My underlay says it has a built-in moisture barrier. Do I still need a membrane?

Usually not, on a floor the flooring's instructions allow. Check that the joins are taped as the underlay's instructions say. If the floor is new concrete, it must be dry enough first, and the flooring's instructions give the test.

Underlay and damp-proof membranes: what to check first

Whether you need underlay, and whether you need a moisture barrier under it, depends on the flooring and on what it is going down onto. The flooring’s instructions decide; this guide tells you what to look for before you buy.

Three questions

How is the floor fitted? A floating floor, where the boards click together and sit loose on the subfloor, usually needs underlay. Boards with underlay attached need none. A glued-down floor takes none at all; it takes adhesive instead.
What is under it? Concrete or screed holds moisture. A suspended timber floor over a ventilated void does not usually need a barrier. Boards or sheets on an upper floor usually need underlay only.
Is it a ground floor? A concrete ground floor is the usual case for a moisture barrier.

Answer those three and most of the list writes itself. Where it does not, the flooring’s instructions are the source, not a rule of thumb.

Underlay

It is sold in rolls, and roll sizes vary a lot: thin foam commonly comes on rolls of 15 to 20 m², while denser acoustic underlay and underlay for vinyl planks comes on smaller rolls, often 6.5 to 10 m². Take the figure off the roll.
The area to cover is the same as the flooring. BuildBatch applies the same allowance to both.
Joins between strips are taped. One roll of joining tape usually does a room; the roll length and the number of joins decide.
Underfloor heating: both the flooring and the underlay say whether they suit it. Check first.

Damp-proof membranes and moisture barriers

On concrete or a ground floor, a moisture barrier usually goes under the underlay unless the underlay has one built in. It covers the same area as the floor, with the overlaps taped as its instructions say.
New concrete must be dry enough before any floor goes down. The flooring’s instructions give the test. Do not guess a drying time.
A membrane is not a cure for a damp floor. If a floor is damp, or you are not sure, find the cause first and get advice.
A suspended timber floor above a ventilated void does not usually need one. The flooring’s instructions decide.

Worked example

The example room’s floor is 15.12 m²; with 10% extra the area to buy for is 16.632 m².

Underlay on rolls of 15 m²: 16.632 ÷ 15 = 1.11, so 2 rolls, 30 m², about 13.4 m² over
Underlay on rolls of 10 m²: 16.632 ÷ 10 = 1.66, so 2 rolls, 20 m², about 3.4 m² over
Underlay on rolls of 7 m²: 16.632 ÷ 7 = 2.38, so 3 rolls, 21 m²

Same floor, three roll sizes, three different amounts left over. If a membrane is needed, it covers the same 15.12 m² of floor plus its overlaps. The roll says what it covers; divide and round up in the same way.

How BuildBatch lists it

Underlay is marked Need, with the rolls worked out, once you say you need it and give the roll coverage. Say you already have it and it is marked Already have. Say the boards have it attached or the floor is glued and it is marked Not needed. Otherwise it is Check first, with the area to cover beside it.
Damp-proof membrane or moisture barrier is Check first for a concrete or ground floor, with a note to look at whether the underlay already includes one. For a suspended timber floor above the ground floor it is marked Not needed.
Underlay joining tape and expansion spacers follow the same answers.

Do it with BuildBatch

Open the Room Refresh Batch with flooring chosen, answer what the floor is going onto and whether it is a ground floor, and the underlay and membrane lines change to match.

What this guide does not cover

Finding the cause of damp. Screeds, levelling compound and subfloor repairs. The design of underfloor heating. Sound insulation rules for flats. Adhesives for glued floors. For any of those, the flooring’s instructions and a professional come first.