How do I fit underlay correctly?

Underlay looks like the trivial part of a flooring job and it is where several expensive faults originate. The two that matter are overlapping the joins and getting the damp barrier wrong.

Safety

Step by step

1. Check what the flooring manufacturer specifies.Thickness, density and whether an integrated membrane is required. Fitting the wrong underlay is one of the most common reasons a flooring warranty is refused.
2. Sort out damp on concrete.A concrete or screed floor needs a damp-proof membrane, either a separate 1000-gauge polythene sheet lapped up the walls or an underlay with one bonded on. Without it, moisture rises into the flooring.
3. Sweep and check the floor first.Every crumb and screw head telegraphs through. It is thirty seconds with a brush and it is the difference between a smooth floor and a lumpy one.
4. Roll it out in the same direction as the flooring.So the joins in the underlay do not sit under the joins in the boards. Running them the same way is standard for laminate and engineered wood.
5. Butt the edges — never overlap.Overlapping doubles the thickness in a line across the room, which shows through the flooring permanently and breaks the joints above it. This is the single most common underlay mistake.
6. Tape every joint.Proper underlay tape across each butted seam. It stops the sheets creeping apart while you lay the floor over them, which they otherwise do.
7. Take it up the wall a little where there is a membrane.About 50mm up, to be trimmed after the flooring is down and hidden behind the skirting or beading. A membrane that stops at the wall lets damp in at the edge.
8. Do not stack underlays for extra thickness.Two layers makes the floor spongy and stresses click joints until they fail. If more insulation is needed, use a single thicker product rated for it.
9. Match it to underfloor heating if fitted.It needs a low tog rating so heat gets through. A thick insulating underlay over underfloor heating largely defeats the heating.
10. Cut around obstacles rather than folding.Pipes, door frames, hearths. A folded piece leaves a lump that shows through, and cutting it takes a minute.
11. Lay only what you will cover that day.Walking on bare underlay compresses and tears it, especially the softer types. Roll out a few metres ahead of the flooring rather than the whole room.
12. Let solid wood and laminate acclimatise on top.Boxes flat in the room for 48 hours before laying. It is the underlay stage where this is easiest to remember and it prevents the gapping that follows.

Tips

• Butt and tape. If you remember one thing about underlay, that is it — overlapping is the fault that shows for the life of the floor.
• Underlay with a foil or membrane backing goes membrane-side down against concrete. It is easy to lay upside down and hard to notice.
• Buy the tape with the underlay. Ordinary parcel tape lifts and the sheets separate as you work.

Common mistakes

• Overlapping the underlay at the joins — It creates a permanent ridge through the finished floor and puts a stress line under the boards that eventually breaks the joints. Always butt and tape.
• Laying on concrete without a damp-proof membrane — Moisture rises constantly through concrete and will reach the flooring. Use a separate polythene sheet or an underlay with one bonded on.

If it doesn't work

Questions people ask