Check the subfloor before you order the flooring. Five tests that take an afternoon

Pull up a corner of the old floor and spend an afternoon on the deck underneath. Everything below can be corrected cheaply while the room is empty and expensively afterward.

Five tests. A long straightedge, a moisture meter, a screw gun and a tape measure cover all of them.

1. Flatness, measured against the manufacturer's number

Flat and level are different properties and only one of them matters here. A floor can slope noticeably and still take a floating plank without complaint. A floor with a dip in it cannot.

Lay a long straightedge across the deck in several directions and look for daylight underneath. Manufacturers publish a tolerance expressed as a maximum deviation over a given span, and it appears in the installation instructions rather than in the brochure.

What to do with what you find:

• Low spots get filled with a patching or self-leveling compound, priced by the bag and by depth.
• High spots on a wood deck get sanded. On concrete they get ground, which is a rented tool and a dusty afternoon.
• A long gentle slope is usually acceptable. Check the tolerance before assuming it is not.

2. Moisture, measured rather than judged

On a wood subfloor, take readings with a moisture meter in several locations, including near exterior walls, under windows, and in front of any plumbing. Compare against the flooring manufacturer's stated limit and against the moisture content of the flooring material itself, because the difference between the two is what causes movement after installation.

On a concrete slab, a meter reading of the surface is not sufficient. The standard methods involve either an in-slab humidity probe or a sealed calcium chloride test, and the manufacturer's warranty states which it accepts.

An elevated reading is information, not a verdict. It tells you to find the source, and the source is usually a fixable thing: a gutter discharging at the foundation, a missing vapor barrier in a crawlspace, a slow supply line fitting.

3. Movement, found with your own weight

Walk the whole deck slowly, listening. Then have someone else walk it while you watch from a low angle.

What you are looking for is deflection, which shows as a visible dip under a footstep, and squeaking, which is a fastener moving in a hole or two boards rubbing.

• Squeaks are fixed now with screws driven into the joists, and they are impossible to reach later.
• Bounce across a wide area suggests joist spacing or span rather than the deck, which is a different conversation and worth having before the finish goes down.
• A soft spot that gives underfoot means water damage or delamination, and that section gets cut out and replaced rather than covered.

4. Fasteners and panel edges

Look at the panel joints. Edges that have swollen or curled produce ridges that telegraph through a thin covering and cause hollow spots under a rigid one.

Check for popped nails and staples. Any fastener sitting proud gets driven or removed. On an older deck, adding a screw at each joist crossing is inexpensive insurance and it is the standard fix for a floor that has been noisy for years.

5. Height, at every doorway

The test everyone forgets, and the one that generates the most annoying problems.

Measure the existing floor thickness and the proposed assembly thickness, including underlayment. The difference is what changes at every threshold.

Then check five places:

1. Under every interior door. A door that drags on a new floor has to come off and be trimmed, which is a real job on a solid door.
2. At the transition to any adjoining floor that is staying.
3. At the bottom stair riser, where a height change alters the riser dimension and can put it outside what the code allows.
4. Under the dishwasher, if the kitchen is involved. Raising the floor in front of it can trap the appliance permanently.
5. At exterior doors and any threshold with weatherstripping.

What the afternoon actually saves

Every item above is cheaper to fix on an open deck than after a floor is down, and most of them are not repairable afterward at any price short of pulling the floor back up.

It also changes the conversation with a contractor. An estimate that includes a subfloor prep allowance and one that does not are not comparable, and once you have your own numbers for flatness and moisture, you can tell which estimate is realistic. That is worth an afternoon on your knees with a straightedge.