Fastening Metal Roofing to Purlins
Are there specific manufacturers that allow metal roofing to be attached to purlins? I am designing a multiplex and was looking to reduce the use of unnecessary structural sheet goods and put that money into steel roofing which will last longer, like a standing seam metal roof. There are plenty of references saying that it can be done, but nothing definitive. Search as I might, it is hard to get an answer to this seemingly simple question. Is there a specific type (I would think there would be a classification) or manufacturer of standing seam metal roofing that I can attach directly to purlins? The roof will be covered with solar panels also.
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markgh1,
I use two different manufacturers who supply a range of profiles of exposed fastener and snap-lock panels. They both provide span tables for the spacing of purlins for all their products. I don't know of any metal roofing that has to be installed on sheathing, although it's my preferred substrate, and here it is required for seismic resistance.
Thanks, Malcolm. Yes, from a performance perspective no metal roof requires a solid substrate deck. I get that from an installation perspective it is an apples to oranges comparison.
Fabral Strongrib and Grandrib metal roofing can be fastened to purlins: METAL ROOFING AND SIDINGALUMINUM AND GALVANIZED STEEL.
Thanks, Kiley
I'm curious - what do you expect to gain with the purlins?
Hey Jon,
Thanks for the question, it gives me an opportunity for solicate a critique of options. This started out as a simple comparison of a standard shingle roof vs standing seam metal over the life of the solar installation that is going on top of it. This is two large single pitched, ventilated roofs oriented solar south utilizing 4/12 pitches. The top chord runs are 48' with the width of building at 56'. I like to future proof my buildings whenever possible and even though the solar arrays will not cover the roofs initially, it does not take that many electric vehicles to consume it, especially with vans and trucks getting into the electric vehicle market.
The orignal plan was to utilize shingles to limit costs, however after analizing the numbers a little further and accounting for the cost of removing the solar arrays to replace roofing 30-35 years from now it became apparent that if I could design the steel roof to limit the cost by eliminating the plywood deck it becomes a much better cost comparison. If I can hold off or the removal of the roofing for an additional 15-20 years it makes a big difference even with the cost of capital.
If this is a wood framed structure the plywood roof deck is a structural element of the building. It is part of the lateral force resisting system. To future proof a building I think you need to build above code level design forces, so it won't be damaged in a design event.
Most metal roofs I've seen installed over commercial or farm buildings are installed over purlins. When $/sqf matters, it seems to be lower cost option. I can't see any reason why it wouldn't work for your project. Make sure there still is an underlayment under the metal panels otherwise you can get condensation drips that can sometimes make it through the insulation into the ceiling bellow.
At least around me, for residential, most of the cost of the metal roof is the install, so make sure you have trades lined up and are willing to install over your proposed structure.
Since you mention durability, make sure to spec hidden fastener panels, exposed fastener panels usually need screws replaced well before the panel wears out. Also check that you can get standard seam clamps for the PV array that are compatible with the metal panels.