Can I install a metal roof over my now asphalt roof?
What type do you recomend, steel or aluminium
Will I need to instal a vapor barer between the two roofs

 

It "can" be done but it is not recommended. We covered this question a few times, perhaps try a search as I know the answer to your question, in detail, is here. Good luck.

 

Fred:
The answer to your question is YES YOU CAN. It is done everyday. Here is how it is done. You leave your existing asphalt roof there,(make sure you only have one layer of asphalt. If you have 2 or more layers you will have to strip your roof.) Then you cover your roof with fan fold insulation. Then lay 1x4's on top of your fanfold and space them every 2' up the roof. Once all of the roof is covered and all the 1x4's down, you can begin putting down your metal. I would use steel, but that is just me. The gap between the metal and the fanfold will act as a barrier. It will help keep your inside both cooler and warmer depending on the season. This is 2 or 3 person job. All metal must be screwed into the 1x4's which in turn are screwed into the roof with 3" screws.
If not done correctly, leaks could develop. I do about 3 roofs a year like this, and it is getting more popular. Many roofing companies kind of try to stay away from this since it is a pretty new concept. Good Luck

 

Jack does that meet your local building codes? What you described about the installation seems accurate but it's been my understanding you should not put a metal roof on a shingle roof.

 

Other reason for the air space inbetween is to help with condensation. Cold metal and warm attic air can cause condensation, its more of a problem in a cabin or garage that is only heated time to time. I can tell you that our cabin stays about 5-10 degrees cooler in the summer with a metal roof. Handling the metal is best done by 3 people, one on the ground to stand them up and 2 on the roof to grab them and set them in place. Gloves are a must.

Jack: Now for an interesting question, ever get called back on a metal roof because mice or flying squirrels got in the air space? I had one hell of a time with this, i kicked flying squirrels out of the chimney the same time we redid the roof. So they figured out a way under the ridge. They kept eating the foam until i got annoyed and took the ridge off both ends and installed galv wire mesh at both ends.

 

Grumpy & Hellrazor:

Yes, it is within most building codes. In fact, I have not run across one that will not approve it, but that does not mean that there are not those jursidictions who will not approve it. If they do not approve it, we will go to Plan B.

Hellrazor. I have never encountered that problem. We put the seals in both the bottom and top. At the top after we put on the ridge cap, there is no way anything can get it. At least on ours there is not.

Have a good evening gentlemen. See you later.

 

Jack, i put all the seals on the sheeting. The buggers were getting under the ridge cap at the ends. And flying squirrels are persistant little pains in the butt. Once they get in someplace, they will chew back in until you get rid of them or frustrate them enough. I know people who have problems with them chewing through cedar siding and living in the walls.

 

For the ends of the ridge, I take a piece of ridge cap, the flat part, cut it bend it, and rivit it two the ends to make matching caps on the ends. Nothing gets in. The best thing I have found for flying squirrels is a flying shotgun. Have a good day.

 

As a contractor I would never install a new metal roof over and existing asphalt roof. This is a lazy practice designed for a quick sale. For one thing unless you can see all the sheathing from the interior of the building you really do not know what condition it is in. It is far better to remove the existing asphalt shingles to inspect the sheathing and replace sheets if necessary. Also it is better to remove the weight of the asphalt shingles from the roof. On average they weight about 240 lbs per 100 sq. feet. You will get a much better and longer lasting roof this way. And installing fan fold insulation on top of the shingles serves no purpose except costing you more money. There is minimal insulation value to fan fold and you would be better off putting the money into insulation on the interior where it is designed to be. Also make sure you have adequate ventilation both in the soffit and ridge area of your new roof system.

 

As a roofing contractor specializing in metal roofing, I agree with "It wasn't me!" (side note: interesting handle - what's the story behind it?). Steel comes in long panels or shingles. Aluminum comes in panels or shingles as well.

The long panels are either standing seam, or overlapped at the seam and are usually installed over 1x3 or similar strapping. If you are choosing this type of roof, go one further step and lay 2x4 vertically to create 3.5" vertical air channels, then the 1x3 horizontal strapping that will support the panels. This method will create the soffit to ridge ventilation. Without this extra step, you do have air cells below the metal, but being horizontal they don't allow vertical air convection.

If you choose shingles, then there are again two methods: over strapping, or on solid sheathing. Each has its own installation technique.

We always strip the roof, for the same reason "It wan't me" said - you can't tell whether the decking is in good shape or not until you strip, and by stripping, you get rid of a lot of weight.

The vapour barrier belongs on the warm side of the insulation which should be between your living space and the attic. If the vapour barrier is anywhere else, it will just trap moisture and contribute to problems.

The choice between aluminum and steel is like discussing religion. There are benefits to each metal, and modern manufacturing methods make both very long-lasting products. The key in metal roofing installation is the quality of workmanship, since 80% of the final quality comes from the installer's skill and knowledge of the craft. Find the best installer of the product you're interested in, check their references, and go with their suggestions.

 

I also agree with you pgriz
I also considered the fan fold acting as a vapor retarder and trapping moisture . Fan fold insulation is not intended for this purpose and will only add to the already poor roof design.

 

Ok, I'm wimping out on this one. I'm getting bids to have a contractor install a metal roof. All the contractors here want to overlay the metal roof on my cedar shake roof (in pretty bad shape, we have had metal shingles insert to stretch it for a couple years).

Does this cause the same problem as the asphalt shingling? Or does cedar shake react differently?

Thanks.

 

Although some contractors do go over an old cedar shingle roof, I would not recommend your doing business with them. Especially if your roof was "stretched", it's full of rot, fungus and mold. Get rid of it, down to the decking, and have them check the decking. Secondly, if you're considering vertical panels, make sure they quote you proper ventilation (ie, provision for air channels that get air in from the soffits and out at the top through ridge vents or equivalent). Too many installers just put the horizontal battens, and claim that the airspace below the panels is ventilation. It isn't. Note the method I gave you in the last post for ventilation. If the contractors are not quoting something like this, then as far as I'm concerned they are not legitimate contractors.

Please spend as much time checking the contractors' credentials as you have done doing research on this project. Remember, 80% of the quality of your roof is determined by your installer, so choose your installer VERY carefully.

 

Pgriz. Thanks for the reply.

This overlay just didn't pass the smell test. My big worry wasn't mold and rot (although those may be a concern) but moisture. Where is going to go?

The sad thing is, I will probably start getting bids for new cedar shake or composition because every single steel roofing contractor in this area wants to do an overlay.

Too bad you are not in my area.

 

Rusty, I may not be in your area, but I am in contact with a network of good roofers, and maybe one of them is in your area. If you wish to pursue this, drop me a private mail, or e-mail me at the address shown on my profile. Regards, Paul.