Field & trade terms
Underlayment
Underlayment is the in-between layer that a finished surface sits on. Under roofing it's the felt or synthetic sheet over the sheathing that sheds water if the roof covering ever lets some through. Under flooring and tile it's the board, membrane or panel that gives a flat, sound, correctly bonded base and can add crack isolation or waterproofing. It's specific to the assembly above it — a roof underlayment and a tile underlayment do different jobs and aren't interchangeable.
How it shows up on site
On a roof you see underlayment rolled and lapped over the deck, shingle-style, before the roofing goes on. On floors you see cement board, a decoupling membrane or panel underlayment set over the subfloor before tile or finish flooring. Laps, seams and fastening all follow the manufacturer's method for that product.
What an inspector looks at
The check is that the right underlayment for the assembly is installed with correct laps or seams, fastened or bonded as specified, and integrated with the flashing or waterproofing it works with. On roofs the overlap direction is the whole point; on floors it's flatness, bond and any waterproofing at wet areas.
RTI is photo-gated, so underlayment is documented before the finish covers it — laps, seams and the tie-in to flashing or waterproofing shown in place, the record that the layer under the finish was done right.
Checklists that put underlayment to work
Free, field-ready checklists, each with the photo every item needs before it can be submitted.
Related terms
- SheathingThe structural panel skin over studs, rafters or joists that braces the frame and gives cladding, roofing and the barrier a base to attach to.
- FlashingThin sheet material lapped into an assembly to direct water back out at joints, edges and openings before it can get behind the finish.
- Dry-inThe point at which a building is closed to weather — roof, wall barrier and openings sealed enough to keep water out and let interior work start.
- Damp-proofingA coating on the outside of a below-grade wall that resists soil moisture — enough for damp ground, but not a waterproof barrier.
- MEP (mechanical, electrical, plumbing)The mechanical, electrical and plumbing trades — the systems run through a building's structure that get inspected at rough-in and again at final.
- FirestoppingSealing openings where pipe, conduit or cable pass through a fire-rated wall or floor so the assembly's rating is restored.
Knowing the term is step one. Proving it is the job.
RTI holds the submit button until every mandatory item is answered and the photos are attached, then chains the record so it can be proven untouched later.