How a Roof Works
Lesson 3. Roof Slope, and Why It Decides Everything
Roof slope is written as rise over twelve, so a six in twelve roof climbs six inches for every twelve inches it runs, and that number rules out more materials than anything else.
At a Glance
- Pitch is rise in inches per twelve inches of run
- Under three in twelve, shingles are not allowed
- Over eight in twelve, the crew needs staging and costs more
- You can measure yours from the attic with a level and a ruler
How to Measure Your Own
- Take a two foot level and a tape measure into the attic
- Hold the level against the underside of a rafter and get the bubble centered
- Measure twelve inches along the level from the end touching the rafter
- Measure straight up from that point to the rafter
- That measurement in inches is your pitch, over twelve
If the gap is six inches, you have a six in twelve roof. This works from inside, in slippers, in five minutes, and it is exactly how a roofer would do it.
What Each Range Allows
| Slope | What It Means | What Can Go On It |
|---|---|---|
| Under 2 in 12 | Effectively flat | Membrane only. TPO, EPDM, modified bitumen |
| 2 to 4 in 12 | Low slope | Membrane, metal, shingles only with doubled underlayment |
| 4 to 8 in 12 | Normal | Anything at all |
| 8 to 12 in 12 | Steep | Anything, and the labor costs more |
| Over 12 in 12 | Very steep | Anything, with staging and a real labor premium |
Worth knowing. Putting shingles on anything under three in twelve is the single most common serious mistake on porch and addition roofs. It will leak, and it will not be the shingles' fault.
Why Steep Costs More
Past about eight in twelve nobody can stand on the roof unassisted. The crew works off roof jacks and planks that have to be built, moved and taken down. Material has to be carried up rather than stacked on the deck. The same square count takes noticeably longer.
This is legitimate and it is not padding. If two bids differ and one of them clearly has not looked at the pitch, that is the bid to question.