Galvalume directly over Shingles

Ron Lalonde
Metal Roof
1/6/2011
Hi My son had a metal roof installed last May, and I recently had one installed last month. Both our roofs were installed by different contractors, but the Galvalume was purchased at the same manufacturer. Both roofs were installed directly over top the shingles, no strapping or underlayment. Can there be a problem with the rough shingles scratching the galvalume causing it to rust from the the inside out due to the movement caused by expansion and contraction with temperature changes? The contractor and steel manufacture (where the steel was purchased) said this is the way they install these roofs all the time. Other than that, both roofs look great. How long do you think the galvalume will take to rust through, if this is indeed a problem? There is a 40yr paint warranty, but is the paint on the back side of the galvalume good enough protection from the shingles. Am I worrying needlessly or will I have to reroof in the near future, due to my not being educated BEFORE I had the roof installed?? Thanks Ron Lalonde Nova Scotia, Canada
Ken Buchinger
NCI Building Systems, Inc.
1/7/2011
I have never seen this be a problem on a house. The panels typically are not long enough to be moving very much. For instance, a 100' long metal panel that changes temperature by 100 degrees, will change length by about 3/4". Living in Canada, I am sure you see temperature differences of over 100 degrees but I would bet that your panel lengths are less than 40'. Also, if the panel used is a through fastened type, they will eperience thermal bow instead of floating as standing seam panels with floating clips do.
Ron Lalonde
Metal Roof
1/7/2011
Thanks Ken, I was worried about too much movement due to temperature change and the shingles scratching the underside of the Galvalume causing it to rust. It is the through fastener type and the length of the longest piece is 16 feet. Thanks again for the answer to my question. My mind is now at ease, especially getting an answer from someone with your qualifications and experience. Ron
james mcrae
1/15/2011
hi there, i am looking for some feedback on information i am hearing from a reputable roofer here on vancouver island (where it rains a lot) that putting a metal roof on top of an existing shingle roof is not something he would ever do.. his reasoning is that you've created a 2nd vapor barrier that creates moisture that has no where to go other then into the roof panels and as a consequence gives you rot which will have to be dealt with 10-15 years down the roac... long sentence, whew, but i am curious to know if anyone would like to comment on this? thanks - james
Ken Buchinger
NCI Building Systems, Inc.
1/17/2011
An asphalt shingle roof is not a vapor barrier. It will actually allow vapor to escape from the attic. However, if you are worried about condensation on the underside of the metal roof, install it on a batten/counter batten system and provide an air intake at the eave and air exhaust at the ridge. This will reduce heat in the summer and help reduce the chance of ice dams in the winter. Don't use treated wood as it contains copper which is a dissimilar material and will cause the metal panels to corrode.
james mcrae
1/18/2011
thanks ken! actually the fellow who was going to put the galvalum over the shingles was going to lay roofing paper - a thicker type - in order to not have the bumps in the shingles show... this 2nd slice of paper while perhaps cutting out any bumps in the look of the galvalum, will essentially operate as a 2nd vapor barrier as i understand it..
Eric Novotny
An informed customer is the Best Customer!
1/19/2011
You would be well served to install as Ken is suggesting (on a system of battens). You roof layer is not designed to be vapor retarder/barrier. In this case, the vapor barrier/retarded is the attic floor. If you are worried about humidity and condensation issue, look at sealing up the attic floor and any penetrations.
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