metal roofing

I'm thinking about reroofing some sheds with metal. The sheds have 1/2" plywood sheeting with 90 lb.roll roofing. Will it be ok to strip the old roll roofing off and install the metal right over the plywood sheeting? I've always used asphalt roofing and know nothing about metal roofing.Thanks for any advice. Jay
 
I would think the 1/2" plywood MAY be a bit thin for the roofing screws to get a secure grip to avoid having the tin tear off easily in a wind, but maybe not???
 
If the surface of the 90 pound roofing presents a smooth surface that will not distort the tin, then you might as well leave it and put the tin over it.
 
I'd leave the tar on if in good shape.

Might want to ask, _some_ metal reacts badly & would need craft paper over the tar. Usually not a problem...

Wood is thin, hope it's good shape, use screws not nails.

A more common way to do this is put 2x4s flat across your roof every 2 feet, screw them well into the rafters, then put the tin onto the top of the 2x4s. Don't bother any at all with whatever is on the old roof, just strip over it.
 
You have to have a full 1 1/2 inches of metal roofing nail into 2x4 runners.

A 1 1/2 inch roofing nail into 1/2 inch plywood is like DUCT TAPING the metal to the roof.

He needs to have the nails go into 2x4 runners placed horizontally along the roof... just like any other metal roof shed.
 
I also want to add that you do not want
to use treated lumber for runners unless
you put a barrier between the tin and
the treated wood. The metal will react
with the treatment (copper sulfate) ane
will prematurely corrode your tin.

I second the 2 x 4 runners. You need the
air space to let the tin breath. Even though the
tin will sit 1 1/2 " higher on the roof, there
is a camp at the rake ends which will cover
it and extend down over the edge.

Tim, Ohio
 
I helped my late father-in-law to put a metal roof
over his asphalt roof that had several layers in 1963 and that roof is still on that house. Hal
 
I was told about a month ago not to put metal roofing over old shingles , that it would cause the shingles to crystallize due to the intense heat. Is that true or have you ever heard someone say that?
 
A couple of years ago I watched the wind peel off metal roofing that had been nailed through the shingles and into the sheeting. It was a wicked sight to see those sheets curl up and fly away like lethal weapons. Now, I know they were nailed and not screwed, so screws might have held, but if the sheeting is beginning to rot the screws won't hold either. If you put down 2x4's first the chances of losing the metal are much less, and the 2X4's do help make a crooked roof look flatter. Removing the old roofing might be necessary if there are three or four old layers and the roof isn't strong enough to hold the extra weight of another layer. The old shingles do act as an insulator that will keep the new steel from dripping condensation if the sheeting is spaced apart for wooden shingles.Jim
 
I agree with the rest. 2x4 purlin boards laid flat at 2 ft. O.C. Make sure you lay the sheets in the right direction according to prevailing winds. You don't want the edges of the overlap facing into the wind. Oh, and drive the screws on the ridges of the metal, not in the valleys.
 

You will be fine with the over lay of the sheet metal. I would screw it down in lieu of the nails. I also would leave the rolled roofing on.
 
Are you planning on using 2X4 purlins?
If so, leave the old roofing. Screw the purlins into the rafters, then install your tin.
If you're not going to use 2X4's you should tear off the roll roofing so you can inspect the sheathing. Replace any that's questionable. You have to use bigger screws when fastening tin to the sheathing. Something like #12 or #14 screws VS #9 for fastening to purlins.

Only nail if you're using 2X4's. Nails in the ribs.
Screws in the flats.

Lap so the prevailing wind won't peel the laps. Use the specified caulk in the laps.

Another thing to consider when deciding whether or not to tear off the roll roofing is weight. Will your roof handle the extra weight of the purlins and tin?
 
I believe the manufacturers recommend 5/8" plywood not OSB and there is a special screw for putting the metal on short and fat not long and thin
 
I have built a few new buildings here in the last few years, all have had metal roofing screwed into 1x rough cut furring strips from the sawmill. I have never had any trouble with them coming loose, but 1" of lumber is way more to bite than 1/2" of plywood.
Zach
 

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