How to pick insulation board for copper-sheathed roofing

Q. I am thinking of covering some dormer window roofs with copper sheathing grounded to thick copper cable. What kind of solid insulation board should be used under the copper so it will not dent under pressure from walking on it? Henry Dunseith Mattapoisett, Mass.

A. Check with your local building-supply firm for a rigid inorganic insulation board. There are many types and thicknesses available.

One particular thickness of insulation board may suit the field roof conditions better than another. Use material which is at least three-quarters of an inch thick, preferably thicker. The thicker the material, the better its insulation or R-value.

The copper sheathing should cover the outboard edges of the insulation to keep runoff moisture from entering it. Naturally, copper or nails need to be long enough to pierce the insulation and enter the wood below.

Be sure to flash, counterflash, and caulk where the dormer roof meets its adjoining vertical wall.

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