WHAT GOES UNDER ROOF SHINGLES?

In the UK felt shingles are normally used on shed and garden buildings, so for these applications a 3B bituminous felt is used as an underlay as the buildings below, not being subject to habitation, do not require the installation of a breathable membrane under the shingles.

In the United States, as felt shingles are the most popular roof covering on houses, a breathable membrane is used under the shingles.