metal roof framing

[tom_benelli]

Browsing through this forums pics I have seen rafters 24OC, 16 OC, with and without ridge beams. I am installing a 12/12 metal roof. Is there certain criteria for the above applications such as pitch or size? My roof will cover 24 x 38 with a collar 4" down from the peak. Any suggestions

[tom_benelli]

Sorry...thats 4 feet down from peak

[Redoverfarm]

Tom the only restrictions that come to mind is that the roof should not be any less that 3/12 and perlins should not be any wider that 24"-36".  Not exactly sure what info you were looking for.  A little tip to make it go smooth is to keep the perlins at a specified distance from either the bottom or the top of the roof. Then you can measure the same distance from the top or bottom of the sheet (keep consistant) and pre-drill your screw holes (I use a drywall square). It not only keeps the screws in line but makes for attachment easier by not having to fumble with a drill or impact driver. It also helps eliminate scratches from slips

[tom_benelli]

Thanks for the tip but I was questioning rafter and ridge beam construction options, 16oc or 24 oc, with or without ridgebeam.

[peter_nap]

Tom...a llot depends on your type of building, the amount of help you have and your abilities.

Generally speaking, a ridgeboard makes it easier to frame by yourself or if you are a novice. It also depends on the size. On my barn.cabin, the center roof is so small it's easier for me to make up rafters with collar ties on the ground and carry them up as a unit. On a larger roof, I wouldn't be able to do that by myself an I would have to use a ridgeboard.

As far as spacing, it depends on how the house ifs framed, IMHO. The way I was taught, everything fits over the studs to prevent unsupported weight in the centers. In other words, 24' studs=24' rafters.

There are a lot of exceptions to that....like trusses. But for stick rafters, I've always held with that rule.

[tom_benelli]

Thanks Peter.if I am sheathing the roof with 3/4 ply can I go with 24 O.C. 2x6 ? The roof is 12/12..24' wide and 38' long with collar at 4' down from peak to create attic air space. The rafters should be appx. 19' long with the 2' overhang.

[tom_benelli]

I forgot to add in the roof is sitting on 2x6 16 O.C. framed walls

[peter_nap]

Are you using perlins and what size?

I'd have to look at Dons span calculator but for that distance, 2x6 seems a little light but should work at a 12/12 pitch. If I were doing it, I would still prefer to have them on 16' centers but that's just a personal quirk.

Are you doing this from One of John's plans. What do they show?

[tom_benelli]

I was thinking of blocking with 2x6 flat spaced  for sheathing. This is my own design so any structual help is welcomed.

[tom_benelli]

My 2nd option was 2x8 rafters...also better insulation room

[peter_nap]

The reason I asked about the Perlins is that they add some strength to the overall roof and I don't like putting metal roofing directly on the sheathing. The space between the sheathing and the metal gives some ventilation.

Most of what I'm telling you are personal preferences. In the past, I just followed what the architect speced out. That's why I asked if it was John's design. He's the expert here.

I'd go with the 2x8's. The 6's will work I think, although I'm not sure they will get by the inspector. Even at a steep pitch, I worry about snow loads and I tend to be the most conservative with the parts of the building that may fall on my head.

[tom_benelli]

After using 2x8's how would you suggest installing metal roof if not on top of sheathing?

[peter_nap]

Perlins over the sheathing and felt.

[tom_benelli]

Thanks so much for your help and effort. Any specific size on purlins

[Redoverfarm]

Just a play on words. Lay the felt on the sheeting before perlins. I normally use 1X4 for perlins which should be nailed with ring shank nails. Then use self tapping gasketed screws to secure the metal to the perlins.  If you use my method of pre-drilling measure so that the screw hits mid width of the perlins.