Installed over strapping or directly on roof

Tania H
3/20/2013
My husband and I would like to install metal roofing. There are 2 systems that interest us. One is smaller sheets that have a polyester coating and are applied over strapping. The other is metal shingles with a kynar coating that are applied directly to the roof. There are pros and cons to both but the part of the equation we have yet to figure out is in regards to the strapping and ventilation. We own a 1 1/2 story house, although the top part of the attic is a walk up and the roof is all open (insulation is under the floor) there is no flow from this space down to the eaves. This part of the roof goes directly over the bedrooms with no insulation (our house is of brick construction, 80 years old and has no insulation in the walls either) and the heat loss in the winter causes a freeze/thaw cycle that produces large ice dams along the edge of the roof. We were told by the one contracter that the strapping would provide much needed ventilation and solve our problem, also decreasing our airconditioning costs. However another contracer who sells both types of roof, said it wouldn't make a lot of difference and the metal shingle is a much better design with hidden hurricane clips that float. Help, is one system better than the other for our situation?
Todd Miller
Isaiah Industries, Inc.
3/20/2013
Good question. Unfortunately most companies when they strap a roof provide only horizontal straps. This really does not create vertical airflow up the roof and out at the ridge. For ultimate effectiveness with ice damming, you would need actual airflow and ventilation. This would require vertical straps followed by horizontal straps, creating this ventable chamber and bringing air in at the bottom and exhausting it out at the top. Strapping the roof also involves raising the level of the roof which can require different trim pieces, raising the gutters, etc. Also, most products that go on strapping have exposed fasteners, overlapping panels rather than interlocking panels, little if any allowance for expansion and contraction, and sometimes these products have lower quality raw materials. Trying to somehow enhance ventilation and insulation in the home as it now is would, to my way of thinking, be most helpful. The metal shingle option, without doing this, may help your situation a little as it tends to shed snow and ice faster than standard shingles, but it will not end ice dams.
Guest User
3/20/2013
Thanks for your reply Todd, The company that was talking about the strapping actually does strap in both directions, horizontal and vertical. Their price is more expensive and I don't think their product is quite as good but they have a 100% transferable waranty, that along with the strapping is making us consider them. The company that has the metal "shingles" that go directly on the roof are maybe a bit better quality and actually a little bit cheaper but the waranty isn't transferable and then because they don't use strapping we were not sure which product/company to choose. Any advice for us metal roof "newbies" is much appreciated.
Todd Miller
Isaiah Industries, Inc.
3/20/2013
Thanks Tania. The double strapping definitely helps and, if vented properly, should help with ice damming. I still prefer to address ice damming with attic ventilation and insulation as those methods have some other benefits (especially the insulation) but I understand that some houses are built in ways to make that very difficult. I will say this ... there are metal shingles out there with transferable warranties so that may be one thing to think about ... perhaps look at other options.
Guest User
6/13/2016
I have a bungalow with a single layer of shingles. We are going to put on a metal roof. Would you recommend we strap it first or just put the metal over the singles?
Todd Miller
Isaiah Industries, Inc.
6/14/2016
Here is a video I recently recorded on this subject. I hope that it is helpful. https://www.youtube.com/watch?v=pEWcwNg29Ag

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