Construction subtypes A vs B: protected vs unprotected

One-line orientation

Construction Types I, II, III, and V each have A and B subtypes. The A subtype is more protected; the B subtype is less protected. Within the same construction type, A always provides more fire resistance than B. (Type IV, Heavy Timber, is the exception to the A/B split.)

Key points

• A = higher fire resistance; B = lower. The letter is a protection tier inside the Roman-numeral type: IA > IB, IIA > IIB, VA > VB.
• Type IA provides more fire resistance than Type IB — the top of the whole table (e.g. can permit unlimited height for some occupancies).
• Type IIA is non-combustible with fire-resistance ratings; Type IIB is non-combustible but has no fire-resistance requirement for structural frame, floors, and roofs.
• Type VA carries fire-resistance ratings; Type VB has no fire-resistance ratings — the lowest tier overall (common light wood-frame housing).
• “B” subtypes are frequently unprotected: IIB and VB typically have a non-rated structural frame, while their A counterparts are rated.
• Type IV is the exception to the A/B split. Heavy Timber traditionally stands alone as Type IV-HT (no A/B pair); the IBC 2021 mass-timber provisions add subtypes IV-A, IV-B, and IV-C alongside HT instead of an A/B pair.

Confusions / comparison

Pair	More fire-resistant	Frame
IA vs IB	IA	IA highest of all; IB rated but less
IIA vs IIB	IIA	IIA rated; IIB no required frame rating
VA vs VB	VA	VA rated; VB no required ratings
Rule	A > B within every type	A = protected/rated; B = often unrated

Related

→ Construction types (this module): the I–V ladder these subtypes refine · Occupancy groups overview (this module): occupancy + construction type set allowable height/area.