Laying Metal Roof Over OSB?

[blueridge]

I Have A 26 Ft. Long Carport X About 15 Ft. With Not A Lot Of Slope, And Have Asphalt Shingles On It Now, They Do Not Last Long, Maybe 7 or 8 Years Because Of Not Enough Slope, Was Wanting To Put Metal On It. Can I Tear Off Old Shingles And Lay Tin On OSB Or Tar Paper Or Do I Need To Put Down Strips For An Air Space, Which Way Will Last Better???? Thanks Gene

 

[Richard G.]

I Have A 26 Ft. Long Carport X About 15 Ft. With Not A Lot Of Slope, And Have Asphalt Shingles On It Now, They Do Not Last Long, Maybe 7 or 8 Years Because Of Not Enough Slope, Was Wanting To Put Metal On It. Can I Tear Off Old Shingles And Lay Tin On OSB Or Tar Paper Or Do I Need To Put Down Strips For An Air Space, Which Way Will Last Better???? Thanks Gene

Just had a guest house reroofed back in Feb.
Roofer took the shingles off and put down a blue paper over the wood and screwed the metal on.
Put the screws in the ridge like it is supposed to be done also.

 

[MJMJ]

For a carport, lay felt over the shingles and put the metal on. Doesn’t need to be complicated.

 

[Dusty MI]

Just mumbling. I have some lead-head nails, that were used for steel roofs before O-ring screws that are used today.

Dusty

 

[E.B. Haymakin']

I Have A 26 Ft. Long Carport X About 15 Ft. With Not A Lot Of Slope, And Have Asphalt Shingles On It Now, They Do Not Last Long, Maybe 7 or 8 Years Because Of Not Enough Slope, Was Wanting To Put Metal On It. Can I Tear Off Old Shingles And Lay Tin On OSB Or Tar Paper Or Do I Need To Put Down Strips For An Air Space, Which Way Will Last Better???? Thanks Gene

Depends on how well framed the structure is and if there is deterioration of the structural components that support it. I would strip the shingles off as they are very heavy. My carport is 24x15 light slope. It was originally shingles over tar paper plywood deck. Took it all off down to rafters put down 1x4s metal to them. Been 10 years now don't regret it. Less weight carried by structure

 

[bmack95hd]

Neighbor put metal over old shingles. Now he has a lumpy wavy roof. I wouldn’t do it.

 

[john in la]

Follow the metal manufactures directions or take a chance of voiding your warranty.

But for a low slope roof I would leave the shingles on the roof.
The shingles will protect the OSB as the metal sweats.
Lay 2x4 over the shingles to take the wave out of the roof and screw down the metal per manufactures directions.

 

[leroy in ne]

Follow the metal manufactures directions or take a chance of voiding your warranty.

But for a low slope roof I would leave the shingles on the roof.
The shingles will protect the OSB as the metal sweats.
Lay 2x4 over the shingles to take the wave out of the roof and screw down the metal per manufactures directions.

That's what my contractor did on my house. Shingles and spacing also cuts down on rain noise.

 

[MJMJ]

Neighbor put metal over old shingles. Now he has a lumpy wavy roof. I wouldn’t do it.

Then he had a lumpy wavy roof before. It has nothing to do with the metal, just made it noticable.

 

[Sean Feeney]

I Have A 26 Ft. Long Carport X About 15 Ft. With Not A Lot Of Slope, And Have Asphalt Shingles On It Now, They Do Not Last Long, Maybe 7 or 8 Years Because Of Not Enough Slope, Was Wanting To Put Metal On It. Can I Tear Off Old Shingles And Lay Tin On OSB Or Tar Paper Or Do I Need To Put Down Strips For An Air Space, Which Way Will Last Better???? Thanks Gene

https://www.amazon.com/Firestone-RubberGardTM-EPDM-Roofing-Rubber/dp/B00KLFWEW6?th=1

Go with rubber

 

[DanielW]

Proper practice (at least when I was doing roofing quotes and mat'l take-offs when I worked for a building supply while in school) was always to strap it so the metal can breathe underneath. One reason is the risk of moisture wicking its way to the underside of the steel, sitting there without being able to vent/evaporate, and rusting it. Another reason is to allow the roof breathing room beneath to keep it more uniformly cool, so you don't get as significant heat/expansion/contraction cycles of the steel. Almost all steel roof failures are not because the steel has rusted nor torn off, but because the expansion/contraction cycles make the fastener holes oblong over time and can work the washers out from under the screw heads (especially if lousy screws are used).

That being said, lots of folks have (and still are) screwing steel straight down to the sheathing or onto shingles, and many of these roofs are doing just fine. Personally, for a house I'd always strip the shingles and strap with 1X4, but for something as simple as a carport, I wouldn't get too fussed about specifics - Good steel will probably outlive you, regardless of how you put it down. I'd still strap it myself, but if there's just one layer of shingles on there now I wouldn't bother stripping them off. And even if you didn't strap it, because it's unheated underneath it's probably going to be fine.

Just be sure to get decent screws with the domed steel cap washers that capture the rubber sealing washer. There are a lot of lousy roofing screws out there that leave the rubber washers too exposed around the edges: The rubber can walk out from under the heads during expansion/contraction cycles of the roof, and they also leave the rubber exposed to sunlight so it degrades and deteriorates over time. Steel roof installed properly with good screws shouldn't have to be touched again during your lifetime, but you hear all sorts of stories of leaks forming around the screws when the washers fail. Contrary to many opinions, thicker steel is actually sometimes worse for this: the smaller sub-ribs (between the main ribs) that are supposed to take up most of the expansion/contraction don't flex as easily with thicker steel, and thus it ends up expanding/contracting over the main ribs and wearing the holes oblong.

There's always some debate of screwing on ribs vs. screwing on flats. Back in the day, proper practice was always to screw roofing on the ribs and siding on the flats between ribs. But many of the largest steel roof manufacturers now recommend screwing roofing on the flats right next to the ribs, saying it lasts much longer and doesn't have nearly as much risk of leaking around the screws after a few years. I still screw on the ribs because I'm old fashioned, but that's no longer supposed to be proper practice.

 

[TerryfromSWIowa]

Just had a guest house reroofed back in Feb.
Roofer took the shingles off and put down a blue paper over the wood and screwed the metal on.
Put the screws in the ridge like it is supposed to be done also.

Metal manufacturers say to fasten on the flats and to use a drill set to just compress the rubber. An impact driver will over tighten and squeeze out rubber gasket.

 

[bmack95hd]

Then he had a lumpy wavy roof before. It has nothing to do with the metal, just made it noticable.

I believe it was from curled shingles? Removing them would have given him the opportunity to get a better look at the OSB and given him a better finished result.

 

[dhermesc]

Just mumbling. I have some lead-head nails, that were used for steel roofs before O-ring screws that are used today.

Dusty

My dad used those in the 1950s to build his machine shed. They were good for 20-30 years then the leaks started and never stopped. We ended up pulling the roof completely off and screwing down a new one. Turned out instead of tin - the roof was actually aluminum that looked like corrugated tin. The trusses and nailers were still good even after getting wet on and off for years because they were old growth yellow pine and as hard as iron. Good thing se planned on using screws because driving a nail in to those things was chore.

 

[dhermesc]

I had a wooden carport that covered the entry to my garage. It was only 12- 13 foot wide and 16 foot long with a single shallow slope. The previous owner had used the wide asphalt roles that are about 3' wide to cover it. I was going to replace with tin until I talked to a neighbor - the wide shingles were in poor shape but he assured me they had been on that roof for at least 25 years. I peeled it off (pretty easy being that wide) and replaced with similar covering. I drove by there a while back and it still looked pretty good 25+ years later.


https://www.lowes.com/pd/GAF-Liberty-3-28-ft-W-x-34-ft-L-100-sq-ft-Black-Roll-Roofing/999976746

 

[Eman85]

My shop building and my house has metal over the shingles, house roof is been done for almost 20 years. Don't use felt under the metal, they make a membrane designed for it which my shop has. On the house we used foil/foam insulation under the metal. Both are screwed in the ribs which isn't always agreed on by many but it works. If screwed to the ribs it doesn't give the roof as much flexibility for expansion and contraction which the roof does expand and contract a lot. But screwed in the ribs keeps the screws out of the water flow and lessens the chance for leaks. I did have one porch roof screwed in the flats and did have a leak. The installer didn't pre-drill and the metal curl from the screw piercing the metal cut the rubber washer as it tightened down. Pre-drill the holes, it works better.

 

[redtom]

No mention of standing seam steel. That's the only thing I would use on any structure. Not much more and none of the screw head/washer issues. After all, would you put shingles on if the nails were exposed?

 

[Blackhole49]

For a carport, lay felt over the shingles and put the metal on. Doesn’t need to be complicated.

I've put metal roofs

over shingles 8 years ago and have been very happy. Back then they made a special fabric you stapled to the shingles to prevent abrasion, then screwed the metal on.

 

[Charles in Aus.]

No mention of standing seam steel. That's the only thing I would use on any structure. Not much more and none of the screw head/washer issues. After all, would you put shingles on if the nails were exposed?

Standing seam or Clip lock decking as we call it is certainly the best steel roofing material to use on low pitch roofs . Cyclone rated and comes in a huge range of colours .