Overdriven Nails Cut the Mat, and the Spec Should Name the Gun Setting

A coil nailer set at the wrong depth can do more harm to a shingle than a missed nail. When the head sinks through the top surface and cuts into the mat, the shingle loses the strength it was rated for, and the sealant may not bond where the crew needed it to. When the nail stands proud of the surface, the shingle above has nothing firm to lock against and can lift in the next wind. Neither fault shows from the driveway, and most homeowners learn about it only after the first hard storm.

What overdriving does to the mat

An overdriven nail punches a hole through the shingle's surface and the reinforcing mat beneath it. The hole is small, but it sits where the shingle is supposed to hold together under wind load. Wind does not pull a cut shingle off in one piece. It tears along the cut, which is why a crew can leave a slope that looks finished and still lose shingles from the same line the next season.

Overdriving is most common when a crew works fast with a pneumatic gun whose pressure is turned up to drive nails in hard, cold or dense decking. Sun-warmed shingles are softer and forgiving, but a cold morning on a St. Louis roof can make the same gun setting cut deeper. Freeze and thaw do not cause the cut, but a cut shingle with water behind it will show the damage sooner. That is one reason a fall or spring install in this climate should be checked for depth before the crew leaves.

What underdriving leaves behind

The opposite fault is a nail that does not seat. A head left proud of the surface holds the shingle above it up off the one below, so the tab does not lie flat. In a wind, that gap lets the edge flutter, and the flutter wears the sealant strip and the granules at the edge. Underdriven nails are also more likely to back out over several winters as the deck expands and contracts.

Underdriving is less dramatic than overdriving, but it is easier to spot from a ladder. Look for heads that sit above the surface of the shingle, and look for a row where the tabs do not lie down when you press gently at the edge. Do not press hard on a fresh roof, because that can break the seal you are trying to check.

Material and spec

The manufacturer's installation instructions set the nail count, the nail line and the placement of each nail relative to the sealant strip and the cutouts. The install sheet in your Atlas or Malarkey bundle carries those instructions, and the crew should be working from it. A spec that says only shingle brand and product name leaves the depth to the crew's habit, which is not a spec.

Write the nail requirement into the proposal in plain terms. Name the nail type, the count per shingle, and the requirement that heads sit flush with the surface, neither cutting the mat nor standing proud. A proposal that says the roof will be installed per manufacturer instructions is a reasonable start, but add the nail depth as its own line so it is not buried in a general statement. If the crew uses hand nailing on steep sections or valleys, the spec should say so, because hand nailing has its own depth habits.

Ask whether the gun is adjusted for the decking under your roof. A plank deck, an older OSB deck and a new plywood deck take a nail differently. The gun setting should change with the deck, and the crew should be able to tell you what they set it to on your house.

What it costs

Setting the gun correctly costs the crew a few minutes of adjustment, which is not a line item on most bids. The cost shows up later, when a slope has to be re-nailed or cut shingles replaced. Re-nailing a section means labor time, a few bundles of shingles and a second trip up the ladder, and a bid that does not say whether that work is covered can leave you paying for it after the crew has left.

Ask the roofer to state in writing what happens if depth is found wrong at the final walk. Some bids say the crew will correct any cut or proud nails before final payment. Others are silent. The difference is worth a sentence in the proposal, because a silent bid makes the correction a negotiation after the job is finished.

What to ask

• What nail depth setting does the crew use on this deck, and who adjusts the gun on your roof?
• Does the proposal state the nail count per shingle and the requirement that heads sit flush?
• Will the crew hand nail any section, such as steep slopes, valleys or edges, and if so, where?
• Who walks the roof for nail depth before the crew leaves, and will you get photos of a sample slope?
• If a nail is found cut or proud at final walk, does the bid include re-nailing and any replacement shingles?
• Is the install sheet from your bundle on site during the work, so the crew can check placement against it?

Keep the answers in writing. A roofer who can explain the gun setting, the nail line and the walk-through without hesitation has usually done this many times. A roofer who says depth is handled by experience has not written it down, and that is the part of the job you will need to point to later.

If you want a second look at an installed roof, the easiest check is a dry-day walk at a low angle with the sun behind you. Overdriven nails show as small dark dots in the shingle surface, and proud nails show as bright metal heads above the line. Photograph any you find and send the photos to the roofer with the row and slope marked. Keys Roofing can attend the walk and put a written scope with photos and measurements in your hands. The office line is (314) 220-2333.