How to Install Felt Underlayment: 15 vs 30 lb, Laps, Nailing and Wrinkles
Felt is the traditional underlayment. It is inexpensive and proven, but it wrinkles, tears and drinks water if handled badly.
- Intermediate
- 2 to 5 hours for a typical roof
- Updated
The short answer
Install asphalt felt by rolling it horizontally from the eave up, lapping each course at least 2 inches over the one below and ends at least 4 inches (check the product and code), and fastening with cap nails or roofing nails with tin caps just enough to hold it flat. Use 15 lb for standard pitches and 30 lb where extra strength is wanted. Cover it with roofing promptly, because wet felt wrinkles.
Asphalt-saturated felt, often called tar paper, has been the standard shingle underlayment for generations. It is inexpensive, easy to cut, and familiar to every roofer. It gives a second layer of protection if wind drives rain under shingles or a few shingles go missing, and it provides a clean, slightly grippy surface for the roofing above.
Felt also has weaknesses. It absorbs moisture and wrinkles, tears in wind and foot traffic, and weathers quickly if left uncovered. In the wet Fraser Valley fall, a felt-covered roof left overnight can ripple enough to show through shingles. Many roofers have moved to synthetics for that reason. See how to install synthetic underlayment and synthetic vs felt underlayment.
Felt still makes sense in some cases, and it must be installed correctly to perform. This guide covers the process, including 15 versus 30 lb, laps and avoiding wrinkles. The eave membrane that goes under it is in how to install ice and water shield.
Tools
- Hook blade utility knife
- Hammer or cap nailer
- Chalk line
- Tape measure
- Roofing boots
- Sandbags for wind
Materials
- Asphalt-saturated felt, 15 lb or 30 lb, in rolls
- Cap nails or roofing nails with plastic or tin caps
- Drip edge
- Ice and water shield for eaves and valleys
- Roofing cement only where specified
Safety first
- Felt is slick when wet, damp or dusty. Wait for a dry deck, stay tied off and watch your footing.
- Rolls are heavy, especially 30 lb felt. Lift with your legs and carry them carefully on a roof.
- Wind makes unrolled felt a sail. Anchor each course right away and do not unroll long lengths in gusts.
- Felt and asphalt products can cause skin irritation. Wear gloves and long sleeves.
Step by Step
Step 1: Choose 15 lb or 30 lb felt
The weight refers to the asphalt saturation. 15 lb felt (No. 15) is thinner and cheaper, and commonly used under asphalt shingles on standard pitches. 30 lb felt (No. 30) is thicker, stiffer, tears less and resists wrinkling better, and is used where more durability is needed, such as under tile, slate, or shakes, or left exposed longer. The shingle manufacturer and the BC Building Code set the minimum, so confirm it for your roof. Some areas need 30 lb felt or a membrane on low slopes.
Step 2: Prepare the deck and the eave details
Remove loose debris and hammer down any proud nails. Felt over a rough or wet deck shows every bump and wrinkles quickly. Install eave drip edge and the eave membrane. The felt laps over the membrane, never under it.
Step 3: Snap a line and run the first course at the eave
A straight first course keeps every lap consistent up the roof. Unroll short lengths, tack the felt at the top edge and keep it taut without stretching it. Trim it so it does not stick out beyond the drip edge where it could collect water or blow loose.
Step 4: Lap each course at least 2 inches horizontally
Work up the roof so each course laps over the one below. Most felt has a printed line to show the 2 inch lap. The BC Building Code and manufacturers give minimum laps, and common practice is about 2 inches at the sides and 4 to 6 inches at the ends. Some codes and shingle makers call for double coverage on low slopes. Confirm for your roof.
On slopes under about 4/12, felt alone is not adequate and a full membrane or double layer is used, following the shingle manufacturer's low-slope rules. See how to shingle a low-slope roof.
Step 5: Fasten just enough to hold it flat
Felt needs only enough fasteners to stay put until the shingles go on, because the shingle nails then hold everything down. Put nails along the top edge of each course, about 12 inches apart, and along the laps. Driven flush, a cap nail holds without tearing. Extra fasteners make more holes.
Do not use staples that pull through in wind, and do not skip caps in windy or exposed areas.
Tip: Keep nails a few inches inside the lap line so the next course covers them without leaving a visible bump.
Step 6: Keep it flat and avoid wrinkles
Felt expands when it absorbs moisture or warms in the sun, which makes ripples. Install in dry conditions and lay out the roof in sections that the shingles can cover promptly. A wrinkle bigger than about a quarter inch shows through and creates a dip in the shingles. Cut it out and lap a patch, or re-fasten the course.
Some roofers snap lap lines and lay out the roof so that the lap falls at the same place every course, to avoid a stack of nail heads.
Step 7: Cut around penetrations and cover it promptly
Lap felt over eave and valley membrane, and up against chimneys and walls where the flashing goes over it. Keep nails out of the valley centre. Felt is not a roof, and most manufacturers limit exposure to a few days to a few weeks. If the roof will sit open overnight, see how to dry in and tarp a roof overnight.
When Felt Still Makes Sense
- Budget jobs: it is inexpensive and plentiful.
- Under cedar shakes and tile, where breathability and traditional installation are preferred.
- Where a shingle or metal manufacturer specifies it, or the roof assembly requires a breathable layer.
- Small roofs and repairs, where a whole roll of synthetic is wasteful.
- Where traction matters: dry felt is less slippery than some smooth synthetics.
Why Felt Wrinkles
Felt is organic or fibreglass mat saturated with asphalt. It absorbs moisture from damp decks, rain and humid air, and swells. When it dries it shrinks, but by then the ripples are set. In the Fraser Valley fall, morning dew is enough. The fix is to install in dry conditions and cover quickly.
Common Mistakes
- Installing over a wet deck. Trapped moisture makes the felt ripple and promotes rot.
- Laps too small or running the wrong way. A lap under 2 inches at the sides or an upper course under a lower one lets water in.
- Leaving felt exposed too long. It degrades in sun and rain and tears in wind.
- Over-fastening. Extra nails make extra holes, and raised heads show through shingles.
- Ignoring wrinkles. They telegraph through the shingles and create low spots.
- Using felt alone on a low slope. Below the shingle maker's minimum slope a full membrane is required.
When to Call a Professional
If the roof is steep, tall, has a low slope, or has many valleys and penetrations, call a roofer. Felt on a steep roof is hard to handle and easy to slide on, especially when damp.
If you are not sure whether felt, synthetic or a membrane is right for the roof and its slope, a contractor can match the underlayment to the shingle warranty and the code.
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 Felt Underlayment: Questions
What is the difference between 15 lb and 30 lb felt?
The number refers to the weight of the asphalt-saturated felt per 100 square feet, and 30 lb is thicker, stronger and stiffer than 15 lb. 15 lb felt is the usual choice under asphalt shingles on standard slopes. 30 lb is used where more durability is wanted, such as under shakes or tile or where it may be exposed longer.
How much should roofing felt overlap?
Horizontal laps are commonly at least 2 inches over the course below, and end laps at least 4 to 6 inches, staggered from course to course. Low-slope roofs may need larger laps or double coverage. Check the BC Building Code, the felt manufacturer and the shingle manufacturer for the requirements on your roof.
Why does roofing felt wrinkle?
Felt absorbs moisture from dew, rain or a damp deck, swells, and ripples. It can also stretch if fastened tight in cool weather and then expand in the sun. Install it on a dry deck, keep it flat without over-stretching, and cover it with shingles quickly so moisture and sun do not get to it.
Is felt or synthetic underlayment better?
Synthetic is stronger, lighter, more tear resistant and does not absorb water, and it stays flat on a roof left open for days. Felt is cheaper and breathes better, and is well suited to some roofing types. For most shingle roofs either is acceptable if installed properly, and many roofers now prefer synthetic for its durability.
How long can felt underlayment stay exposed?
Not long. Felt degrades in sun and rain, wrinkles when wet and tears in wind, so most manufacturers limit exposure to a few days or a couple of weeks at most. Cover it with shingles or metal promptly. If the roof has to sit open, use a drying-in plan with tarps or a more durable 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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