Under the slab the crew has spread 4-mil polyethylene sheeting over the base course with the seams butted edge to edge and no overlap, and the sheeting is cut oversize around the plumbing penetrations. What should the inspector require before concrete is placed?
- ANothing further, because polyethylene sheeting of any thickness is accepted as a vapor retarder beneath a slab.
A minimum thickness is specified, and thin film also tears under traffic before the concrete arrives.
- BThat the sheeting be removed, because a vapor retarder laid over the base course rather than under it traps water in the stone.
The retarder belongs between the slab and the base course, so its position here is already correct.
- CThat a second layer of the same 4-mil sheeting be laid at right angles to the first to make up the thickness.
Two thin layers do not equal one of specified thickness, and the code names a material, not a sum.
- A vapor retarder of at least 6-mil polyethylene or an approved equivalent, with joints lapped not less than 6 inches.
Why D is correct
The under-slab vapor retarder must be at least 6-mil polyethylene or an approved equivalent, with joints lapped not less than 6 inches. The 4-mil sheeting with butted seams meets neither condition, so it has to be replaced or overlaid with compliant material before the pour.
What this question is testing
Whether the candidate recalls both halves of the vapor retarder requirement, material thickness and joint lap, and can distinguish it from the separate requirement for a base course beneath the slab.
On the job
This is a five-minute inspection that pays for itself in flooring callbacks. Walk the slab area looking for the roll stamp, which prints the mil thickness right on the film, then look at the seams and at the penetrations. Damaged sheeting is normal by the time the reinforcement is set, so expect to ask for taping and patching. Remember the exception for unheated accessory buildings and exterior flatwork, which is why a detached garage slab may be poured without one.
Memory technique
Six and six: six mil thick, six inches lapped.
Exam tip
Six mil and six inches: the two numbers travel together in this provision.
Where to look it up
IRC Chapter 5, the vapor retarder paragraph under the concrete slab-on-ground provisions at R506.2.