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Question DetailsAsked on 8/30/2016

metal roofing on a semi flat roof

I have been told that you can and that you cant use metal roofing on my semi flat roof

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1 Answer


Many manufacturers state you can use their product on roofs as flat as 1/4:12 slope - a 2% slope, which is sort of a rule-of-thumb for decent drainage on very low-slope membrane roofs and strip metal roofs EXCEPT for wind-driven rain and ice damming situations. 2:12 slope is a common industry rule-of-thumb for "sheet and seam" products like normal metal sheet roofing. That should be enough to prevent backing up of water under the overlaps in theory - but on roofs flatter than 4:12 commonly the end overlap of sheets is increased, and commonly below 4:12 and certainly below 2:12 slope gasketing at the ends is needed. (Some makes use end gasketing always, some never, some only on flatter slopes).

One exception - for raised seam products with continuous positive compressed gasket seal on the seam, IF the sheets are full-length in one piece (each panel continuous ridge to gutter, without any mid-sheet seams) then a high-ridge raised seam roof should perform well except in ice damming situations on a simple valley-free single ridge roof. However, on the flattear slopes valleys form potential for water infiltration under the ends of the sheets. Here are images of high and low-ridge standing seam roofing FYI -

My recommendation - for areas with hard blowing rain or ice damming potential, for anything less than 2:12 use a membrane or built-up roof - and for 2:12 to 4:12 use only high-profile gasket seam metal roofing, with end gaskets at sheet end seams. And definitely use a good water barrier under it.

Note - for metal SHINGLES, same rule as for wood or asphalt - anything under 4:12 slope is asking for trouble.

If you do decide to use metal sheet roofing on a very low slope (under 2:12 say, or under 4:12 if low-profile and in blowing rain or icing area, you can beef up your protection by using full-coverage ice and water shield under the roofing rather than water barrier membrane - costs maybe $0.50/SF more but is FARRR more watertight in the event you get leaks through the roofing. (Also a good idea in hurricane areas where it will almost always provide much better protection of the sheathing in the event of roofing blowoff).

Answered 4 years ago by LCD

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