underlay vs underlayment

underlay and underlayment both are nouns.

Word	Noun	Adjective	Verb	Adverb
underlay	Yes	No	Yes	No
underlayment	Yes	No	No	No

As nouns, underlay and underlayment are synonyms defined as:

Other synonyms of underlay include carpet pad, rug pad.

underlay (noun)	underlayment (noun)
a pad placed under a carpet	a pad placed under a carpet

underlay (verb)	underlayment (verb)
provide with a base, support, lining, or backing
put (something) under or beneath
raise or support (the level of printing) by inserting a piece of paper or cardboard under the type