Screws into wood: 5/8 in
Under 2018 IRC R702.3.5.1, gypsum board screwed to wood framing takes Type W or Type S screws complying with ASTM C1002, and they must penetrate the wood at least 5/8 in (15.9 mm).
The code names specific screw types, so "any drywall screw" does not meet it. The 5/8 in figure is a minimum: a Type S screw at exactly 5/8 in complies, and so do Type W screws at 3/4 in or 1 in. A Type W screw penetrating only 1/2 in falls short. There is no 1 in minimum; the figure is 5/8 in.
Steel and SIPs use different figures
The same section sets two other penetrations:
- Cold-formed steel framing thinner than 0.033 in: Type S (ASTM C1002) or bugle-head (ASTM C1513) screws penetrating at least 3/8 in.
- Wood facings of structural insulated panels (SIPs): at least 7/16 in.
The trap is mixing framing types. 5/8 in is for solid wood, not steel; 3/8 in is for thin steel, not wood; 7/16 in is only for SIP facings. Neither 1/2 in nor 3/4 in is a minimum penetration in R702.3.5.1.
Where water-resistant board is barred
Under R702.3.7.1, water-resistant gypsum backing board may not be used where it will be directly exposed to water or in areas subject to continuous high humidity. That is why it is not a tile backer inside a shower compartment.
It is not banned from bathrooms in general, only from those locations. It is also not barred from ceilings: R702.3.7.1 allows it there. Exterior walls behind siding are not the limitation the section states.
Vapor retarders, edges and the board standard
- It may not be installed over a Class I or II vapor retarder in shower or tub compartments. Taping the joints does not change that.
- Under R702.3.7, cut and exposed edges must be sealed as the manufacturer recommends. A water-resistant core is no excuse to leave them untreated.
- The board itself must meet ASTM C1178, C1278 or C1396. ASTM C1002 is the screw standard, not the board standard.