Cap Nails or Staples: How the Underlayment Gets Held Down

Homeowners rarely see the underlayment once the shingles are on. It is a thin sheet laid over the deck before the first course, and it is held down by fasteners driven through it and into the wood. Those fasteners decide whether the sheet stays flat and sealed through a St. Louis windstorm, a winter that works water into the seams, and the first July when the sun heats the deck. Two common choices are cap nails and staples. Both can work. They do not behave the same way, and the spec should say which one the crew will use, how long it is, and how far apart the fasteners go.

Material and spec

A cap nail has a wide head, usually plastic or metal, that spreads the load across the sheet so the material does not tear around the fastener. A staple is a U-shaped wire driven by a pneumatic stapler. Staples are quicker to drive, and some crews use them on felt. The concern with staples is that the legs can pull through a thin sheet under wind load, or sit too deep and cut the material. A cap nail takes longer to set, but it grips a wider patch of the sheet.

The underlayment type changes the answer. Felt is heavier and takes a staple or a cap nail well, though a head that is too small can tear it. Synthetic sheets are lighter and can be slick, and some products are sold with instructions for cap nails only. The fastener choice belongs to the underlayment manufacturer's installation instructions, not to whatever the crew happened to load on the truck. Read the product data for the exact sheet going on your roof, then write the answer into the spec.

Length matters because the fastener has to reach solid wood. The spec should give one length, not a range that lets the crew choose on the day, and the foreman should be able to say how far the point goes into the deck. Deck condition matters too. Older board decking has gaps, and a fastener that lands across a gap holds less than one set in solid sheathing. A deck that has been patched in places needs a closer look before the fastening pattern is set.

Spacing is where many underlayment specs go vague. Manufacturers often give one spacing along the sheet edges and a different spacing in the field. The edges, the laps and the eave are where wind gets under the sheet first, so the closer pattern belongs there. The spec should show the numbers for each zone. A line that says fasten per manufacturer leaves the crew to decide what that means while the roof is open.

Why St. Louis wind and freeze change the answer

St. Louis roofs take gusts from spring thunderstorm fronts and the occasional straight-line wind that comes through without a tornado warning. A corner of shingles lifting after a storm is common, and the sheet under that corner is where water goes next. A fastener that held in calm weather may not hold once the sheet has been flexed by wind for several seasons. Wood decks also swell and shrink with moisture, and the seams between boards move a little each year. A sheet fastened only at its center can work loose at the edges over time.

Freeze and thaw matter as well. Water that gets under a sheet at the eave can freeze and push material upward, and a fastener that has backed partway out gives the water an easier path. The fastening pattern does not need to be complicated to do its job. It does need to be written, measured and checked before the shingles go on top of it, because the shingles hide every fastener from the ground.

What it costs

Fasteners are a small line in a roof bid. A box of cap nails usually costs more than a box of staples, and the difference per square is small. The bigger gap is labor time. Setting cap nails at the spacing a manufacturer calls for takes longer than stapling the same field, so a bid that uses the faster method may come in lower for a reason that has nothing to do with the material. Compare bids by the method and the spacing written in each one, not by the total alone.

The fastening plan can also change after a bid is signed. If the crew finds that the chosen underlayment calls for a different fastener than the bid assumed, the difference should be written down before the work changes, not discovered on the final invoice. Ask what the bid assumes about fastener type and spacing, and ask whether a change to either one would be a written change order with a stated price.

Cheaper fasteners do not make a cheaper roof. A staple driven a little short of the deck saves a few seconds and loses holding power for as long as the roof stands. Ask the crew to show one finished section before the rest of the field is covered. That check costs nothing extra if it happens on the first day of the install.

What to ask

1. Which fastener type does the underlayment manufacturer approve, cap nail or staple, and does the spec name that same type?
2. What fastener length is written in the spec, and how far into the deck does it reach?
3. What is the field spacing, and what is the edge spacing along the eaves, rakes and laps?
4. Will the crew set a few fasteners on the first sheet and show the head seating before the rest go down?
5. Does the bid price cap nails or staples, and if the method changes, will that be a written change order?
6. Does the crew carry the exact underlayment named in the spec, and will the fastener boxes on site match the approved type?

Ask the office at (314) 220-2333 to put the fastener type, the length and both spacings into the written scope before a start date is set.