What Is Under Your Alabama Floor: Raised, Slab or Plank
The substrate decides more about your flooring quote than the flooring does — and in Alabama it also decides how much of it anyone has ever been able to see.
Three substrates, and how to tell
Raised floor over a crawl space
The traditional Alabama build and still common in the older housing of Birmingham, Mobile and Montgomery. Plywood or plank decking over joists, with clearance underneath. It is the only one of the three where an inspector can see the underside of your floor at all — which is both an advantage and the reason a clean report can feel more conclusive than it is, since the top face of the decking is still hidden.
Concrete slab on grade
Very common in postwar Alabama construction and in most of the newer suburban housing. Tap it — no hollow sound, no movement underfoot. A slab takes tile, stone, rigid-core vinyl and engineered wood well, and solid hardwood not at all. Its own issue is moisture from below rather than insects in the decking.
Plank decking
Boards laid across the joists, found in prewar housing. At a floor vent you will see individual boards with gaps between them rather than a continuous sheet. Plank takes nailed solid hardwood and carpet directly. Everything else — tile, vinyl, laminate — wants an underlayment or a plywood overlay first, because the gaps telegraph and the boards move independently.
What it costs you
- An underlayment over plank adds roughly a dollar a square foot.
- A plywood overlay adds two to four, and costs you height at doors and at the bottom stair riser.
- Grinding and levelling a slab varies enormously.
- Replacing damaged decking is the line that is missing from most quotes, and in this state it is the one worth agreeing in advance.
This is why two quotes for the same material in the same city can differ so much. One installer has looked underneath and priced the preparation; the other has not.
The five-minute check
- Lift a floor register and look at the cut edge of the material.
- Walk the room barefoot and feel for give — especially along walls, in front of the dishwasher and around toilets and tubs.
- If there is a crawl space, look up at the underside of the decking: staining, sagging, mud tubes on the piers, or a sill plate that looks wrong.
- Tell the installer whatever you found, before they quote rather than after.
Common questions
How do I know if my house is on a slab?
Whether there is a crawl space beneath answers it directly. Failing that, drop something heavy-ish on the floor — a slab sounds dead, a framed floor sounds hollow.
Can I put tile straight on plank decking?
Not directly. The boards move independently and the gaps telegraph. Tile over plank wants a plywood overlay and usually an uncoupling membrane, plus a deflection check first.
Does the subfloor change what the floor costs, or just what is possible?
Both, and the cost side is the one people underestimate. Preparation is routinely the difference between a quote that looked competitive and a final bill that did not.