How to Install Synthetic Roof Underlayment: Laps, Cap Nails and UV Limits
Synthetic sheets are stronger and lighter than felt, but they have their own rules for laps, fasteners and exposure.
- Advanced
- 2 to 5 hours for a typical roof
- Updated
The short answer
Install synthetic underlayment over clean, dry sheathing by starting at the eave and rolling horizontally with the printed side up, lapping each course over the one below by the marked line (commonly 4 inches at sides and 6 inches at ends), and fastening with cap nails or cap staples about every 12 inches along laps and in the field as the manufacturer specifies. Cover it with roofing within the UV exposure limit on the roll.
Synthetic underlayment, usually polypropylene or polyethylene, has mostly replaced felt on many roofs. It is lighter, tougher, tears less, lies flatter and does not absorb water. One roll covers much more roof than a roll of felt, and most brands handle foot traffic better. That said, it has rules of its own that are easy to break.
Each product has its own lap lines, fastener schedule, exposure limit and approved uses. Some are vapour-permeable and some are not. The numbers in this guide are common values, and the instructions printed on the roll take priority. For a wider comparison with felt, see synthetic vs felt underlayment and underlayment materials.
This guide covers the field of the roof. The eaves, valleys and other vulnerable spots get ice and water shield first, and the underlayment laps over it.
Tools
- Hook blade utility knife
- Cap nailer or cap stapler
- Hammer tacker or hammer
- Chalk line
- Tape measure
- Roofing boots with good tread
- Sandbags or weights for windy days
Materials
- Synthetic roof underlayment in rolls, typically 4 feet wide
- Plastic cap nails or cap staples as recommended for the product
- Ice and water shield for the eaves and valleys
- Drip edge
- Compatible seam tape or sealant where the manufacturer calls for it
Safety first
- Synthetic underlayment can be slippery, especially when wet or dusty, although many brands have textured surfaces. Stay tied off and wear footwear with clean tread.
- Wind catches rolls and sheets like a sail. Unroll short lengths, anchor them at once and stop work in gusts.
- Do not leave rolls or scraps on the roof. They are slip hazards and blow away.
- Do not walk on it when it is wet or frosty.
Step by Step
Step 1: Prepare the deck and install eave protection
The deck should be clean, dry and free of raised nails and splinters. Install drip edge on the eaves, then the self-adhering membrane at the eaves, in valleys and around penetrations as described in how to install ice and water shield. Synthetic underlayment laps over the membrane, not under it.
Step 2: Check which side faces up and the direction of the lap
The printed side carries the lap lines, the brand and the nailing pattern. It is also usually the higher-traction side. Some products have a coated underside to help them lie on the deck. Installing one upside down can change traction, UV performance and the way the lap lines line up. Follow the roll's instructions.
Step 3: Snap a line and start at the eave
Use a chalk line to make sure the course is straight, because an error at the eave grows as you go up the roof. Run the first course horizontally along the eaves. On a roof with drip edge installed beneath the membrane at the eaves, the synthetic generally laps over the eave membrane and stops short of the edge of the drip. Follow the detail for your product and for the shingles above.
Step 4: Roll out a course and fasten it flat
Pull the sheet tight enough to lie flat, without stretching it, and fasten it so it does not flap. Use plastic cap nails or cap staples, which spread the load so the fastener does not tear through the sheet. Common spacing is about 12 inches along laps and on the top edge, with extra fasteners in the field where the manufacturer specifies. See how to use a cap nailer.
Do not use plain roofing nails without caps, because the thin sheet tears around them. On windy days, put in extra fasteners and work in shorter lengths.
Tip: Drive cap nails flush, not sunk. An overdriven cap cuts the membrane, and an underdriven one sits proud and shows through the shingles.
Step 5: Lap each course over the one below
Work up the roof so that each new course laps over the one below, like shingles, shedding water down the roof. Side laps are commonly 4 inches, and end laps 6 inches, staggered by at least 6 feet between courses. Some products call for 3 inches of side lap or for seam tape. Confirm this on the roll.
On low slopes, some manufacturers and codes call for larger laps or sealed laps. Shingle manufacturers also set minimum slopes for using standard underlayment.
Step 6: Keep the sheet flat and avoid wrinkles
Synthetics wrinkle less than felt, but they can ripple in heat or if fastened unevenly. A wrinkle under a shingle creates a bump and a low spot where water collects. Re-tack the sheet or cut and lap over it. Keep the lap lines straight as you go, since drift makes the laps uneven.
Step 7: Cut and seal around penetrations, valleys and ridges
At valleys, lap the synthetic over the valley membrane and keep the nails at least 6 inches from the centre of the valley. Around pipes and curbs, cut the sheet to fit and lap it so water runs over the flashing, not behind it. Flashing details are in how to install a pipe boot and how to flash a chimney.
At the ridge, lap over from both slopes, leaving any ridge vent opening clear.
Step 8: Cover it with roofing within the exposure limit
Each product has a stated maximum exposure, from about 30 to 180 days for many products and shorter for some. UV degrades the surface, and the underlayment is not a roof. Plan the job so the roofing goes on quickly. If there is a delay, check the manufacturer's rule. Also remember that underlayment does not substitute for drying in the roof in a storm when seams are not fully sealed.
Synthetic vs Felt in Practice
- Weight and coverage: a roll of synthetic covers 10 to 20 squares or more, where a felt roll covers 2 to 4.
- Strength: synthetic resists tearing in wind and foot traffic, and felt tears and wrinkles.
- Moisture: synthetic does not absorb water, and felt does and ripples.
- Permeability: felt breathes somewhat, and some synthetics are not breathable. Check for roofs that need vapour to escape.
- Cost: synthetic costs more per roll, but often less overall in labour.
Wind on the Roof
Wind is the main hazard when installing any large sheet. Outflow winds off the Fraser Canyon can rise quickly in the afternoon. Unroll in short lengths, fasten as you unroll, and stop if gusts lift the sheet. A sheet that blows off can tear and injure someone below.
Common Mistakes
- Installing with the wrong side up. Traction, lap lines and UV performance depend on the top surface.
- Using plain nails instead of cap nails. The sheet tears around the nail head and the nail pulls through in wind.
- Laps running the wrong way. An upper sheet under a lower one directs water behind it.
- Leaving it exposed past the UV limit. The sheet degrades and loses strength before the roofing goes on.
- Stretching the sheet or leaving wrinkles. They show through shingles and collect water.
- Working in wind. A loose sheet is a hazard and is torn easily.
When to Call a Professional
If the roof is steep, high or complicated, with valleys, dormers and penetrations, a roofer will do the underlayment faster and with fewer mistakes. The slipperiness of synthetic on a steep roof is a real fall hazard.
Underlayment also affects the warranty on the roofing above it, so get the brand and the installation noted on the contract.
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 Install Synthetic Underlayment: Questions
Which side of synthetic underlayment faces up?
The printed side, which carries the brand, lap lines and fastening pattern, is the top surface on nearly all products, and it usually has more traction. The underside is smoother or coated. Check the instructions on the roll, because installing it upside down can affect traction, UV resistance and lap alignment.
How long can synthetic underlayment be exposed before roofing?
It depends on the product. Many are rated for 30 to 180 days of UV exposure, and some are rated for less. The limit is printed on the roll. Underlayment is a temporary cover, not a finished roof, so install the shingles or metal as soon as possible and follow the manufacturer's stated limit.
How much should synthetic underlayment overlap?
A common side lap is 4 inches, following the printed lap line, and end laps are commonly 6 inches and staggered between courses. Some products specify 3 inches or require seam tape on low slopes. Always follow the manufacturer's instructions and any requirements of the shingle maker or building code.
Do you need cap nails for synthetic underlayment?
Yes, in most cases. Cap nails or cap staples spread the holding force over a wider area so the thin sheet does not tear around the fastener. Spacing is commonly about 12 inches along laps and the top edge, with more in the field where specified. Plain roofing nails without caps are not recommended.
Is synthetic underlayment slippery?
It can be, especially when wet, frosty or dusty, even though most brands have a textured surface. Footwear with clean tread and a harness tied off to an anchor are essential. On steeper pitches, roof jacks or toe boards add footing. Never walk on wet synthetic underlayment.
Related Guides
All Tear-Off, Decking and Underlayment 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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