Check the Minimum Pitch in Your Shingle Spec Before You Sign

Every asphalt shingle has a lowest roof slope it was designed for, and that number lives in the installation instructions printed on the bundle wrapper or the install sheet folded inside it. Most homeowners never read it, because it sits between the color chart and the warranty language and looks like fine print. It is the part of the spec that decides whether the shingle you picked can go on the roof you have.

Roof pitch is measured as rise over run. Hold a four-foot level against the underside of a rafter, keep it level, and measure how far the rafter drops over twelve inches of horizontal distance. A roof that rises six inches over that twelve-inch run is a 6 in 12 pitch. Most St. Louis houses fall somewhere between 4 in 12 and 8 in 12, but a porch, dormer, or addition can sit well below that.

Material and spec

Many asphalt shingle installation instructions set a standard minimum of 4 in 12 for ordinary application. Below that line, the manufacturer may allow the shingle only with a stated underlayment system, extra sealing, and sometimes a different fastening pattern. The exact figure for each product belongs on its own install sheet, so read that sheet rather than relying on a number from a neighbor's roof or a web article. Keys installs Atlas and Malarkey shingles only, and the written scope should name the product line and point to its instructions for the pitch on your house.

Why does the floor matter? A shingle lies flat and sheds water through overlap. On a steep roof, gravity pulls water down the face quickly and the overlap does its job. On a low roof, water moves slowly, can sit against the seams, and can be driven up under the course above by wind. Snow and meltwater in St. Louis winters stay longer on a low slope, and the freeze-thaw cycle works on any seam that water can reach. The shingle is not wrong for the roof because it is cheap or old. It is wrong because the water has more time to find a gap.

The minimum on the install sheet is not the same as the lowest pitch on your house. Measure the lowest slope on the roof, not just the main field. A porch roof tied into a 9 in 12 main roof may be 2 in 12, and a dormer cheek or shed addition may be lower still. Each plane needs its own number, and each plane needs its own spec line in the quote.

Ask whether the manufacturer's minimum applies to the shingle alone or to the shingle installed over a specific underlayment. Ask what underlayment the quote assumes for each low plane, how far up the deck it runs, and whether the seams are sealed where the shingle is laid over it. Those are spec facts. A quote that says "low slope area, match existing" without naming them has left the decision open.

Pitch also changes nailing. A shingle applied at a low slope may need the nail count or nail placement that the install sheet specifies for that slope, and some instructions call for more fasteners or a different layout when the pitch is below the standard line. If the quote names the shingle but says nothing about fasteners on the low plane, ask for the count per shingle and where each one goes.

What it costs

A low-slope section usually costs more per square than the main roof, and the reasons are real. The crew has to lay more underlayment in that area, add sealing or cement steps that the steep field does not need, and work the detail where the planes meet a wall or valley. Waste can rise too, because the cut lines around a small low plane are rarely clean. A line for the low section should show those items separately so you can see what the extra money buys.

The cost of skipping that work usually shows up later as a leak, most often at the seam where the low plane meets the steep roof above it. Repairing that leak means opening shingles that were laid to a spec that did not match the roof. A roof that needs an unbudgeted second visit has cost more than the quote line it saved.

If the low plane is a small addition, ask whether the quote prices it inside the square count or as an allowance. An allowance is a number that is not yet a price. Ask what it covers and what would move it up or down once the crew measures the plane on the roof.

What to ask

Bring the quote and your own measurements to each roofer and ask these questions in writing:

• What is the pitch of each plane on my house, in rise over run, and how was that number measured?
• What is the minimum slope printed on the install sheet for the shingle you are quoting?
• Which planes sit below that minimum, if any, and what underlayment and sealing does the quote assume for each?
• How far up the deck does the low-slope underlayment run, and does it overlap the steep field?
• What is the fastener count per shingle on the low planes, and does it differ from the main roof?
• Where does each low plane meet a wall, valley, or steeper slope, and how is that joint flashed?
• Is the low-slope section a line item, a unit price, or an allowance?

Ask for the install sheet itself, not a summary. A roofer who has the product in hand can show you the page that governs the work. If the answer is a verbal assurance with no page behind it, the spec is not settled.

You can call the office at (314) 220-2333 to put these questions to Keys before you sign. Ask the same questions of every bidder and compare the answers on the same sheet. A roofer who names the minimum slope, shows the install sheet, and writes the low-plane steps into the scope has given you something you can check on the day the crew arrives.