How to Use a Cap Nailer or Cap Stapler to Fasten Roofing Underlayment
Plastic caps spread the load so the underlayment does not tear. Load the caps, set the depth and follow the fastener spacing on the roll.
- Intermediate
- 10 minutes to set up
- Updated
The short answer
A cap nailer or cap stapler drives a fastener through a plastic cap, spreading the holding force over the underlayment so it does not tear or pull over the head. Load the cap strip, set air pressure and depth so the cap sits flat without cutting the membrane, and fasten along laps and the field at the spacing printed on the underlayment roll.
A plain roofing nail through a sheet of underlayment does a poor job. The head is small, the membrane is thin, and wind and foot traffic tear the sheet around the nail. A plastic cap under the head spreads the load over a wide ring, and holds the underlayment flat until the shingles cover it. Cap nails are the standard fastener for synthetic underlayment and for felt on many jobs. Cap nailers and cap staplers make them quick to install.
Like any pneumatic tool, a cap nailer must be set correctly. Driven too hard, the cap cracks or the fastener cuts through the membrane. Driven too soft, the cap stands proud and lets the sheet flap. This guide covers loading, depth and the fastening pattern. The underlayment itself is covered in how to install synthetic underlayment and how to install felt underlayment. For the air supply, see how to set up an air compressor.
Tools
- Pneumatic cap nailer or cap stapler
- Air compressor, regulator and hose
- Safety glasses
- Hearing protection
- Scrap deck and underlayment for testing
- Hammer for a few hand-driven caps
Materials
- Plastic cap strips or cap nail coils sized for the tool
- Ring shank or smooth nails or staples matching the caps
- Synthetic or felt roofing underlayment
Safety first
- Disconnect the air before loading, clearing a jam or adjusting the tool. A cap nailer fires like any other nailer.
- Use a sequential trigger if the tool offers one. A bump trigger is hard to control on a slick underlayment.
- Synthetic underlayment is slippery. Fasten with both feet set, and avoid reaching. See walking a roof safely.
- Wear safety glasses and hearing protection. Caps and nail fragments can fly out.
Step by Step
Step 1: Match the caps and fasteners to the tool and the underlayment
Caps come in different diameters, and the fastener length must reach well into the deck. Cap nails are usually ring shank or smooth shank roofing nails with a plastic cap, and cap staples are narrow crown staples through a cap. Many underlayment manufacturers specify the type and spacing on the roll. Check the instructions before you start.
The fastener should penetrate the deck by the depth your local code or the manufacturer requires. Short fasteners that barely reach the sheathing are a common cause of pulled-out underlayment.
Step 2: Connect the air and set the regulator
Cap tools usually run at a lower pressure than coil nailers, often somewhere around 70 to 100 psi, but the label on the tool is the authority. A lower pressure setting is kinder to the cap and the membrane. You can raise it if the fastener will not seat.
Step 3: Disconnect the air and load the caps
Load a strip or coil of caps, then the fasteners where the tool takes them separately. Some tools use pre-assembled cap nail strips, and others use a cap magazine with a separate nail or staple magazine. Check the diagram in the manual. Make sure the lead cap sits squarely and the feed spring is working.
Step 4: Set the depth on a scrap test
The cap should lie flat against the underlayment with the fastener head seated inside it. If the cap is cracked or the fastener has cut through the underlayment, the depth is too deep. If the cap stands proud or rocks, it is too shallow. Adjust a notch at a time, and test again. Do this on the real deck type, since decking varies in hardness.
Test again after a rest, and in cold weather, when the plastic caps are more brittle and need a lighter setting.
Step 5: Fasten the laps and edges first
Laps hold the sheet edge down and keep water out, so fasteners along the laps go in first. Typical spacings for cap fastening are closer along the laps and edges than in the field, with spacing printed on the roll. Follow the manufacturer's pattern for the wind exposure. Avoid placing a fastener in the lap where the next sheet will go over the cap, which can leave a bump.
Step 6: Fasten the field in a consistent pattern
A consistent pattern matters because the underlayment may be left exposed for days or weeks, depending on the job. Weather, wind and foot traffic can all tear a poorly fastened sheet. Keep fasteners at least a short distance from the edge so they do not tear out. See how to dry in a roof overnight for protecting an open roof.
Step 7: Keep your footing and control of the tool
Synthetic underlayment can be very slick, especially with dew or dust. Use toe boards or roof jacks on steeper slopes and stay clipped in. Carry the tool by the handle with the air disconnected when moving between areas. See how to walk a roof safely.
Step 8: Clear jams and maintain the tool
Plastic caps leave fragments in the nose, which cause jams. Disconnect the hose, open the nose, remove the broken cap and debris, and reload. The procedure is the same as for any nailer, see how to clear nail gun jams. Drain the compressor and keep a water trap fitted.
Cap Nails Versus Cap Staples
Cap nails hold better in most decks because the shank grips the wood, and they suit roofs where the underlayment will stay exposed for a while. Cap staples are faster to drive and cheaper, and suit shorter exposure under a full roof system. Some manufacturers allow only one type, and building codes may specify the fastener for some applications. Follow the underlayment's instructions, and do not substitute a short staple on a steep roof or a windy site.
Hand-Driven Caps
Cap nails can also be driven by hand with a hammer. It is slower and the depth is harder to control, but it is useful for touch-ups, valleys and places where a nailer will not fit. Drive the nail straight and stop when the cap sits flat. Do not hit the cap hard enough to crack it.
Common Mistakes
- Over-driving the cap. It cracks the cap or cuts the membrane.
- Leaving caps proud. The sheet flaps in the wind and tears around the nail.
- Short fasteners. They do not reach the deck and pull out.
- Fastening too far apart along laps and edges, where the wind gets under the sheet.
- Using staples or nails the manufacturer does not approve.
- Clearing a jam with air connected. A cap nailer fires like any other.
When to Call a Professional
If the roof is steep, the underlayment will stay exposed in wet weather, or you are not sure which fastener and spacing apply, a roofer can set the system up correctly. A poorly fastened underlayment can tear in the first windstorm and let water into the deck before the shingles go on.
Decks with damaged sheathing or old nail holes also change how the fasteners hold. See the deck inspection guide.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Use a Cap Nailer or Cap Stapler: Questions
What is a cap nailer used for?
A cap nailer drives a nail through a plastic cap to fasten roofing underlayment, especially synthetic underlayment, to the roof deck. The cap spreads the holding force so the membrane does not tear around the nail head. It is much faster than hand driving cap nails and gives more consistent depth.
What is the difference between a cap nailer and a cap stapler?
A cap nailer drives a nail with a plastic cap, which grips the deck by its shank and holds well in wind. A cap stapler drives a narrow crown staple through a cap, which is faster and lighter. Underlayment manufacturers often specify which they allow, so check the instructions before you choose.
How far apart should cap nails be on underlayment?
The spacing is printed on the underlayment roll or in its installation instructions, and it is closer along laps and edges than in the field. The figures vary by product and wind exposure, so follow the manufacturer's pattern. Do not guess, because too much spacing lets wind lift the sheet.
What pressure should a cap nailer be set to?
Use the range printed on the tool. Cap nailers are often run at a lower pressure than coil roofing nailers, and it is best to start low and raise it in small steps until the cap sits flat. Too much pressure cracks the cap or cuts the membrane, and too little leaves it standing proud.
Can you use regular roofing nails on synthetic underlayment?
Not as the main fastener. A plain roofing nail head is too small and the underlayment will tear around it. Synthetic underlayment manufacturers specify plastic caps or cap nails so the load is spread. Check the product's instructions, because the approved fasteners and spacing are part of its warranty and wind rating.
Related Guides
All Roofing Tools guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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