Central Roof Repair · Kentucky code
Mineral-surfaced roll roofing is permitted on slopes down to 1 in 12 — an 8 percent slope. That is the lowest minimum of any non-membrane roof covering in the residential code Kentucky has adopted, and it is half the minimum slope asphalt shingles require.
There is a narrow band of roof — steeper than a flat roof, shallower than a shingle roof — where a lot of Central Kentucky homeowners get told two contradictory things. A porch roof, a shed-roof addition, a low dormer tie-in. Shingles are not permitted down there. A full membrane feels like overkill for eighty square feet. The code has an answer for that band, and it is the covering almost nobody brings up.
What is the minimum slope for mineral-surfaced roll roofing in Kentucky?
One unit vertical in twelve horizontal. Section R905.5.2 states that mineral-surfaced roll roofing “shall not be applied on roof slopes below one unit vertical in 12 units horizontal (8-percent slope).”
That is the code floor, not a recommendation, and it sits well below the 2:12 floor for asphalt shingles in Section R905.2.2. Both sections live in Chapter 9 of the International Residential Code, readable at the Kentucky IRC 2015, Chapter 9, Roof Assemblies.
Which version of the code applies in Kentucky?
The 2015 IRC, as amended. Kentucky’s residential code regulation adopts the 2015 edition and lets the Kentucky amendments override it where the two conflict — not the 2018 or 2021 edition most national articles quote.
The regulation says it directly: under 815 KAR 7:125, Kentucky Residential Code, “the 2015 International Residential Code for One (1) and Two (2) Family Dwellings (IRC) shall be the mandatory state residential building code for all single-family dwellings, two (2) family dwellings, and townhouses constructed in Kentucky, except that the Kentucky amendments in the 2018 Kentucky Residential Code shall supersede any conflicting provision in the 2015 IRC.”
This matters more than it sounds. Search for a roof slope minimum and most of what comes back cites the 2018 or 2021 IRC. The roll-roofing section numbers happen to line up across editions, so you can land on the right answer from the wrong book — but that is luck, not method. The book that governs a Kentucky permit is the 2015 IRC plus the Kentucky amendments, and the local building official is the one who reads it.
Between 1:12 and 2:12 there is exactly one shingle-family covering the code lets you use — and the manufacturer may still say no.
Where does 1:12 sit against every other roof covering?
Between the membranes and the shingles. The membrane systems go lower, at a quarter unit in twelve; everything in the shingle family needs 2:12 or more. Roll roofing is alone in the gap.
| Roof covering | Section | Minimum slope |
|---|---|---|
| Built-up roof (other than coal-tar) | R905.9.1 | 1/4 : 12 — 2% design slope for drainage |
| Modified bitumen membrane | R905.11.1 | 1/4 : 12 — 2% |
| Thermoset single-ply (e.g. EPDM) | R905.12.1 | 1/4 : 12 — 2% |
| Thermoplastic single-ply (e.g. TPO, PVC) | R905.13.1 | 1/4 : 12 — 2% |
| Mineral-surfaced roll roofing | R905.5.2 | 1 : 12 — 8% |
| Asphalt shingles | R905.2.2 | 2 : 12 (double underlayment from 2:12 to 4:12) |
| Photovoltaic shingles | R905.16.2 | 2 : 12 |
| Clay and concrete roof tile | R905.3.2 | 2 1/2 : 12 |
| Metal roof shingles | R905.4.2 | 3 : 12 — 25% |
| Metal roof panels, lapped non-soldered seam without applied lap sealant | R905.10.2 | 3 : 12 — 25% |
| Wood shingles | R905.7.2 | 3 : 12 — 25% |
| Wood shakes | R905.8.2 | 3 : 12 — 25% |
| Slate and slate-type shingles | R905.6.2 | 4 : 12 — 33% |
Does the code minimum settle it?
No — and this is the part that trips people up. Section R905.5.5 requires installation “in accordance with this chapter and the manufacturer’s instructions,” so a product rated to a steeper minimum than 1:12 governs over the code floor.
Read that way, R905.5.2 and R905.5.5 work as a pair: the code sets the lowest slope the code will ever allow, and the manufacturer sets the lowest slope that product will ever allow. The stricter of the two is the real answer for a specific roof. Which means “code says 1:12” is only half an answer until somebody has named the actual material and read its instructions. On a shallow porch roof that distinction decides whether the job is compliant or whether it is a leak with a warranty disclaimer attached.
What else does R905.5 require?
Four more things, and one of them surprises people: ice barriers apply to roll roofing too.
- Solid sheathing (R905.5.1). “Mineral-surfaced roll roofing shall be fastened to solidly sheathed roofs.” No spaced decking.
- Underlayment (R905.5.3). Per Section R905.1.1 — for roll roofing, Table R905.1.1(1) calls out ASTM D226 Type I or II, or ASTM D4869 Type I, II, III or IV.
- Ice barrier (R905.5.3.1). Where required, per Section R905.1.2 — and R905.1.2 names mineral-surfaced roll roofing explicitly in its list of coverings. Our walk-through of when an ice barrier is required under Kentucky’s roof code covers how that requirement gets triggered locally.
- Material standard (R905.5.4). The product must conform to ASTM D3909 or ASTM D6380, Class M. That is a specification you can ask a contractor to point to on the wrapper.
In my own experience the conversation that produces this question almost never starts with the covering. It starts with a homeowner who has been told their porch roof “can’t be shingled” and has no idea what the alternative is, or with a roof that was shingled at 1.5:12 by somebody who did not measure. The slope is the first thing to establish, with a level and a tape, before anyone talks about material. Everything in the table above follows from that one number.
If you are trying to work out which category your roof falls into, start with flat versus low-slope roofs under Kentucky’s code, and if you are in the 2:12 to 4:12 band instead, see when a low-slope shingle roof needs double underlayment in Kentucky.
Is mineral-surfaced roll roofing the same thing as “rolled roofing”?
“Rolled roofing” is a loose shop term that gets used for several products. The code term is specific: Section R905.5 governs mineral-surfaced roll roofing, and R905.5.4 ties it to ASTM D3909 or ASTM D6380, Class M. Smooth-surfaced and saturated-felt roll products are not the same thing and are not what R905.5 permits at 1:12.
Can shingles be used below 2:12 if extra underlayment is added?
No. R905.2.2 sets 2:12 as the floor for asphalt shingles and requires double underlayment only in the 2:12-to-4:12 range. Below 2:12 the extra underlayment does not buy permission — the covering itself has to change.
Who decides whether a particular repair needs a permit?
The local building official with jurisdiction over the address. 815 KAR 7:125 sets the statewide code, but permits, inspections and certificates are handled locally, and requirements differ between a city with its own inspection program and an unincorporated area. Ask before the work starts, not after.
Last updated: October 10, 2026
Need a slope measured and an honest answer about what the code allows on it? Call or text (859) 310-1209, or use the Central Roof Repair contact page.
By Marcos Gil, REALTOR® — Keller Williams Commonwealth · KY Real Estate License No. 296259 · Call or text (859) 310-1209.
Code summaries are educational and are not a substitute for a determination by the building official with jurisdiction over your address. Central Roof Repair is owned by Marcos Gil, who also publishes marcosgilrealty.com, forsalelexingtonky.com and investinthegorge.com.
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