A rowhouse job in Gowanus called me back about eighteen months after I’d poured a new stoop and entry apron, and the complaint had nothing to do with the concrete. The wood door threshold sitting on top of it had gone soft and dark at both ends where it met the jambs. The slab itself had cured fine, held its pitch, and hadn’t cracked anywhere. The trouble was that I’d matched the old sill height instead of dropping the new pour, and eighteen months of splash-back off a canal-side block with almost no air gap under that wood had done exactly what water does to timber sitting where it shouldn’t sit.
The concrete usually isn’t what fails first
On most of these jobs, the slab or apron outlasts every piece of wood that touches it. A properly placed and cured 4-inch stoop or landing at 3,500 to 4,000 psi will still be sound decades after the sill above it has rotted out twice. The failure point is almost always the joint where a hard, water-shedding surface meets a soft, water-absorbing one, and that joint gets built wrong far more often than the slab does. As a paver, I’m usually the last trade on site before the finish carpentry goes in, which means the grade and pitch I leave behind either sets the wood up to dry out or guarantees it never will.
Clearance and pitch are the two numbers that actually matter
Framing lumber and sills generally want somewhere around 6 to 8 inches of clearance above finished grade or the top of a slab, and a lot of older Brooklyn stoops were built with almost none of that margin. Where clearance can’t be had because the door sill has to land at a fixed height, the slab itself has to do more of the work: a slope away from the threshold of at least 1/4 inch per foot, carried out far enough that splash-back from rain doesn’t bounce straight up onto the underside of the wood. A flat or back-pitched landing at a door is one of the more common mistakes I get called to fix, and it’s a grading problem, not a concrete-strength problem.
What belongs at the slab-to-timber line
Pressure-treated stock or a composite sill, never plain lumber, is the minimum at any point where wood touches masonry or concrete. Behind and under it, a self-adhering membrane should wrap up the back of the jamb and down over the top edge of the slab like a small pan, so any water that does reach that joint runs back out onto the concrete instead of soaking into end grain. The gap between the slab and the sill framing itself should be an isolation joint, not a rigid mortar bed, backed with rod and sealed with a flexible sealant that can move with both materials without opening a path for water.
Getting the pour and the woodwork on the same page
Most of the failures I see trace back to the pour happening before anyone nailed down exact sill elevations, so the concrete crew guesses and the finish carpenter is left shimming a threshold to make it work. On a rowhouse restoration or anything with paneled jambs, a routed sill, or a transom over an old parlor-floor entry, that detailing needs to be set before the forms go up, which means talking to whoever is building it early rather than late. For that side of the job, I’ve taken to sending clients to a Brooklyn millwork shop for the custom sill and jamb work, because stock lumber from a box store isn’t cut to the tolerances that kind of threshold detail actually needs.
Frequently asked questions
How much clearance should wood have above a concrete slab? Roughly 6 to 8 inches where it’s achievable; where a fixed door height won’t allow it, the slab needs a stronger pitch away from the threshold instead.
What pitch should a slab have at an entry door? At least 1/4 inch per foot sloping away from the threshold, carried far enough out that rain splash-back doesn’t reach the underside of the sill.
Should the gap between a slab and a wood sill be filled solid? No. It should be an isolation joint with backer rod and a flexible sealant, not rigid mortar, so both materials can move independently without trapping water.