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INSTALLING METAL ROOF OVER SHINGLES
In order to install a metal roof over an existing shingled roof, is it best to have the roof "stripped" with strips of wood, or just place metal right on top of shingles? Also, for a home, is 26 guage metal okay?

You should always put a new underlayment over the old shingles. Whether boards are put down or you go direct over the old shingles is very much a function of the metal roofing product you choose. You will need to confirm with the roofing manufacturer how their product has been tested and how it is to be installed.

Metal gauge, again, is dictated by product design. Check into what testing your metal roofing has passed.
Can you apply A 26 gage metal roof directly over exzistig shingles
In many cases, yes. It depends largely upon the design of the metal panel. Check with the manufacturer for a final answer.
can i install metal over asphalt shingles
This will be determined by the condition of the shingles and the type of metal roofing being installed. The manufacturer of the roofing you choose should be able to help with this question. Generally, the more heavily formed metal roofing profiles are better suited for this.
Some of the less textured metal roofs in particular might show unevenness if installed over old shingles. Also, in any event, you need to take steps to ensure secure fastening through the old shingles.
Is ther a chance of the asphalt singles melting under the new metal roofing?
I have never run into anything even remotely like that.
I have heard that if you don't provide an air space between the metal and shingles, mold/mildew could occur in that space because of condensation. Comments?
I have never seen this occur in 20+ years in the industry.

That said, though, I am a firm believer in new underlayment beneath every new metal roof and I am also a firm believer in good attic ventilation.
I was called to inspect problems a homeowner had after having this roofing installed on a 35 yr. old FmHA house.

New panels were screwed directly to existing single layer of asphalt shingles on approx. 4 in 12 pitch hip roof with one hipped wing turned out to cover a front porch.

The roof looked OK from the outside, except for some swag on one of the hip ridges that probably pre-existed the metal.

Owner showed me photos of water streaming down the inside surface of one whole exterior wall. In addition, the inside surfaces of the plywood roof decking had become black, apparently from mold, though they felt dry to the touch.

A manufacturer's site (Fabral) I checked recommended furring strips over existing shingle roofs, but only as a "best case" scenario.

Contractor did eliminate two turbine vents from the house without replacing them, and soffit vents were in place but occluded by deep dish fiberglass batts.

The loss of vent looks to me like a reason for blackening of decking, but as for water streaming down outside wall I am at a loss.

I suspect that blown rain may be finding a way below tops of panels at hip ridges, and don't know the condition of the covered shingles.

Have any of you seen problems associated with this kind of roofing?
Bad things can happen when good products are not installed properly.

Obviously, there is an issue with ventilation and moisture control.

Next, there is a problem in that no underlayment was installed over the old shingles.

Finally, I do suspect there may be some flashing problems allowing water intrusion as well.

Feel free to email me pictures (close up) of the flashings, etc. My direct email address is below. I really suspect this whole roof will have to be removed and re-installed.

tmiller@classicroof.com
Hello, Mr. Miller:
I'm thinking of having a metal roof installed over my office of ~1600 sqft. The existing hip roof is not steep and not extremely flat and is in need of repair. The contractor I contacted told me he'd put the 1x4 wood strips and install the metal over that. I see you recommend an "underlayment". What is that and what material is it made of? I have a ridge vent in the old roof and have extreme heat problems in the summer. Will a metal roof installed this way improve the situation?
Thanx.
Jim
Jim,

Thanks for your question.

I am assuming that the existing roof and decking will be left in place. In that case, I would cover it with underlayment and then vertical battens followed by horizontal battens. This is provided that you choose a metal roof which can be installed over battens.

The reason for vertical battens first is that this will create an air chamber which can be vented from the bottom to the ridge. You will want to work in an eave detail which allows for venting at the bottom.

This vented air chamber will help keep your office cooler and it will also help to avoid any accumulation of unwanted moisture.

As far as underlayment ... 30 pound roofing felt is normal but there are also many good synthetic underlayments available today. A quality roofing contractor will know what these are.

A couple of other things -- it sounds like your roof is low pitch. Make sure that the metal product being installed is approved for use on roofs that are the same pitch as yours. You may wish to explore the full world of metal roofing products before making a final decision. Keep in mind the many differences in product appearance, metal, finish, warranty, and installation details.

Always choose an experienced and qualified installer with good references.

Note to other readers: My response is particular and specific to Jim's situation. Please do not assume that what I have described here is the way to go in all situations. I have had readers before sense that I was inconsistent in my answers. I try my best to be consistent but every situation is different and may merit a different response.

Todd
Mr. Miller:

I have a roofing problem similar to one described by Jim (message from 04/09). Briefly I want to install metal shingle roof over two layers of old aspalt shingles on my cottage. Roof slope is 3:12 and there is no attic. Would it make sense to add foam insulation between old roof and new metal shingles to keep house cooler in a summer. Also would it reduce room between old roof and metal shingles and affect roof ventilation?

Thank you,

Vadim
Most, but not all, metal shingle style products require solid decking. I understand your need for insulation but I would not choose insulation over ventilation.

If the structure can handle the weight, you could do something like build up with foam and an airspace and then new decking on top of that. Vent the airspace.

You may wish to get a structural engineer involved to do some analysis.
Im installing a 26 ga metal roof on an exsting asphalt roof . I put 30 felt down and after installing 5 panels there is uneveness is this typical or should the aspalt be removed for a better fit ?
This is pretty much impossible to answer "long distance" but, yes, going over the old shingles is likely a contributing factor.
can i put a metel roof over a roof that has a layer of ceder shakes and two layers of asphalt shingles is there a load restriction
First of all, building codes in effect in most areas restrict to two layers maximum.

If you are in an area where there is no building code in effect, you might be able to go over this. I would consider having a structural engineer review the home first and do some load studies. It all depends on the construction and condition of your home and roof.
Hi Todd - I just posted our situation on another string, but seems you are active so I'll raise it here too - as it is consistent.
We bought a log cabin that had apparent water stains on the ceiling inside. The seller told us, when we inquired on the date of closing, that this all related to the former roof which had been a problem. He advised that the bank had required them to install a new roof (a year or so earlier) and they put on a metal roof (not standing seam). The water instrusion continued. We have had water coming in through where the stove pipe exists through the roof (a round pipe but surrounded by a squared-box and no flashing we have just learned); and where the main roof of greater slope joins with a more gradual sloping part of the roof that goes over the front porch; on the porch interior and we have observed water running off the roof from under the metal roof. Three contractors have recently told me that this building should never have been fitted with a metal roof because 1) metal roofs were not designed for residential use (colling and heating), 2) it is difficult to properly flash around skylights such as we have, and in general, 3) the roof slope was too shallow to avoid ice freezing and melting and seeping through at the referenced joint/channel . . . etc. Each roffer said that asphalt shingles would have been the appropriate application/material for this situation. Can you explain these comments? Also, where, exactly, might I go to see the applicable codes that should have governed this construction (in NY state)? Appreciate ANY help/advice you can give! I will try to send you an email with a photo or two of the roof I am referring to (hope you don't mind me doing that!) THANK YOU - Jen
Thanks Jen. Looks like we're in contact by direct email now, too, so I will respond that way instead of here
I live near the ocean and am using Fabral aluminum panels over asphalt shingles. I would prefer not to use 1X4 furring strips and go right over the shingles. What do you think? Also can you walk on this type of roof without denting it, after it is on?
You really need to check with Fabral but I believe that their preference is for that product to be installed over solid deck, meaning I think they'd prefer you put underlayment over the old shingles and then go over that. You really need to check with them though. It should be walkable though.
Hi Todd, Please settle this debate between my husband and myself. We are going to install a new metal roof. I say we need to remove the exciting shingles, my husband wants to leave them on. It is only one layer but the shingles are so bad that if you touch some of them they fall off. You could take a broom and sweep some of them off because they are so rotten and loose. Under the shingles our roof is made out of cedar decking not plywood. He is planning on putting down 1x4 boards or 50 pound roof felt. I don't think it is a big deal to remove the shingles since they are in such bad shape. I am worried since some are loose what will happen once the metal roof is installed. He told me to write to you, so your answer will settle this. Do we remove the exciting roof or not? Also no one around here has put any roof vents on with the metal roof. Why? Does roof vents leak with metal roofs? Thanks Jill
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