course.
You won’t have much luck trying to convince a roofing contractor with 30 years experience that his starter courses have been wrong, but it’s important to note if it does not comply with the manufacturer’s specifications. It could have an influence on the limited warranty, but it doesn’t void the warranty be-cause shingles warranties are for product defects not installation error.



Overhangs – Inadequate shingle overhang is an indication of amateur workmanship. If there is drip edge flashing installed, the overhang can be less (minimum 1/2"). Without drip edge flashing, the minimum is 3/4". As a general practice, roofers overhang 1" on rake and 1 - 1 1/2" on eaves. Insufficient overhangs typically cause water damage to fascia and rake boards. Excessive overhang is also common. When shingles overhang too much, they are more likely to have wind up lift and to blow off. Also over time, the excessive shingle will curl over to create an unsightly view.

Nailing – Most shingle manufacturers prefer nails to staples. A minimum of four nails or staples per shingle is the usual requirement. Proper nail placement is critical in shingle performance. Incorrect nailing or lack of nails can result in shingle blow off and/or shingles slipping out of place. On vertical (steep) applications or in high wind areas, shingle manufacturers recommend additional nails (six), and the use of small quarter size dabs of roof cement is necessary to seal down shingles. Shingles will not self-seal on vertical surfaces, which usually ends in blow offs. A small dab of roof cement under the tabs helps hold shingles down. Fasteners should be located directly above the cutouts, at least 1/2" above them. The nail should not be in the self-sealing strip or above it. Roofers commonly ignore that specification and nail in the sealant. On shallow slopes many roofers nail high (above the line). Laminated (dimensional) shingles usually have a nailing line. Sometimes nailing high will cause the laminated part to separate during hot weather and slip down. If you see several laminated shingles that have slipped down, it's probably because the installer nailed too high. Fasteners should not be exposed. The necessary exposures, such as the finish ridge cap or bottom flashing flanges, should be sealed over with sealant. Nails and staples should be flush with shingle. With the wide spread use of pneumatic fasteners, it’s common to find over driven, crooked and under driven fasteners. Also the smooth shanks on coil roofing nails have been blamed for back outs or nail pops.

Exposure – The exposure on standard 3-tab shingles is typically 5", and it varies with other types of shingles. On nail-overs (shingle over shingle), the first course will have a 3" exposure, due to the nesting method. Roofers sometimes shorten the exposure on shallow slopes to 4", which is all right on 3-tab shingles except it moves the self sealing strip, and it increases the chance of wind blow off. It is not recommended on the irregular dimensional shingles. The irregular voids it creates increases the chance of wind blow off and wind driven rain backing up under shingles. I routinely see short exposures (sometimes 3") on both dimensional and 3-tab shingles on shallow slopes.

Installation method –
Most shingle manufactures recommend installing shingle using the diagonal method (see Figure 1). The vertical method (called racking) is also approved, but typically shows patterns on a finished roof, which is more noticeable on dimensional style shingles. Roofing contractors often install 3-tab shingles in the vertical (racking) method. Most dimensional shingles are installed diagonally.

Additional common problems

After the precursory inspection of the new roof, it’s time to check for a few more common problems that can be seen by the observant eye.

Check for holes. On steep roofs, roofers will often install a toe board (2x4"nailed through shingles) to hold materials and/or roofers on the roof. After shingles are complete, the board is removed and the nail holes are either forgotten or caulked. I’ve seen white latex caulk used to fill holes in a black shingle roof. What good is caulk on a 20-year shingle roof? If the shingles were installed during cold weather, they may not seal down until summer. You may want to point that out to your client, because until shingles seal they are more prone to wind damage.

When inspecting older shingles, it can be difficult to distinguish between what is “normal for the age” decline and “premature” decline. Research shows shingles rapidly age during the first years after installation, some granule loss, minor curling around edges and even small blisters may develop, all considered normal. After this initial curing stage, the shingles enter a long, slow aging process that usually last the better part of the shingles’ life. During midlife, shingles may lose more granules, but not a noticeable amount, and surface cracking may develop. After the midlife period, the shingles enter a declining stage, during which deterioration accelerates again, ending in failure.

To make informed judgments on the condition of the roof, it helps to understand the violent conditions shingles are exposed to including sun, wind and seasonal weather changes. Think about a hot summer day, when the shingle surface temperature reaches 150 plus degrees, and along comes a thunderstorm. The cool rain quickly drops the material temperature 50-75 degrees. Thermal shock occurs. The shingles and roof sheathing expand and contract like this time after time, year after year. Little by little the shingles tear, and sometimes fail. The shingle must be able to expand and contract along with everything else. When they don’t, it’s often thought to be a manufacturer’s defect, referred to as thermal splitting or tearing. There was an excellent article on thermal splitting in the January 1998, ASHI Reporter on “Roof Shingle Failures.”

Ventilation – In addition to the harsh environment, there’s the issue of ventilation. Research has shown improperly vented attics or air space inhibits air movement and usually increases moisture content. Under these conditions, heat builds up and shortens the shingle life. Moisture builds up, causing sheathing movement and/or deterioration, resulting in shingle failure. This hostile environment is one of many factors that affect the longevity of the shingle. Air Vent, Inc. offers seminars on the topic, which I attended and found useful. The company serves as a valuable information resource, and authored an article published in the September 2001 ASHI Reporter.

What happens as shingles age?

As shingles age, the petroleum in the asphalt is drawn up through the shingle, where rain can wash the oils away. The asphalt hardens and the granules begin to break away. Granules serve two basic functions: to give the roof its color and to serve as a sun shield, protecting the shingle from harmful ultraviolet rays. As this process advances, the asphalt has a shrinking effect that often causes the shingle to curl up around the edges. Surface cracking, which is different from thermal splitting, is also common. Surface cracking occurs only on the surface, while thermal splitting is a complete tear through the shingle. Blisters may appear as shingles age. The blisters may be as small as peas or slightly larger – open or closed. I call open blisters “pits.” Blisters often are caused by improper attic ventilation, although some roofers believe they are due to a manufacturer’s defect. I’ve seen blisters on three-year-old shingles.

The big picture – When inspecting a roof, I find it wise to first look at the big picture. I step back far enough to get a clear view of the house and its roof. As I slowly walk around the house looking up at the roof, I make mental notes of anything unusual, such as unevenness, sags, dips, or obvious damage. These observations will be worthy of a more focused look later, because dips or irregularities can be prone to leak. Obviously sags or unevenness could be an indication of a structural problem, but that’s yet another subject. Inspectors may find it difficult to distinguish between normal aging, prematurely aging and worn out. Many roofs fall in a gray area. A roof may look good from a distance and is without leaks or stains. How do you know the difference between the good the bad and the ugly?

Trying to determine the age of the roof is a good place to begin. Check the layers of shingles or attic roof sheathing for empty nail holes, which would indicate a prior roof had been removed. Looking at neighboring homes can often suggest roof age, especially in neighborhoods that were built at the same time. Often it’s a guessing game, especially in older homes. Keep in mind there may have been an early replacement due to weather, etc. Shingles carry limited manufacturer warranties from 20-40 years depending on the type and quality of the shingle. It is a mistake to confuse the limited warranties with actual life expectancy. The projected life expectancies vary depending on the type of shingle and the environmental conditions in the area they are located, ranging from 12-20 years on a standard shingle.

Typically southern states have shorter projected life spans due to the hot ultra violet light. It is a questionable practice to emphasis shingle warranties with your client. A limited manufacturer’s warranty covers only manufacturer defects. If a defect is proven, it typically covers only the cost of the shingles. If the shingle is not defective, the manufacturer has no responsibility. The wind warranty gets tricky and is usually much shorter – usually 5 years.

More on asphalt shingle roofs coming in the next issue: What affects roof life and problems that need attention.

The illustrations used in this article were taken from “Certainteed Shingle Applicators Manual.” Reprinted with permission.