Glossary / assembly
Roof and wall assemblies
The combination of parts as tested or installed, such as a roof covering plus underlayment plus deck. A covering alone is not an assembly.
An assembly is the combination of parts as tested or installed: a roof covering plus its underlayment plus the deck; a wall covering plus sheathing plus the framing behind it; a vent as a manufactured unit. Fire ratings attach to assemblies because the tests are run on them. ASTM E108 and UL 790 burn a roof covering applied as intended on a deck and class that covering-on-deck A, B or C; ASTM E2707 exposes a wall assembly to ten minutes of direct flame; ASTM E2886 tests a vent assembly for ember and flame intrusion; SFM 12-7A-4 tests a deck assembly, under-deck flame and then a burning brand on the boards.
Why it matters for a purchase. A bag of shingles is not a Class A assembly: the class needs the underlayment and deck the listing names, and a metal covering can be Class A in one listed system and not in another. California's OSFM handbook lists roof assemblies in category 8180 by ID, each naming covering, deck and underlayment; how to read a Class A roof listing walks one row, and covering versus assembly has the drawing-note version: write the covering, the underlayment, the deck and the listing ID, never "Class A shingles".
The codes use the word deliberately. California CWUIC 504.2 requires a Class A roof assembly; the 2024 IWUIC specifies a Class A tested roof assembly for Class 1 and 2 construction; Oregon R327's floor is a minimum Class B-rated roof assembly equivalent. IBHS Wildfire Prepared writes Class A roof covering, and the covering it means is still the one tested on a deck. A one-hour fire-resistance-rated exterior wall, which Oregon R327 accepts as one wall path, is likewise a tested wall assembly under ASTM E119, not a rating of the siding board. The roofing hub holds the listed assemblies.
Questions
What are the differences between a Class A and Class B roof?
Both are ASTM E108 / UL 790 classes for a covering on its tested deck; A is the more severe exposure. California CWUIC and IBHS ask for A; Oregon R327, where adopted, sets B as the floor and bans wood shakes.
Are all metal roofs Class A?
No. Some metal systems are Class A assemblies on a named deck and underlayment, some are not; two Metal Sales Image II rows on this site name different decks for the same covering.
What is a Class A shingle?
A marketing phrase. The class belongs to the covering installed on the tested deck with the named underlayment, so the same shingle can be inside a Class A system and outside one.
What is the difference between class A and class C roofing materials?
Exposure level under the same test: A is the most severe, C the least. Kittitas County requires no less than Class A in high and extreme hazard areas and Class C in low and general ones; the material family is not what changes.
Do not use as a synonym: Class A shingles.
Related
Where this term is used
- A Class A rating is not a service-life clock
- Arizona WUI code status
- ASTM E119 versus a WUI wall test
- Bird blocks, boxed soffit, or unvented attic
- Building code upgrade coverage
- California FAIR Plan
- Char rate
- Clark County IWUIC over Nevada’s deleted Chapter 5