Underlayment goes down in strips with a small overlap at the seams, so the usable coverage is slightly less than the roll states. Eight percent covers overlap and wall trimming.
What it actually does
Underlayment smooths out minor subfloor irregularities, cushions the floor slightly, and reduces sound transmission. On concrete it also carries the vapour barrier.
What it does not do is fix an out-of-flat subfloor. Foam compresses under load; a hump or dip stays a hump or dip.
Over concrete you need a vapour barrier
Concrete slabs pass moisture continuously, even ones that look and feel dry. Any wood or laminate floor over a slab needs a 6 mil vapour barrier, either separate or built into the underlayment.
Check whether your underlayment has one attached. Two barriers trap moisture between them, which is worse than none.
Some floors come with it attached
Many LVP and laminate products have a pad pre-attached to the plank. Those need no separate underlayment, and adding one usually voids the warranty.
Read the plank spec before buying rolls — it is a common way to waste a hundred dollars.
Thickness is not where the performance is
Thicker underlayment feels better underfoot in the shop and does not necessarily perform better under a floor. Beyond the manufacturer's specification, extra thickness lets the floor flex, which stresses the locking joints on a click-together product.
Most laminate and rigid core floors want 2 to 3 millimetres. Going to 6 because it felt plusher in the sample is a common and expensive mistake — the joints work loose and the floor develops a hollow, clicking sound.
Follow the flooring manufacturer's spec rather than the underlayment manufacturer's marketing. The floor warranty follows the former.
Sound ratings and what they mean
Underlayment is often sold on two acoustic numbers. IIC measures impact sound transmitted downward — footsteps heard in the room below. STC measures airborne sound through the assembly.
In a single family house neither number matters much. In a condominium they can be a condition of the association's rules, and a minimum IIC rating is a common requirement written into the bylaws.
If you are in a building with rules, get the required number in writing before you buy, because underlayment that does not meet it has to come back up along with the floor on top of it.