How to Shingle a Low-Slope Roof from 2/12 to 4/12
Shingles shed water by gravity. On a low slope, water moves slowly and finds every gap, so the underlayment does the real work.
- Advanced
- 1 to 3 days
- Updated
The short answer
Asphalt shingles are normally installed on slopes of 4/12 and steeper. Between 2/12 and 4/12 they can be used only with added protection: full coverage self-adhered ice and water shield, or a double layer of underlayment with cemented laps, as the manufacturer and code require. Below 2/12 shingles are the wrong roof: use a low-slope membrane system instead. Confirm the current code and product instructions.
Shingles work because water runs off them faster than it can get under them. On a steep roof that is easy. On a low slope, water moves slowly, and in a storm the wind can push it uphill under the tabs. That is why shingle manufacturers and building codes set a minimum slope, and why a roof between 2/12 and 4/12 needs more than the usual single layer of underlayment.
The slope range covered here is 2/12 to 4/12: that is, 2 to 4 inches of rise for every 12 inches of run. It covers many porch roofs, additions, carports and shallow main roofs. Below 2/12, shingles are not approved by manufacturers or codes, and a roof that flat needs a different system, such as torch-on roofing, EPDM or TPO.
The exact rules for low slope shingles depend on the product and the current BC Building Code, so read the manufacturer's low-slope instructions and confirm with your local building department. The numbers below are common practice, not a substitute for them. Where a low-slope section meets a steeper one, see how to flash a pitch change.
Tools
- Roofing coil nailer and compressor
- Hook blade utility knife
- Chalk line
- Tape measure
- Pitch gauge
- Roller for pressing membrane
- Fall arrest harness and anchor
Materials
- Self-adhered ice and water shield, full coverage
- Or two plies of underlayment with cement on the laps
- Laminated or 3-tab shingles approved for low slope
- Starter strip and drip edge
- Roofing nails, 1-1/4 inch galvanized
- Asphalt roof cement
Safety first
- Low slopes feel safe and are not. Snow, moss, wet underlayment and granules are slippery at any pitch, and a fall from the edge is the same height.
- Self-adhered membrane is sticky and hard to reposition. Work with a helper and keep your footing.
- Stay tied off. WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and any fall that could cause serious injury.
Step by Step
Step 1: Measure the pitch and confirm shingles are allowed
Use a pitch gauge or a level and ruler on several points of the roof, since a roof that looks like 3/12 can be 2/12 in a sagging area. Shingles are generally acceptable at 4/12 and steeper. Between 2/12 and 4/12 they require added waterproofing, and under 2/12 they are not allowed.
Look up the low-slope application in the installation instructions for the exact shingle. Some products and warranties do not permit low slope use at all, and some allow it only with specific underlayments.
Step 2: Repair the deck and make it smooth
Water sits on a low slope wherever the deck dips, and a dip under a shingle is a pond. Replace soft sheathing, renail loose panels, and fill gaps. See how to inspect a roof deck and how to replace roof sheathing.
Sweep the deck clean and dry. Self-adhered membrane will not bond to a wet, dusty or frosty surface.
Step 3: Install the drip edge
Eave drip edge goes on the deck, with the membrane over it, and rake drip edge goes over the membrane, as on any roof. See how to install drip edge.
On a low slope, water runs off the eave slowly, so a proper drip edge and a proper overhang matter more than on a steep roof.
Step 4: Cover the entire deck with self-adhered membrane, or double the underlayment
The most reliable method is to cover the whole deck with ice and water shield. Work from the eave up, each course lapped over the one below by the width the membrane manufacturer specifies, commonly 3 to 4 inches, and press it with a roller so there are no air pockets.
The alternative, where the shingle maker allows it, is two plies of underlayment with the laps cemented, typically a 19 inch starting strip and 36 inch full sheets lapped to give double coverage. Follow the wrapper exactly, since the pattern varies. Felt and synthetic products are not interchangeable in these details.
Watch out: Self-adhered membrane in cold weather may not stick. Keep rolls warm, and use primer where the manufacturer calls for it.
Step 5: Install starter and the first courses
Starter goes on the membrane in the usual way. See how to install starter strip shingles. Some manufacturers require hand sealing with roof cement on low slopes, because the wind can lift the tabs before the sealant bonds.
Nail as the wrapper shows, usually 4 nails per shingle on low slopes, and keep every nail in the nailing zone. Low-slope roofs are more likely to fail at bad nailing. See shingle nailing: placement, count and depth.
Step 6: Lay the field and keep the details tight
Run courses as on any roof, with the usual offsets. See how to lay shingle courses and offsets. At walls, use a wider membrane turn-up and step flashing or headwall flashing as the wall requires.
Do not use exposed nails or sealant as a substitute for flashing on a low slope. Water will find them.
Step 7: Finish the ridge and check the ventilation
Low-slope roofs with poor ventilation trap heat in summer and moisture in winter, and both shorten the life of the shingles. See how to ventilate a low-slope roof.
Check the eave and rake, and look for any place where water could pond. After the first heavy rain, look for leaks at the walls and penetrations.
When Shingles Are the Wrong Choice
Below 2/12, shingles are not approved, and a roof like that should be a membrane system. Even between 2/12 and 4/12, shingles are not always the best choice. A roof that holds standing water, has many penetrations, or carries snow loads for long periods is better covered in a continuous membrane or a metal roof with a low-slope panel. See metal roofing and choosing a flat roof membrane.
Snow and Ice on Low Slopes
Low slopes hold snow longer, so melt water sits against the shingles and can be pushed uphill under them by the next freeze. At Hope and up the canyon, where wet snow lies for weeks, this matters. Extra membrane at the eaves, good attic ventilation and insulation reduce ice dams. See how to deal with an ice dam.
Common Mistakes
- Using standard single-layer underlayment on a 3/12 roof. The manufacturer's warranty and the code require more.
- Shingling a roof under 2/12. Shingles are not approved and will leak.
- Skipping deck repair. A sag under the membrane holds water, and ponding shortens the roof's life.
- Membrane over a wet or dusty deck. It does not stick, and wrinkles trap water.
- No hand sealing where it is required. Tabs lift before the sealant activates.
- Ignoring ventilation. A low-slope roof with poor airflow cooks the shingles and traps moisture.
When to Call a Professional
Low-slope shingling is an area where small errors become leaks, and the rules vary by product and code. Have a roofer confirm that shingles are appropriate for the slope and that the underlayment meets the manufacturer's low-slope requirement. Check also with your building department whether a permit is needed.
If the roof is under 2/12, ponds water or leaks already, a roofer should look at it before shingles are considered. A membrane or metal roof is often a better answer.
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 Shingle a Low-Slope Roof (2/12 to 4/12): Questions
What is the minimum slope for asphalt shingles?
Standard shingle installation is for slopes of 4/12 and steeper. Between 2/12 and 4/12, shingles can be used only with added waterproofing such as full coverage ice and water shield or a double layer of underlayment. Under 2/12, shingles are not approved. Always confirm the current code and the product instructions.
Can you put shingles on a 2/12 roof?
Only with a low-slope installation approved by the shingle manufacturer and accepted by the local code, usually with the whole deck covered in self-adhered membrane or two plies of cemented underlayment. Some products and warranties do not allow it. A 2/12 roof is the lowest pitch where shingles can be considered, and a membrane or metal roof is often better.
What underlayment is used for low slope shingles?
Self-adhered ice and water shield covering the entire deck is the most reliable choice. The other accepted method is two plies of underlayment with cemented laps, commonly a 19 inch strip and 36 inch sheets to give double coverage. The shingle manufacturer's low-slope instructions give the exact pattern.
Why do low slope roofs leak more?
Water runs slowly off a shallow roof, so it sits longer, and wind can push it up under tabs and laps. Snow lies longer and melts and refreezes. There is less margin for error in flashing and nailing, so any weakness shows up as a leak. That is why low slopes need extra waterproofing under the shingle.
Is a metal roof better for a low slope?
Often, yes, for slopes at the low end. Snap-lock and exposed fastener metal panels commonly need about 3/12, but mechanically seamed standing seam is designed for lower slopes, in some systems down to about 1/2 in 12 with sealant in the seams and suitable underlayment. Check the panel maker's minimum. A membrane system is the choice for slopes below that. The best roof depends on the slope, the climate and the budget.
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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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