Roof and flashing defects are installation errors that let water get past the roof covering. The U.S. Department of Energy’s Building America Solution Center (BASC) names improper nailing as a common shingle error. Other frequent problems include reversed or missing flashing, poor drip edge and valley details, and attic air leaks that lead to ice dams.
Last reviewed: September 2026. Sources are listed at the end of this page.
What are the most common roof installation defects?
Improper nailing is one of the most common, according to BASC. That includes nails set high or low, angled, overdriven or underdriven, and too few fasteners.[1] BASC says nails should go in straight and flush.[1]
| Roof detail | What BASC calls for | What a defect looks like |
|---|---|---|
| Shingle fasteners | Codes require at least four fasteners per strip shingle. Six nails are generally used in high-wind areas. No staples.[1] | Too few nails, staples, or nails that are angled, overdriven or underdriven |
| Roof slope | Asphalt shingles only on slopes of 2 in 12 or steeper. Slopes from 2 in 12 up to 4 in 12 need a double layer of underlayment.[1] | Shingles on a flatter roof, or a single underlayment layer on a low-slope roof |
| Drip edge | At eaves and rakes. Joints overlap at least 3 inches. Extends at least ½ inch below the sheathing and at least 2 inches onto the deck. Shingles extend no more than ¼ inch past it.[1][2] | Missing drip edge, short overlaps, or shingles hanging too far past the edge |
| Valleys and penetrations | A self-sealing (peel-and-stick) membrane lapped 6 inches shingle-style. Metal valleys are recommended over woven or abutted shingles.[3] | No membrane under valleys or around penetrations |
| Ice barrier (cold climates) | Two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen, running at least 24 inches inside the exterior wall line.[1] | No ice barrier, or one that stops short of the wall line |
BASC says most roof covering blow-off happens at roof edges.[2]
Why does roof flashing fail?
Flashing usually fails because it is lapped the wrong way or left out. Step flashing that is underlapped (reversed) lets water run behind it, and flashing that is not tied into the house wrap sends water into the wall.[4][10]
Kickout flashing turns water away from the wall at the bottom of a roof-wall intersection and into the gutter. BASC warns that without it, water can damage wall sheathing, framing and insulation and cause mold inside the wall. Fiber cement, vinyl siding and brick veneer can hide the evidence for years.[10] BASC cites IRC (International Residential Code) sections R703.4 and R903.2.1 as requiring step and kickout flashing.[10] See the water intrusion guide.
What causes ice dams?
Ice dams form when the roof deck under snow warms above freezing. The meltwater runs down, refreezes at the colder eaves, and backs up under the roofing.[4]
BASC says heat escaping into the attic through air leaks is one of the biggest causes of ice dams in homes. The first fix is to air-seal the ceiling.[4] University of Minnesota Extension gives the same first step and notes that ice dam moisture can lead to mold and mildew.[5]
Ventilation also plays a part. BASC summarizes the IRC attic ventilation rules (2015 IRC R806.1) this way:[6][4]
- Net free vent area of at least 1/300 of the attic area where the vent layout qualifies.
- Otherwise, at least 1/150.
- 40 to 50 percent of the required area in upper vents, at least 3 feet above the eave vents.
What roof problems can you check yourself?
You can spot many roof problems from the ground and from the attic. The American Society of Home Inspectors (ASHI) lists these signs:[8]
- Shingles that do not lie flat, or missing shingles
- Cupped or curled shingles, or nails backing out
- Rusted or damaged flashing
- Stains or peeling paint on ceilings, including closet ceilings
The International Association of Certified Home Inspectors (InterNACHI) says water stains on the underside of the roof deck can show past and current leaks before a ceiling stain appears. Rusty nails poking through the deck point to a long-running moisture problem.[9]
Does an installation defect affect your shingle warranty?
It can. GAF’s Shingle and Accessory Limited Warranty, effective January 1, 2025, excludes damage from improper fastening, from application that does not strictly follow GAF’s printed instructions, and from inadequate ventilation. It also excludes settlement, movement or structural defects in the building or roof base.[7]
Notice rules are strict. GAF requires you to notify GAF of a claim within 30 days after you notice a problem. GAF states that notice to your contractor, dealer or home builder is not notice to GAF.[7] If your builder installed the roof, a letter to the builder alone does not start a GAF claim. GAF also asks for proof of the installation date and ownership, plus photos and samples at your expense.[7] Other manufacturers set their own notice rules, so read the warranty for your product. See how to document a construction defect before you send notice.
This page is general information, not engineering advice. Have a licensed professional engineer or qualified inspector evaluate your home.
Related guides
- Water intrusion and building envelope defects
- Mold after water intrusion
- How to document a construction defect
- New-home builder warranties
- What a home inspection covers
Sources
- U.S. Department of Energy, Building America Solution Center, “Asphalt Shingle Roofs,” last updated December 2, 2022.
- U.S. Department of Energy, Building America Solution Center, “Roof Edge Protection,” last updated September 18, 2020.
- U.S. Department of Energy, Building America Solution Center, “Roof Valleys and Penetrations Sealed,” last updated September 26, 2021.
- U.S. Department of Energy, Building America Solution Center, “Construct Roofs and Attics for Ice Dam Prevention,” last updated December 6, 2022.
- University of Minnesota Extension, “Dealing with and preventing ice dams,” reviewed 2022.
- U.S. Department of Energy, Building America Solution Center, “Calculating Attic Passive Ventilation,” last updated August 23, 2021.
- GAF, “GAF Shingle and Accessory Limited Warranty (RESWT160L),” effective January 1, 2025.
- American Society of Home Inspectors, “Roof System,” January 3, 2025.
- InterNACHI, “What Your Attic Reveals: The Truth About Your Roof’s Hidden Health,” no date shown.
- U.S. Department of Energy, Building America Solution Center, “Step and Kick-Out Flashing at Roof-Wall Intersections,” last updated September 14, 2020.