Rosin Paper Slip Sheet

Seamed metal roofs, especially copper, are traditionally installed over an asphalt-saturated felt underlayment. On a hot roof, asphalt softens and can bond to the underside of the metal. Once bonded, the pans cannot expand and contract freely, which leads to buckling and fatigue cracking. A slip sheet of rosin-sized building paper laid over the felt prevents this bonding.

Specification

Copper roofing guidance in the United States commonly calls for:

• Asphalt-saturated felt of at least 30 lb per 100 sq ft (for example ASTM D226 Type II, No. 30), lapped and fixed.
• Over it, rosin-sized building paper of at least 4 lb per 100 sq ft, lapped about 4 in (100 mm).
• The slip sheet must fully separate the felt from the copper; wet slip sheet should be replaced before metal is laid.

Why rosin paper

Rosin-sized paper is inexpensive, smooth and does not stick to metal or asphalt at roof temperatures. It is used under copper, terne-coated stainless, zinc and other hand-seamed roofs, as well as under some flashings.

Modern variations

Underlayment products have diversified, and metal manufacturers' instructions should be followed for compatible layers. Zinc and lead roofs in Europe often use drainage mats or ventilated decks, mainly to control underside condensation.

Installation tips

The paper should be laid smooth, without wrinkles that would telegraph through soft metal such as copper. It is fastened only enough to hold it in place until the metal pans and cleats are installed, and work should be planned so that exposed paper is covered before rain. Because the paper is a separation layer, not a waterproofing layer, the felt or other underlayment beneath it remains the secondary water barrier.