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TOPIC: Roofing over existing roof

adding a metal roof over existing asphalt

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We have a 4 year old cabin in northern MI with asphalt shingles - a 12' wide unheated porch has created ice dams and leakage; we're also getting some condensation over the cathederal ceiling of the cabin. Can we solve both problems by installing a metal roof on top of OSB set on 2x4 'rafters' on top of the existing roof? The 2x4s would provide some air flow from soffit to a vent at the peak. Any other suggestions?
Jim Pifer

2/9/2004
As long as you can get good airflow in this manner, yes, I think you would improve the ice damming considerably. Keep in mind, though, that mother nature is unpredictable and oncontrollable. I would never wnat to suggest that anything might completely end damming problems forever. Weird weather situations can make ice damming unavoidable. However, getting good airflow will be a huge step in the right direction I feel. Regarding the condensation, this might help. Ideally, I'd like to see a vapor barrier behind the ceiling. Ultimately, if you have a lot of warm moist air in the house and if we can keep this new vented airspace the same temperature as outside, you might still end up with some condensation behind the current asphalt shingles. Removing the existing shingles would probably help to make sure that doesn't happen. Perhaps some of my colleagues will weigh in with an opinion on this as well. Al Reid knows a LOT about this stuff and can certainly give you some great direction as well.
Guest User

2/9/2004
It would be difficult to get a vapor barrier above the ceiling at this point - would it help reduce the possibility of condensation if we drilled holes in the existing roof (instead of removing the asphalt shingles)? Jim
Guest User

2/11/2004
I think you need to bring this up with the manufacturer of the roofing being installed. Generally, I am not sure I'd do this unless you set up a gridwork of first vertical and then horizontal battens on top of the current roof, allowing you to also install a quality underlayment on top of the existing shingles. My concern, frankly, would be that, even with a vented airspace, you might still have condensation from time to time on the back of the metal which might drip off and run into these holes.
Guest User

2/13/2004
can i install metal roofing directly over asphalt shingles.
Guest User

2/14/2004
This varies somewhat from product to product but, yes, many metal roof systems can be installed over existing shingles. This is a huge benefit not only in terms of cost and mess savings but in terms of environmental impact. Check with the manufacturer of your metal roofing, as well as the local contractor and local building inspector to determine whether your roof is a candidate for re-roofing without removing the old shingles. Todd Miller
Todd Miller
Isaiah Industries, Inc.

2/15/2004
I am going to be installing a metal standing seam roof on my home. It has a gambrel roof with poor ventillation and frost in the attic. I would like to install the new metal roof over the existing shingles. Could you please provide some suggestions on how to obtain proper ventillation? Is it feasible to put metal roof over existing asphalt roof?
Guest User

3/9/2004
I have to assume that you have living areas in the gambrel and that part is attic and part is not. Part of your problem may be heat loss from inadequate insulation and improper or non existant interior air barrier. Without knowing the extent of this and your area, applying a metal roof may only treat the symptoms and not cure the disease. Having said that the new building code in the US is now dealing with these issues and requiring ventilation. They require a minimum of 1" of vented air space over vaulted ceilings. This is adequate if you have a moderate heat loss and ice problem however more is always better. I would then suggest strapping the roof vertically with 1 1/2" material over the framing members. Then I would choose a SS System that can install over strapping. I would install a reinforced underlament ove the vertical strapping , then strap horizontal and install the roof. Ventilate the eaves well as intake and then the ridge. If you have an attic in the uper gambrel portion, cut the ridge open to ventilate it up through your new vented ridge. If you need more contact me directly at [email protected]
Allan Reid
Dura-Loc Roofing Systems, Inc.

3/10/2004
I am having a metal roof installed over one layer of standerd roof shingles. Installer is placing 2x4s over the old roof, then installing metal. THERE SEEMS TO BE a new difference of opion / should the metal roof be installed directly to old roof or use the 2x4s - several people have told me they do not use the 2x4s any more. HELP
Guest User

4/12/2004
If the roofing panels being installed are approved by the manufacturer for this type of application, then it is fine. And, in fact, if you put down vertical battens first followed by the horizontal ones, you can ventilate this area and further enhance the energy efficiency of your new metal roof.
Guest User

4/12/2004
Would the battens have to be 2x4s' or could they be 1x4s'? How would that increase the energy efficiency of the roof? I just found this site and I am planning on putting a metal roof over my 18 yr. old shingles this spring. I built the house but I have never dealt with metal before. Is it hard to go around chimneys? Do you know where I could get installation tips? Thanks for the help
Guest User

4/13/2004
The extra airspace can help to eliminate heat transfer by conduction down through the roof system. However, realistically, if your attic space beneath this is vented, the effect of adding this venting may not be all that great. If you do decide to add this, just 1 x 4's, preferably fastened through the roof to the rafters below, would be fine. Chimney flashing must be paid attention to and the exact method depends upon exactly what profile of roofing is being installed. Generally, I prefer to see the flashing be cut into the brick of the chimney. If the metal roofing manufacturer cannot provide complete installation instructions, including chimney flashing, find another manufacturer.
Guest User

4/13/2004
Is it alright to put standing seam metal shingles over
Guest User

5/7/2004
Is it alright to put metal roofing over two layers of asphault shingles? It has been about 26 years since the last layer was put on. We have talked to two contractors, one says yes, other says no. Which is it? Thanks
Guest User

5/7/2004
Several things must be looked at before proceeding. 1) Any signs of an existing weightload problem or other structural and decking issues? If so, those must be addressed. 2) Is there a local building code which cites a "two-layer maximum" rule when it comes to roofing? 3) Is the product you're choosing deemed appropriate by its manufacturer for installation over existing shingles? Generally, the design and low weight of metal roofing helps facilitate installation over one or two layers of old shingles. Todd Miller
Todd Miller
Isaiah Industries, Inc.

5/7/2004
What is advisability of covering a flat, asphalt coated roof with a new hip roof of raised rib steel? Building is very close to seacoast - just a few yards away. Of special concern are the rain canopies attached to the landward side of the building. These are sloped (45 degrees, est.). They provide a kind of rain shield for persons entering the building, but are not part of the roof that is one story above them. They jut out below second story windows. They are covered with asphalt shingle roofing. Do you forsee any problems, or special installation requirements?
Guest User

5/11/2004
You will need an experienced metal roofing contractor and, in all likelihood, also an engineer or architect to help ensure that this is done in a way which can be supported by the existing structure. Overall, though, this type of thing is done on a regular basis with excellent results. One of our members, ATAS International, has a system for this type of re-roof project. Todd Miller
Todd Miller
Isaiah Industries, Inc.

5/11/2004
Live in Northern Alabama. Roof slope is 1:4. A Contractor has installed a vert. panel metal roof(similar to MasterRib) directly (no 30#felt) over my existing asphalt shingle roof that varies in age from 2 to 18 yrs old. Old roof is 1/2 plywood with felt and single layer of shingles. Problem 1 - He used 1" screws (in flats) minus approx. 1/8" for washers (rubber & metal) the length is approx. 7/8". From the attic in most cases the screw did not penetrate the full thickness of the 1/2 plywood. Is this a concern? Problem 2 - His helpers told me due to the shorter than usual screws (they used 1 1/2" screws in the past) they increased the torque on their screw drivers to help penetrate the 1/2" plywood. This in my opinion caused the old shingles to be compressed and cause a dent/dimple in the metal at most of the screws. Needless to say the roof looks like crap. Problem 3 - To reduce the dents/dimples the contractor had his helper back off a little on each of the screws. This helped the appearence a little, but what does that do for the integrity of the screwed connection as far as backing out or leaking? Your comment/opinions would be helpful. The contractor & I are discussing the above problems I have. Roger
Roger Ferguson

5/28/2004
Sorry to hear about your concerns. Normally a screw needs to penetrate the wood decking by 1/2" in order to meet the pull out strengths. One wants to be careful not to use too long a screw as in colder climates, the screw could draw the cold in quicker into the attic and condensate however that does not appear to be an issue here. Screws for through fastened panels have a rubber washer with a metal cap. Care should be taken to not overtighten the screw as the rubber washer could be distorted out of place and allow the water to enter. Metal sheets do expand and contract slightly with the hot and cold temperature changes and the the rubber is designed to compensate. Overtightening could cause problems if not in the short term, then the long term. A cure for this would be to replace the screws with proper length screws however it will do nothing to improve the appearance of the dimples at this point. Hope this helps.
Allan Reid
Dura-Loc Roofing Systems, Inc.

5/28/2004
Thanks for your quick and helpful response. Should I be concerned that no vapor barrier (i.e., 30# felt) was applied to the roof prior to installing the metal roof. 80% of the roof covers a heated space. Thanks again, Roger
Guest User

5/28/2004