Before you buy flooring
The floor fails at the substrate, not the plank
Three measurable things decide whether a floor lasts: what the slab reads, how flat it is, and whether the transition heights were worked out before demolition. None of them are about the product you picked.
Before you buy flooring
The floor fails at the substrate, not the plank
Pick what you want to lay and this shows what the slab has to test at, how flat it has to be, and how thick the finished assembly is, which is what decides every doorway in the house.
- Flatness required
- 3/16 in. in 10 ft
- Slab moisture limit
- Commonly 85 to 90 percent RH limit, or 3 lb calcium chloride
- Substrate
- Slab on grade or plywood
- Underlayment
- Attached pad or a separate 1 to 1.5 mm pad
Finished build-up
6 to 8 mm plus pad
Relative to carpet with pad, the thickest common assembly on this list.
Forgiving on moisture, unforgiving on flatness. Long planks telegraph every dip in a slab.
- Flatness required
- 1/8 in. in 10 ft
- Slab moisture limit
- Adhesive-dependent, often 85 percent RH
- Substrate
- Slab on grade
- Underlayment
- None, adhesive direct to prepared slab
Finished build-up
2.5 to 5 mm
Relative to carpet with pad, the thickest common assembly on this list.
The flattest substrate requirement of the resilient products, and the thinnest build-up at a doorway.
- Flatness required
- 3/16 in. in 10 ft
- Slab moisture limit
- Typically 75 to 80 percent RH limit
- Substrate
- Slab with a barrier, or plywood
- Underlayment
- Vapour retarder or moisture-control adhesive
Finished build-up
9 to 15 mm
Relative to carpet with pad, the thickest common assembly on this list.
The tightest moisture limit of the common products. Test the slab before you order it, not after.
- Flatness required
- 1/4 in. in 10 ft
- Slab moisture limit
- Tolerant, but movement joints are mandatory
- Substrate
- Slab, backer board or an uncoupling membrane
- Underlayment
- Uncoupling or crack-isolation membrane over slab
Finished build-up
8 to 10 mm plus 6 mm mortar
Relative to carpet with pad, the thickest common assembly on this list.
Moisture-tolerant, which makes it the usual answer on a slab that tests high.
- Flatness required
- 1/8 in. in 10 ft
- Slab moisture limit
- Tolerant, with movement joints
- Substrate
- Levelled slab or membrane
- Underlayment
- Uncoupling membrane, often required by warranty
Finished build-up
10 to 12 mm plus 8 mm mortar
Relative to carpet with pad, the thickest common assembly on this list.
The tightest flatness requirement on this list, and the reason self-levelling underlayment dominates large-format budgets.
- Flatness required
- 3/8 in. in 10 ft
- Slab moisture limit
- Tolerant, but moisture under carpet grows odour
- Substrate
- Slab or plywood
- Underlayment
- Separate pad
Finished build-up
10 to 14 mm plus 8 mm pad
Relative to carpet with pad, the thickest common assembly on this list.
The most forgiving product and the thickest build-up, which is what makes carpet-to-tile doorways awkward.
Transition rules we design to
Under 1/4 inch of difference
No ramp needed. A flush reducer or a simple T-moulding handles it and nobody feels the change underfoot.
1/4 to 1/2 inch of difference
Needs a bevelled reducer. Anything steeper than roughly a 1 in 2 slope becomes a trip point, especially for anyone using a walker or a cane.
Over 1/2 inch of difference
Fix the substrate instead of trimming over the problem. Recess the subfloor, build up the lower side, or change the assembly so the heights meet.
Where the transition belongs
Under the closed door leaf, on the centreline of the jamb. A transition strip in the middle of a doorway opening looks like an afterthought because it usually is one.
Perimeter expansion gap
Floating floors need a clear gap at every vertical surface, typically 1/4 to 3/8 inch. It gets covered by base and shoe, never filled with caulk or adhesive.
Movement joints through tile
Tile needs movement joints at the perimeter, over any slab control joint and at interior field intervals. A joint that stops at the tile edge has not done its job.
Preparation, in order
- 01
Test before you order
In-situ relative humidity probes in the slab, set and read on the manufacturer's schedule. Surface meters are a screening tool, not a compliance test.
- 02
Sweep for flatness
A 10 foot straightedge across the whole area, with every deviation marked on the floor and mapped. This is what the levelling quantity is priced from.
- 03
Grind the highs
Old adhesive, paint over-spray and high spots at former wall lines all come off first.
- 04
Prime and level
The primer the levelling compound requires, then the compound at the depth the map calls for, in one pour where possible.
- 05
Mitigate if the readings demand it
A roll-on or trowel-applied moisture barrier, or a change to an assembly rated for the reading. Re-test after application.
- 06
Acclimate the material
In the conditioned space, for the period the manufacturer requires, with the HVAC actually running.
- 07
Set the layout
From the longest visible sightline, with the transition heights already fixed at survey stage.
Side by side
Every product on one table
| Product | Build-up | Flatness | Moisture limit | Best substrate |
|---|---|---|---|---|
| Luxury vinyl plank, floating | 6 to 8 mm plus pad | 3/16 in. in 10 ft | Commonly 85 to 90 percent RH limit, or 3 lb calcium chloride | Slab on grade or plywood |
| Luxury vinyl plank, glue down | 2.5 to 5 mm | 1/8 in. in 10 ft | Adhesive-dependent, often 85 percent RH | Slab on grade |
| Engineered hardwood | 9 to 15 mm | 3/16 in. in 10 ft | Typically 75 to 80 percent RH limit | Slab with a barrier, or plywood |
| Porcelain tile, up to 12 in. | 8 to 10 mm plus 6 mm mortar | 1/4 in. in 10 ft | Tolerant, but movement joints are mandatory | Slab, backer board or an uncoupling membrane |
| Large-format tile, 24 in. and over | 10 to 12 mm plus 8 mm mortar | 1/8 in. in 10 ft | Tolerant, with movement joints | Levelled slab or membrane |
| Carpet with pad | 10 to 14 mm plus 8 mm pad | 3/8 in. in 10 ft | Tolerant, but moisture under carpet grows odour | Slab or plywood |
Values are typical and illustrative. Always use the specific manufacturer's published limits, which govern the warranty.
Flooring questions
Questions
What clients ask about floors
Because age does not dry a slab. Many Jacksonville slabs poured before the 1990s have no vapour retarder beneath them, so ground moisture moves up through the concrete continuously. We have measured 92 percent relative humidity in a slab poured in 1974.
As a screening tool on a first visit, yes. As a compliance test, no. Manufacturers want in-situ relative humidity probes set into the slab and read after equilibration, and that is what a warranty claim will be judged against.
It is priced by area and depth, so the flatness survey is what produces the number. On a 2,700 square foot Ponte Vedra floor it came to 71 bags. On a 900 square foot two-room job it might be four. This is why we sweep the floor with a straightedge before we quote.
Usually, and it looks considerably better than stopping at doorways. It costs more because the preparation area grows, and because every existing height difference at a threshold now has to be resolved rather than hidden under a strip.
Either the substrate heights were never worked out before demolition, or a transition strip is covering a difference too large for it. Anything over about half an inch should be fixed in the substrate, not ramped over. It is a genuine trip hazard for anyone using a cane or a walker.
As long as the manufacturer says, in the conditioned space, with the air conditioning actually running. Skipping acclimation is one of the few flooring mistakes that shows up within weeks rather than years.
Related checks
The other three we run before design
Electrical service capacity
A NEC 220.83 demand calculation for the finished plan, before rough-in rather than during it.
HVAC sizing for added area
Sensible and latent load, duct static pressure, and when a separate zone beats extending the system.
Windows, doors and wind load
Design pressure per opening, corner zones and the approval numbers a permit set needs.
We test before we quote
Probes in the slab, a straightedge across the floor, and a levelling quantity you can see the arithmetic behind.