Roofing Underlayment

Roofing underlayment (in Britain and Ireland usually called underlay, and in Australia often sarking) is a sheet material installed between the roof deck or rafters and the visible roof covering. Shingles, tiles, slates and metal panels are the primary weather barrier; the underlayment is the backup that sheds water driven past them by wind, ice dams or flashing failures, protects the deck during construction, and in some systems contributes to wind resistance.

Main types

• Asphalt-saturated felt: the traditional product, sold in North America as #15 and #30 felt under ASTM D226 Types I and II, with lighter grades under ASTM D4869.
• Synthetic underlayment: woven or spun-bonded polymer sheets that are lighter and more tear-resistant than felt. Products are typically evaluated against code acceptance criteria rather than a felt standard.
• Self-adhered membranes: rubberised or polymer-modified bitumen sheets with a sticky back, used as ice barriers, in valleys and around penetrations, or as full-coverage underlayment.
• Vapour-permeable (breather) underlays: common in the UK and Europe, these resist liquid water but let water vapour escape from the roof space.
• Vapour-impermeable underlays (HR): traditional bitumen-based sheets that block vapour, used with a ventilated loft.

Installation basics

Underlayment is laid in horizontal courses starting at the eaves, with each upper course lapping over the one below so water runs outward. At the eaves of a North American shingle roof it goes over the drip edge; at the rakes the drip edge goes over it. On low-slope shingle roofs, codes call for additional laps or double coverage.

In the UK, underlay is usually draped over the rafters under battens. BS 5534 refers to a nominal 10 mm drape so that water can drain beneath the battens to the gutter.

Choosing a product

Selection depends on the roof covering, pitch, climate and code. Roof covering manufacturers often specify compatible underlayments, and some coverings, notably metal in hot climates, may need high-temperature-rated products.