How to Install Roll Roofing: Exposed Nail, Concealed Nail, Laps and Slope Limits
A low-cost covering for sheds, garages and low slopes. The method, the cement and the temperature decide whether it lasts.
- Intermediate
- 4 to 8 hours for a small roof
- Updated
The short answer
Roll roofing is asphalt sheet roofing in rolls about 36 inches wide. Let it relax flat in mild weather, install drip edge, lay the first sheet at the eave, and either nail the laps (exposed nail method) or hide the nails under cemented laps (concealed nail method). In BC the building code sets minimum slopes of 3/12 for single-coverage roll roofing and 2/12 for double-coverage 19 inch selvage. Cement every lap, make end laps about 6 inches, and keep nails out of the exposed field.
Roll roofing is a one-piece asphalt sheet, with a mineral granule surface, sold in rolls. It is the cheapest common roof covering, fast to install and useful on sheds, garages, workshops and other low slopes where shingles are not suitable. It is not as long-lived or as attractive as shingles, and most of its leaks come from poor laps and exposed nails.
There are two methods. In the exposed nail method, nails go through the top of the cemented overlap and the heads show. In the concealed nail method, the nails go into the lower sheet and a cemented overlap covers them. Double coverage, using 19 inch selvage roll with the overlap wider than the exposed portion, gives two layers of roofing over the deck and is the low-slope version. Product instructions in some markets allow the exposed nail method from 2/12 and concealed or double coverage down to about 1/12, but the BC Building Code sets higher floors for houses and small buildings: 3/12 for smooth or mineral surfaced roll roofing and 2/12 for 19 inch selvage double coverage. Use the higher of the code and the product instructions.
For choosing a covering for a small building, see how to roof a shed. If the roof is nearly flat, a membrane system is more suitable, see how to choose a flat roof membrane.
Tools
- Hook blade utility knife
- Roofing hammer
- Chalk line
- Tape measure
- Trowel or putty knife for cement
- Caulking gun
- Push broom or stiff roller
- Extension ladder, tied off
Materials
- Mineral surface roll roofing, commonly 36 inches wide
- Asphalt lap cement (plastic cement)
- Roofing nails, galvanized, 1 inch or 1-1/4 inch with large heads
- Drip edge for eaves and rakes
- Asphalt primer (if required for the edges)
- Felt underlayment, if the maker calls for it
Safety first
- Roll roofing is slippery when wet, and a hot, sticky surface scuffs under boots. Work in dry, mild weather on a dry deck, tied off where a fall could cause serious injury.
- WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more. Even low sheds need a secure ladder and care at the edges.
- Roofing cement and primer are flammable and the fumes irritate. Work in fresh air, away from flames and pilot lights, and keep them off skin.
Step by Step
Step 1: Check the slope, the deck and the weather
Roll roofing is stiff when cold and sticky when hot. The usual installation range is roughly 45 to 85 degrees Fahrenheit, about 7 to 30 degrees Celsius, but check the label. The deck should be dry, clean and flat, with nails set flush and any rotten boards replaced. See how to inspect a roof deck.
Choose the method by slope. At 3/12 or more, single-coverage roll roofing with the exposed or concealed nail method meets the BC Building Code floor. From 2/12 to 3/12, use 19 inch selvage double coverage with cemented, concealed-nail laps. Below 2/12, roll roofing is the wrong product in BC: use a membrane such as EPDM or torch-on.
Step 2: Let the rolls relax
A roll stored on end or in cold weather holds a curl. Laying pieces flat for a few hours lets them relax so they lie flat on the deck and do not buckle later. Cut lengths about 1 inch longer than the run, using a hook blade and a straightedge.
Step 3: Install the drip edge and any underlayment
See how to install drip edge for the order. Some makers allow roll roofing directly on the deck, and some call for a layer of felt. Check the instructions for yours.
Step 4: Lay the first sheet at the eave
Align the edge with the drip edge and with a chalk line so it runs straight. For the concealed nail method, the first course is often a starter strip of half-width roll roofing, nailed along the top. Nail with large head galvanized roofing nails, spaced about 12 inches apart along the nailing line, following the instructions.
Do not nail in the field of the sheet. Nail only where the next course will cover the heads, or in the cemented lap for the exposed method.
Step 5: Apply lap cement for the next course
Use a trowel to spread a consistent layer. Too little cement leaves dry spots that will leak, and too much squeezes out and stains the surface. Follow the width of the lap called for in the instructions, commonly about 2 inches more than the exposed portion for double coverage. Check the instructions for your product.
Step 6: Lay and nail the next course
Exposed nail method: nail through the cemented lap, about 1 inch from the edge, spacing nails about 3 to 4 inches apart. Concealed nail method: nail along the upper edge of the lower sheet, then cement the next sheet over it so all nails are covered. Roll or brush the lap to press out air.
Stagger the end laps, which are commonly 6 inches wide, so they do not line up from course to course, and cement them fully.
Watch out: Do not leave nails or staples in the exposed surface, and do not step on fresh cement.
Step 7: Finish the edges, ridge and penetrations
At the rake, turn or trim the roofing over the drip edge and cement it down. At a peak, lap the top sheets over the ridge and cement and nail them, or use a ridge cap. Seal all exposed nail heads with a dab of cement. Penetrations need flashing, see how to install a pipe boot and step flashing.
Exposed Nail Versus Concealed Nail
The exposed nail method is faster and simpler, but the nail heads sit in the weather and can leak as the roof ages, which is why it belongs on the steeper end of the range. The concealed nail method hides the nails under the cemented overlap and gives a more weather-tight roof. It takes more time and more cement. On the shallowest pitches allowed in BC, from 2/12 to 3/12, the choice is made for you: double coverage with concealed, cemented laps.
Where Roll Roofing Fits
It suits sheds, garages, workshops and other low-cost, low-slope buildings. It is not often used on a house, where shingles or a membrane give a longer life and a better finish. In the Fraser Valley, where the roof stays wet for months, cement laps are the weak point. Inspect them each spring and renew any that have opened.
Common Mistakes
- Installing in cold or hot weather. Cold sheets crack and hot ones stick and scuff. Stay within the label range.
- Skipping the relax time. A curled roll buckles on the roof and leaves ripples that trap water.
- Not enough cement on the laps. Dry spots let water in. Spread a full, even band.
- Exposed nails or single coverage on a low slope. Below 3/12, use double-coverage selvage with concealed nails, and below 2/12 use a membrane instead.
- Nailing in the field. Nails in the exposed surface leak. Keep nails in the laps or along the covered edge.
- Not sealing nail heads and edges. Every exposed fastener is a future leak.
When to Call a Professional
Call a roofer if the roof is below 2/12, if the deck is damaged, if the structure is attached to a house or has walls and penetrations to flash, or if the building is high enough that falls are a serious risk.
A roofer can also advise whether a membrane, metal or shingle system would give a better result for the building.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install Roll Roofing: Questions
What is the minimum slope for roll roofing?
In BC, the building code sets the floor at 3/12 (1 in 4) for smooth or mineral surfaced roll roofing and 2/12 (1 in 6) for 19 inch selvage laid as double coverage. Product instructions in some markets list the exposed nail method from 2/12 and concealed nail or double coverage down to about 1/12, but on a BC building use the higher of the code and the instructions. Below 2/12, use a membrane roof instead.
What is the difference between exposed nail and concealed nail roll roofing?
With the exposed nail method, nails are driven through the cemented lap with the heads showing, and in BC it belongs on slopes of 3/12 or more. With the concealed nail method, nails go into the lower sheet and a cemented overlap covers them, which suits lower slopes. The concealed method takes longer but is more weather-tight.
Can you install roll roofing in cold weather?
It is not advisable. Roll roofing is stiff and cracks in the cold, and the cement does not bond well. The usual range is roughly 7 to 30 degrees Celsius, but check the product label. In hot sun, it becomes soft and scuffs easily. Choose a mild, dry day, and let the roll relax flat before installing.
How long does roll roofing last?
Roll roofing generally has a shorter life than shingles, often in the range of 10 to 20 years, depending on the product, the slope and how well the laps and nails are maintained. In wet climates, inspect laps and exposed nail heads every year and renew any cement that has opened.
Related Guides
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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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