Shingle Nail Length: Name the Penetration in Your Spec

A nail is the cheapest part of a roof, and it is the part most often left out of the paperwork. The bundle wrapper names the shingle, the quote names the square count, and somewhere in the labor line the words install the shingles appear. None of those say how long the nail is or how far it has to reach into the deck. In a St. Louis metro that gets hot afternoons, hard freezes and spring windstorms, that missing number decides whether a shingle stays down through its first hard winter.

What a short nail does

A roofing nail holds a shingle in two ways. The head pins the shingle to the deck, and the shank grips the wood beneath. If the shank is too short, it grips only a thin layer of sheathing and little else. The shingle can look fastened from the ground and stay put on a calm day. The first gust that lifts a tab puts the full load on a nail with nothing solid to hold. Nails that barely reach the deck also tend to work loose over several seasons, because the deck expands and contracts a little with each temperature swing.

Manufacturer installation instructions for asphalt shingles generally call for the nail to pass through the sheathing or to sit at least 3/4 inch into it. The exact figure for your product is printed on the install sheet inside the bundle, and that sheet is the document any later warranty question will be measured against. Read it before the crew opens the first bundle, because it is part of the spec whether or not anyone quotes it.

The arithmetic on your deck

Nail length is simple subtraction. Add up the thickness of everything the nail must pass through, then add the 3/4 inch of sheathing it must grip, and you have the minimum length. A bare 1/2 inch plywood or OSB deck that takes new shingles directly works with a 1-1/4 inch nail. If the crew lays new shingles over one old layer, the nail has to pass through roughly another quarter inch of old shingle and underlayment, so the same deck often calls for a 1-1/2 inch nail. A 5/8 inch deck needs more length again.

The deck thickness is not always what the house plans say. Some older St. Louis houses have plank decking or boards laid at odd thicknesses, and some have had a previous re-roof that added a layer nobody wrote down. The only reliable check is on the roof, before the nails are bought. Ask the roofer how many layers are on the deck and how the deck was measured, and get the answer in writing.

What to name in the spec

The spec should carry three numbers, not one word. The first is the nail length in inches, along with the deck thickness and the number of layers it assumes. The second is the penetration minimum, stated the way the install sheet states it, so that the crew and the homeowner read the same rule. The third is the nail count per shingle, which the install sheet also sets, since a shingle held by the right length but too few nails still moves in wind.

Head type and shank type belong on the same line. A smooth shank and a ring shank grip the wood differently, and a spec that names only the length leaves the crew to choose the shank. Name the product you want, the length you want, and the shank you want, so that a substitute on delivery day is visible as a substitute.

What it costs

Nails are a small share of a roof bill. A longer nail costs slightly more per box than a shorter one, and the difference across a typical job is modest. The larger cost comes from the wrong length. A shingle held by nails that are too short may need a repair visit within a few seasons, and by then the trip, the labor and the disruption cost far more than the box of correct nails would have. The price of getting the length right is almost always smaller than the price of finding out later that it was wrong.

Watch the quote for where nails sit. Some bids include nails inside the material line, some inside the labor line, and some leave them out and bill them as a separate supply item on the final invoice. A bid that says nails are included should say what length, and a bid that leaves them out should say who buys them and when the count is confirmed. Compare the nail line across bids the same way you compare the underlayment line, because a bid that quietly uses a shorter nail can look cheaper on the total while describing a different roof.

What to ask

• What nail length will the crew use over this deck, and how many layers did you count to reach that number?
• Is the nail length written into the spec, or only the shingle name and the phrase install per manufacturer?
• Does the install sheet in the bundle state a penetration minimum, and will the crew follow that minimum on every slope?
• How many nails per shingle does the install sheet call for, and does the quote assume that count?
• How will the crew check nail length on the first bundle, before they start the second slope? A pulled nail measured with a tape is a simple check.
• Who confirms the deck thickness when the tear-off opens, and if the deck differs from the plan, who decides the new length and in what writing?
• Which nail box is being delivered to the lot, and does its label match the length in the spec?

The nail is a small part of the roof, but it is the part that holds every other part down. A spec that names it in inches, ties it to the deck it will pass through, and checks it on the first bundle gives the homeowner a number to hold the crew to.