Underlay
Underlay or underlayment is a term used to describe materials in both flooring and roofing.
[edit] Flooring
Underlay in flooring is material typically made from rubber, sponge, foam, felt, plastic and so on. It serves as a protective layer between the 'unfinished' subfloor and the final floor covering. Depending on the material selected, underlay can help provide thermal and acoustic insulation, reduce impact noise, reduce wear and enhance comfort. It is sometimes advisable to use higher quality underlay in order to avoid the deterioration that can occur with lower quality products that can harden and crumble over time.
Underlay is most commonly used under vinyl flooring, wood flooring and carpet. Some forms of carpet - such as rubber or foam backed carpet - do not require underlay.
[edit] Roofing
Underlay in roofing is material made from rubber, felt, tar paper, synthetic underlayment and other protective substances. As a roofing membrane, the purpose of underlay is to helps to prevent water that has penetrated through or under the roof covering from leaking into the building.
There are non-breathable underlays and breathable underlays based on ventilation requirements of the roof.
NB The 2018 International Residential Code (IRC), published by the International Code Council (ICC) defines underlayment as: ‘One or more layers of felt, sheathing paper, nonbituminous saturated felt, or other approved material over which a roof covering, with a slope of 2 to 12 (17-percent slope) or greater, is applied.’
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