Metal roofing under

[Bedlam]

Hi , I would like to get some peoples opinions either from first hand knowledge or instructions on the idea of a metal roof over an asphalt roof.
Here is the situation, a garage roof in the northern states (so sees snow) .Presently it has an asphalt roof thats about 8 years old. No leaks but Id like to apply a metal roof over it to have the snow slide off and not require the snow shoveling.
Id prefer to remove the shingles to reduce the weight on the roof, and use some strapping so make level.
so the question is ,
Is there an advantage to leaving the shingles on or removing them? It had a OSB sub straight under the shingles . Thanks for any opinions

 

[oliver28472]

The usual way around here in NC is to leave the shingles on, nail down some 1x3 or 1x4's and screw the tin to that.

 

[cowboy357]

I checked into it for Our place . & was told there could be an insurance issue leaving the asphault shingles on in case of even a small fire .

The Shingles could continue to burn under the tin & not allow the fire department to have access to put it out. So I decided against it , You might check Your Nearest fire department & insurance agent . Bob

 

[orangebluegreen]

I recently did this house and that of a friends roofing over shingles. This link from FABRAL has some good info.
http://www.fabral.com/technical/Reroof.pdf

Technical Information: Metal Wall and Roof Systems – Fabral

I used 2x4's spaced by the precut Styrofoam for radiant heat rejection (Lowes) that gave me a 17" screw spacing and the screws I used #10 x 1.5" ZAC gave me a wind uplift rating approximately 190+ or - MPH (depending on which engineering study you believe). I used 2x4 purlins to help strengthen the 2' gable end overhang which originally was not built properly and was sagging. Rather than tear the shingles, plywood, and gables off, I chose to use the 2x4's over the plywood and rafters, replacing just some wood at the corners that needed fixing. This also saves in having to rent a dumpster and not needing new tar paper. Because I had 1 layer of shingles and 1/2" plywood, in order to get enough fastener pull out depth on the 2x4's required 4" deck screws at a minimum. Hand nailing with bigger nails than available for an air gun would probably work ok as well in areas not as critical for uplift, but in my location uplift is of prime concern for hurricanes. I used a combination of air nailing and 4" screws for the purlin to truss, and 2-2.5" screws into the plywood between each truss. The plywood was sagging from age (not rot) in between many of the trusses and the 2x4's flattened the roof out nicely. Also got rid of a flat roof that was on the back porch which improved the appearance considerably.

My friend used 1x4's and that allowed for air nailing to be more effective, but we also did extensive screwing with deck screws just to be safe. The 3/4 thickness of the Styrofoam and a 3/4 purlin allows for a somewhat quieter roof during rain due to the metal being in contact with the foam. Using the 2x4 with 3/4 foam allows the metal to make a bit more noise. Finding 1.5" foam is pretty hard, and doubling the 3/4 foam is cost prohibitive.

 

[teststrips]

Here in PA, I've helped put metal roofing over several existing roofs... when the underlayment of the roof was in pretty good shape (with no dips, etc) we just put the tin directly over the shingles - no spacers or anything. If the roof was in bad shape, we added 2x4x16 board horizontally (going with the roof ridge), nailed them into the main beams/trusses as best we could + attached the tin to the 2x4x16 boards.

Both methods seem to work very well - the oldest job is about 10 years old + was one we just screwed right on top of the existing shingles (my in-laws house) - they've had absolutely no problem with leaks, show weight damage, or anything like that.

 

[Bedlam]

Thanks for the replies all good ideas. Keep them coming .LOL
I hadn't given much thought to if there was a fire under the metal this is more of a concern to me now come to think of it as I am I guess you'd say 'self insured".
I have done jobs where the metal was put directly over the shingles. I always thought that is wasn't a bad idea if they were in good shape as it would add a slight second barrier.It is in relative smooth even shape. I guess the only way to tell if it would lay flat would be to try a few panels on it.Perhaps ill run a string across it tomorrow and See just how flat it is.
Again thanks and everyone please feel Free to continue to post.

 

[20 20]

If the old roof is in fairly good shape just go over the top, plus it will help with insulation.

 

[PILOON]

I have needed to pull metal roofing during renovation projects.

Whenever the metal is in direct contact with sheething the sweating from condensation causes the underside to rust out rapidly as there is no air circulation to dry up the underside. Worst still, the metal basically sits on a wet surface.

Metal roofing must have the underside exposed to air circulation!

Now I'm talking of 10 years for serious rust to show! (and metal roofing is generally quoted as a 40 year product)

Just a short while ago I questioned why my buddy wanted to change his 15 year old metal roof only to find that it was really rotted (rusted) to the point that it leaked in many places. In fact we had to repair his plaster ceiling in 4 places.

 

[Bedlam]

Very good point on the ventilation. I think I had thought of the condensation but not how the moisture would be trapped under the metal and cause it to rot prematurely.

 

[orangebluegreen]

While possible to lay the metal directly on the shingles, I wouldn't do it because even if they're in good shape you'll probably get "waves" or "oil canning" in the metal from the difference of thickness in the layers of shingles. They recommend putting very heavy felt paper or titanium paper over shingles if you're going to do that, which probably cost as much as just putting down new purlins over top of the shingles to begin with. The metal doesn't breath like shingles (so they claim) so you have to have vapor and thermal barrier of some type. The layer of shingles would be your vapor barrier and "might" work as a thermal barrier, but silver bubble foil or the Styrofoam works. My understanding is the thermal barrier helps keep the metal from condensating on the bottom side due to too large a difference in temp between to top and bottom surfaces. Also should have ridge vent to help get any moisture out. I used Profile-Vent but it is very expensive at $2.40 per lineal foot, you can get by with products such as Cobra-Vent from the big stores which is the same material but not form fitted, it's really intended for shingles but it will work for metal.

Btw, DO NOT place Galvalume directly in contact with ACQ pressure treated lumber (the darker colored PT), it will corrode in months. Galvanized (not Galvalume) does better with ACQ though but doesn't weather as long in service compared to Galvalume. I have had Galvalume siding in contact with ACQ that I removed after 1-2 months to pour concrete and found total corrosion at the contact points. I have used MCQ pressure treated (light natural color and sold at Home Depot in my area) and had no problems with contact.

Ventco - The Ridge Vent Cut To Fit

Roofing : Insulation4less.com