ASTM D1970: Self-Adhered Ice & Water Shield

D1970 specifies self-adhered polymer modified bituminous sheet materials used as steep-slope roofing underlayment for ice dam protection. Two features define the category: it bonds to the deck without torching or mechanical fastening, and it is thick enough to seal around the nails that penetrate it.

That second property is why it is specified at eaves and in valleys — the places where ice dams form and standing water backs up under the shingle course. A mechanically fastened sheet leaves a hole at every nail; a self-adhered sheet reseals around it.

• Thickness — nominal and minimum, since thin spots are where the membrane fails first.
• Tensile strength and elongation — the membrane must stretch over movement without splitting.
• Low-temperature flexibility — it must remain pliable at cold temperatures, not crack when handled or when the deck moves in winter.
• Adhesion to the deck and to itself at the lap — the bond is the product. A sheet that debonds is just a heavy slip sheet.
• Water absorption and water vapour permeance — a near-zero permeance is intended here; this is a barrier, not a breather sheet.
• Thermal stability / flow resistance — on a dark roof in summer the deck can run well above ambient, and a modified bitumen sheet must not slump.
• Sealability around nails — the defining ice-barrier behaviour.

The common applications are eaves and rakes in cold climates, valleys, around penetrations such as chimneys and skylights, and full coverage on low-slope sections where shingles are not permitted. Building codes in cold regions typically require an ice barrier from the eave edge to a point inside the exterior wall line.

Because the material is considerably more expensive per square than mechanically fastened synthetic or felt, it is applied selectively rather than across the entire roof in most residential work. Full coverage is reserved for low slopes and for roofs where the owner wants the redundancy.