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Question DetailsAsked on 4/29/2015

Are furring strips always used when installing a metal roof over asphalt shingles>

We had a roofer tell us that we don't need the furring strips to attach the metal roof over the asphalt roof.

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1 Answer


The question should not be if they are ALWAYS used (and you know the answer to an always question has to be no), but whether they SHOULD be used.

Below is a prior related question with comments similar to below and also a strong opposing opinion, which might be of interest to you - also others on the roofing-over issue can be found in the Home > Roofing link in Browse Projects, at lower left.

If talking metal shingles, then putting them over existing asphalt shingles would be basically the same approach as putting on a second layer of asphalt shingles - same issues, same technique basically except they fasten a bit different, and firring strips would generally not be used in my experience of seeing it done (not that I would generally ever second layer with metal myself).

If talking strip metal roofing, like standing or raised seam, then I can't imaging getting an even final surface or proper fastening without firring strips - not only because putting the metal right on the shingles gives no way to level out the uneveness of the old shingles, but also the underlying shingloes and underlayment will get awful hot in the summer without that airspace provided by the firring (which should have intermittent air gaps in every row), plus generally the screws should be going into thicker material than the thin 15/32 or even thinner sheathing being used these days - and especially if the sheathing is OSB or particle board, which is terrible at holding screws. Used to be fine in 3/4 BDX or CDX sheathing, but the standsards dropped way off in the u70's building boom, allowing sheathing that is marginal even for roofing nails - which is why they increased the required gage and gripping serrations on them, because some were pulling or working their way out.

I would be looking for a couole more bidders if I were you, and if already under contract check out the - let's see - NRCA (National Roofing Contractors Association, as I recall it is called) installation standards book - should be able to find on the web. Also check out, more importantly with respect to not damaging your manufacturer warranty, their installation instructions. Commonly in or on the boxes, and also on the web at probably all manufacturer websites.

Answered 5 years ago by LCD

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