Re-roofing a 1960’s Pole Barn
Reader JEFF in GRESHAM writes:
“I have an older (built in the 60’s?) pole building that needs a new roof. It is a typical metal over 2×6 purlins (Purlins 2′ on center, trusses 12′ apart). Current roof is corrugated metal attached with ring shank nails with rubber washers and it leaks (unsurprisingly). The roof had a spray on coating 15 years ago which helps, but again, it leaks and it needs a new roof. I would like to do standing seam roof but since the roof needs to act as a shear diaphragm If I tear off the old metal, I need to sheet the whole thing so those are two substantial projects in their own right (#1 tearing off and then #2 re-sheeting…) . I tried taking a pry bar to find out the difficulty of removing the old nails and it was stuck so securely I ripped the head off the nail rather than nail of the roofing and I estimate there are about 6000 of them! Thinking about it and doing some research, I think it’s possible to go over the existing corrugated roofing with standing seam using wood strips over the corrugated. I’m still concerned about things like condensation occurring if I do that and not sure if it would be unlikely, infrequent, or maybe much worse than what I see now. Obviously frequent and substantial condensation is just going to lead to rot and problems. I’ve looked at the added weight and its about 1.4 lbs per square foot using 24Ga standing seam = pretty trivial compared to snow loads so I think it’s unlikely to be an issue. I’m interested in knowing what advice, experience, and tips the Pole Barn Guru can offer for this situation. Please see attached .pdf for a visual of the scenario at hand. Thanks!?”
Yes, those old nails are tough to remove. Most standing seam steel is not stiff enough to span between purlins, so it is nearly always recommended to install over solid sheathing. Even if it will carry this span, it is likely to droop between purlins and if stepped upon, by accident, between purlins expect it to buckle.
Next challenge – those roof trusses are probably not designed to carry more than bare minimum loads. You want to add as little weight as possible. It might behoove you to have them inspected by a Registered Professional Engineer to insure they will be structurally adequate to handle any added weight.
Assuming trusses are structurally adequate, I would pull out every nail I could and, for ones where heads break off, once old roofing is removed, pound them in. Sheet roof with 19/32″ CDX plywood, install 26 gauge standing seam over a synthetic underlayment. There is no real reason to go to 24 gauge, other than a salesperson’s commission. Make sure to vent ridge and have adequate air intakes and condensation should then be a non-issue.
