Planning tool 3 of 4
Work out the height stack before you order the floor
A floor is the sum of its layers. Get two rooms out of step by a quarter of an inch and you have decided, without meaning to, what every doorway in the house will look like.
The stack, layer by layer
- Room A total build-up
- 1 1/2 in
- Room B total build-up
- 1 3/8 in
- Clearance left under the door
- 1/2 in
A shop-made reducer with a shallow bevel carries this difference without a trip edge. We cut it from the same stock as the higher floor so it reads as part of the floor rather than as a strip.
Before any of this matters, the subfloor has to be flat: 3/16 in over 10 ft for most wood products, 1/8 in over 10 ft for large-format tile. And the floor has to meet L/360 deflection, or L/720 for natural stone.
Ninety percent of flooring failures are subfloor failures, and most of the remainder are height failures. The height problem is the one nobody warns you about: you choose tile for the kitchen and wood for the hall, both perfectly good products, and end up with a step at the doorway that has to be covered with a strip of anodised aluminium in roughly the right colour.
The fix is a piece of paper. Write down, room by room, the subfloor thickness, any levelling or underlayment layer, the membrane if there is one, and the finish thickness. Add them up. Now you know the finished floor height in every room before anything is ordered, and the transition becomes a design decision rather than an accident.
The three measurements that come before the stack
- Flatness: a 10 foot straightedge on a grid. Most wood products tolerate 3/16 inch over 10 feet, large-format tile tightens to 1/8 inch.
- Deflection: calculated from real joist size, spacing and span. L/360 for most finishes, L/720 for natural stone. A floor can be structurally safe and still crack tile.
- Moisture: wood flooring within 2 to 4 percent of the subfloor moisture content, and a relative humidity test on any concrete slab before anything goes down on it.
- Acclimation: 3 to 14 days in the conditioned space depending on the product, not in the garage and not on the driveway.
What each height difference forces
| Difference at the doorway | The honest detail | Notes |
|---|---|---|
| 0 to 1/16 in | Flush seam or flat saddle | Expansion gap hides under the saddle |
| 1/16 to 1/4 in | Bevelled reducer | Shop-made in the higher floor's stock |
| 1/4 to 1/2 in | Stepped threshold | Check whether levelling the lower room is cheaper |
| Over 1/2 in | Level the substrate first | It will be felt at every crossing |
Scroll the table sideways to see every column.
We make thresholds, reducers and nosings in our shop from the same stock as the floor.
Doors are part of the stack
Raising a floor raises everything that has to clear it. A door with three quarters of an inch of clearance over a bare subfloor has nothing left once a subfloor layer, an underlayment and a three quarter inch solid floor go in beneath it.
Undercutting doors is straightforward work, but it is work, and on a whole-house floor replacement it is twenty or thirty doors. Knowing before the order goes in means it is a line in the scope rather than a surprise in week eleven.
Below-grade slabs have their own rules
On a basement slab the product list narrows. Luxury vinyl plank over a proper underlayment, tile over an uncoupling membrane, or engineered wood over a sleeper and foam assembly all work. Solid hardwood directly on a slab is a call-back waiting to happen, however dry the basement looks in July.
Every below-grade assembly also needs a capillary break, because concrete moves moisture as vapour whether or not the slab has ever looked wet.
Questions
About this tool
Sometimes, and it is often right for a single layer of sound sheet vinyl under a floating floor. It is wrong when it buries a moisture problem, when it pushes the finished height above a door or an appliance opening, or when the existing layer is loose.
Because an off-the-shelf strip is never quite the right colour and never quite the right profile, and it announces itself at every doorway. A reducer cut from the same stock as the floor reads as part of the floor.
It lets a wood-framed substrate move seasonally without transmitting that movement to the tile and grout above it. On a wood floor it is close to essential. On a sound slab with no cracks it is cheap insurance.
Yes, and it is structural work: sistering joists, or adding a mid-span beam and posts down to a pad. It happens before the floor goes down, not after, and it appears as its own line in the scope.
The other three
Load path and footings
Trace an addition's weight from the roof to the soil, and size the pad that catches it.
OpenBasement egress and moisture
Check an opening against the five code numbers, and read what the water is doing.
OpenElectrical service capacity
Run the NEC optional calculation and find out whether 100 amps still carries the house.
Open