Flooring.lol Editor 
I have bought a transition strip before the floor was down. Twice. Both times I stood in the aisle holding a photo of a doorway on my phone, guessed at the height difference, and came home with a profile that was too tall, too narrow, or the wrong shape entirely.
The strip is not trim. It is a movement joint with a decorative top, and the floor underneath it has to stay free to slide. Treat it as a cover-up for a rough edge and you will spend the following summer watching a seam lift two metres away.
The other mistake is one of timing. You cannot choose a strip properly until the floor is down, because the strip’s job depends on a measurement that does not exist until then.
The mistake is in the order of the job
Most people buy the strip with the flooring, in the same trip, because it is on the same aisle and it feels efficient. Then the floor goes down and the height difference turns out to be different from the guess.
The strip has to do three things at once: cover the expansion gap on both sides, sit flush enough that nobody stubs a toe, and let the floor move underneath it. All three depend on numbers you only get after the last plank is tapped home. Buying early means buying twice.
Four profiles, four different jobs
The names vary by shop, but the shapes do not.
- T-moulding is a T in section, a flat top with a stem underneath. The stem sits in the gap between two floors of similar height, and the top covers both expansion gaps. It is the one for a doorway between two hard floors at the same level.
- Reducer is a ramp. It takes a floor down to a lower one, such as laminate down to a concrete step or a thick engineered floor down to sheet vinyl. Its job is to make the step walkable, not to hide a gap.
- End cap finishes an edge. It has a lip that covers the plank end and a face that drops to the floor, and it is what you want against a hearth, a sliding door track or the open edge of a raised floor.
- Threshold is the doorway profile, usually wider and often sitting over a door sill. It handles a bigger height change and a bit of weather, which is why it belongs at an exterior door.
| Profile | Use | Typical mistake |
|---|---|---|
| T-moulding | Two floors of similar height meeting in a doorway or open plan | Bought too narrow, so the top flange pinches both floors instead of covering their gaps |
| Reducer | A real drop from one floor to a lower one | Fitted where the heights were actually equal, leaving a pointless ramp in a flat floor |
| End cap | Finishing an edge against a hearth, track or wall | Screwed through the planks, which pins the floor |
| Threshold | A doorway, especially an exterior one, often over a height change | Chosen before the door is hung, then it fouls the bottom of the door |
Measure the height difference properly
A tape measure held against the edge of a floor tells you almost nothing, because the edge is exactly where the floor is least reliable.
Use a straightedge long enough to bridge both surfaces: a 4 ft level, a straight piece of aluminium, a decent offcut of trim. Lay it across the join and look at the gap under each end. Then slide a scrap of the actual plank under the low side. If the straightedge levels out, the height difference is one plank thickness. If it still rocks, stack a second scrap or measure what is left with a feeler gauge.
Measure at three points across the opening: left, middle, right. Floors are not level, and a strip that sits flat in the middle can stand proud at one edge.
Write the numbers down and take them to the shop: the height difference, the width of the opening, and how much gap the strip has to cover on each side. The profile’s own specification tells you the height range it handles, and the packaging wins over anything you read online.
A screw through a floating floor is a slow puncture
A transition strip is usually fixed with screws through its stem, down into the subfloor. That is fine, and it is how the profile is designed to work. The fixing goes through the strip and into the subfloor, and the floating floor slides underneath the top flange.
The mistake is driving the screw through the plank as well. Now the floor is fixed to the building at that one point. When it expands in summer it cannot slide, so the pressure goes into the nearest seam and something has to give, usually a peak, sometimes a gap at the far end of the room. Peaking at the seams is often a transition strip’s fault rather than the floor’s.
Construction adhesive does the same thing. A bead of grab adhesive along the stem, pressed onto the plank, is a screw in liquid form.
There is a simple test after fitting. Press the floor near the strip with the flat of your hand. It should move a hair. If it feels welded to the building, you have pinned it.
Hide the gap, do not fill it
The expansion gap has to exist on both sides of the strip. The top flange is what hides it, and the flange has to be wide enough to do that without pressing on either floor. No expansion gap is what happens when the gap gets filled or skipped, and fixing that is a bigger job than buying a wider strip.
Trim the plank ends square and leave the gap the carton asks for, commonly around 3/8 in for laminate and 1/4 in for LVP, though your manufacturer’s instructions are the number that matters. Then set the strip so its stem sits in the gap between the two floors, not on top of either one.
Check the fit with a piece of paper. Slide it under the flange and pull. If it drags, the strip is clamping the floor and the gap is no longer a gap.
If the stem is shorter than the gap is deep, pack under it with a scrap of underlayment rather than foam. On a concrete subfloor, use the plugs or the track the strip came with, because one screw into crumbly screed will not hold a strip that gets walked on every day.
Undercut the jamb before you buy anything
The strip has to slide under the door casing, and the door has to clear the strip. Both of those are jamb problems, and both are easier before the floor reaches the doorway.
An oscillating multi-tool with a flush-cut blade does the whole job. Rest the blade flat on a scrap of plank to set the depth, and cut the casing and the jamb so the plank slides underneath instead of butting into it. The full method is in cutting around a door jamb.
Do it before the strip, and before the last row. Cutting a jamb with planks already laid around it means taping the floor, working at an awkward angle, and a good chance of a scratch you will see every morning.
The order of operations
None of this is difficult, and it sits comfortably in the DIY column on can I DIY this?. The order is the part that matters.
- Lay the floor to the doorway, leaving the expansion gap on both sides.
- Undercut the jamb and casing, or confirm somebody already did.
- Bridge the join with a straightedge and measure the height difference at three points.
- Measure the opening width and the depth of the gap the strip has to cover.
- Buy the profile that matches both numbers.
- Cut the strip to length, allowing for the casing and the door swing.
- Fix it to the subfloor only.
- Test that the floor moves and the door closes.
The test that tells you it is right
Before the kettle goes on, do three things. Slide a piece of paper under the flange and make sure it pulls free. Press the floor beside the strip and watch for movement under your palm. Open and close the door and listen for a scrape.
Then leave it alone for a week and look at the nearest seam. If nothing has lifted and nothing has opened, the strip is doing its job. If a seam has peaked, the strip is holding the floor, and the fix is to lift it and find the screw that went too deep. A gap that opens in winter at the same seam is the same message in a different season.
That is the whole trick: the strip is the last thing you buy, not the first thing you hide behind. The wrong transition strip is what it looks like when the order gets reversed.