Q:

I built a timber-frame home eight years ago that is enclosed with structural insulated panels (SIPs). I installed heavy-duty architectural roof shingles over this code-permitted unvented roof, but the shingles have failed miserably, with vertical, horizontal, and crescent-shaped cracking as well as crumbling all over. Is this problem a result of shingling over the SIPs, or did I get a bad batch of shingles?

Scott Young, Fredericton, None

A:

William B. Rose, a research architect at the Building Research Council at the University of Illinois, replies: Horizontal, vertical, and crescent-shaped cracking, i.e., cracking without a directional pattern, sounds like a problem with stresses in the shingle laminations introduced during the manufacturing process. Crumbly shingles sounds like a bad batch of asphalt. If you trace the cracks on a photo or drawing of the roof and there is a pattern, it may indicate other problems such as improperly crowned rafters or overspanned sheathing.

Could lack of venting be at fault? Even with venting, the shingles on any southfacing cathedral-style roof still will be hotter than those on the north side, at least for the top half of the roof. Venting does cool a roof, but cathedral-vent slots are thermal chimneys that become pretty hot toward the top. If the north-facing shingles are bad, then the south-facing shingles probably will be at least as bad. So when you consider everything that affects shingle temperature, such as latitude, solar orientation, shingle color, and roof complexity, you’ll realize that shingle performance should never be pegged only to ventilation.

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