Poured Curbs Best Practices
BEST PRACTICES: Floor level windows and door sills should have curbs poured 1/8 to 1/4" below and 1/8" narrower than the frames/sill. Curb height is determined after lasering the slab and establishing finished floor level and the bottom of every frame. This will vary depending on the door sill riser height (top of riser at or 1/8" above Finish Floor) and now much window frame reveal is needed.
This varying sill height is why your windows and doors have to be ordered at different heights because the tops of all frames should be exactly the same from finished floor and control line. Your windows and doors should NOT all be ordered at 120" from the window and door schedule!!!!!
- Never rely on shell contractor's control lines. Finished floor level MUST be established with a laser and close coordination with GC for flooring type and thickness.
- Before pouring curbs, the slab should be washed clean and bonding agent applied.
- Curb MUST be squared off to the room and best if measured from framer's chalk lines. 1/8" behind whatever reveal the GC wants the window frames set out from the exterior.
- Liquid applied waterproofing should be applied over the curb and surrounding masonry. Bucks should be installed after pouring and waterproofing curbs so they are separated from moisture wicking.
- Tight joint between curb and buck should be sealed and then buck painted with liquid applied waterproofing.
- Curb should NOT protrude inside of the window frame or it will interfere with floor return, though it can at the doors because it is almost always below. Flush drainage sills and ADA sills are special cases.
- Anchors should be long enough to achieve full penetration depth into the SLAB below the curb.
- For curbs over 2" most manufacturers recommend adding aggregate and at 3" or more, may require steel reinforcing (consult your fenestration engineer). The curb is not only supporting the weight of the windows and doors but acting as part of the surrounding structure to transfer the wind load during storms.
