“Cold” Exposed Fastner Metal Roof over old shingles with purlins?
Hello,
I’m in Region 6 (Minneapolis) and need to replace my roof. I am looking at both standing seam and exposed fastener metal roof options. The exposed fastener metal roof represents a significant cost savings and hence my strong consideration. Being that I’m in Region 6 I understand that there is going to be value in installing the roof on 1×4 purlins to provide an air gap, thus making a “cold roof”. I have also been thinking that installing an exposed fastener metal roof over the old shingle roof (possibly adding a roofing membrane over old shingles) may be prudent in the case where the exposed fasteners do begin to leak, as backup protection.
I am hoping that this roof design will allow me to seal the attic space, which has the air conditioner air handler (currently unconditioned space). Further the cost savings on a exposed fastener roof will allow for more closed-cell insulation on the underside of the roof deck.
My roof is 5/12 pitch, stick framed true dimensional (1928 era) 2×4, 2×6 joists. I hope for either flash coat 2″ CC spf and supplemented with wet blown cellulose on shims or rigid, or 4″ CC spf supplement with wet blown cellulose on shims or rigid.
Any thoughts on this roof design? Is exposed fastener “OK” or is it still too much of a risk in residential construction. Would I be better off with 1×4 purlins and new roof nailer asphalt shingles if cost is an issue?
Thanks,
Garret
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Replies
Garret,
All over the U.S., from Vermont to Florida to the far West, homes are roofed with through-fastened metal roofing that is performing well. That's what I have on my house.
If you're going to use a non-permeable insulation under the roof deck, then it's imperative that the roof be able to breathe upward. I would remove existing shingles (both for breatheability and for more secure attachment of purlins), install a breatheable membrane (#15 felt or pricey Roofshield by Vaproshield), strapping and metal roofing. Be sure to have the washer-head screws installed on the flats (not the ribs) of the roofing. That prevents movement and leakage.
It may not be rational but I have a strong discomfort with burying worn-out roofing or other materials under new materials. In the case of an asphalt-shingle roof-over... maybe. In your situation, with metal roofing over purlins, I would strip the old roofing, re-nail the existing sheathing to the rafters (you may have a lot of rusty nails there, my 1921 house certainly did), and then do as Robert suggests and make sure the roof can dry to the outside. You may decide to install the metal directly to the deck, saving a step and making it a little easier to walk on the metal without buckling it.
I've used the metal roffing for years. The leaking fastener problem has all but gone with the switching from rubber to polyprpylene washers. You should check with your local builing official about double layer roof membranes. Some locals wont alow for a shingled roof under the metal because of their local fire codes. R. Hall TN home inspector
I would worry more about condensation on the fasteners if they protrude into the attic space, especially if you condition the space. I would purlin over the existing roof and size the screw length accordingly. For the same reason I would not fasten the metal directly to the existing decking.