Roofing cottage with metal no DIY task

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This article was published 23/12/2007 (6513 days ago), so information in it may no longer be current.

QUESTION — We have a cottage at Delta Beach, on the south end of Lake Manitoba, 30 kilometres north of Portage la Prairie. The cottage is approximately 80 years old. It has a central portion that is about 20 feet square that has a high-pitched roof with a three-foot horizontal square portion right at the top. There is a lean-to portion added onto the front and a lean-to portion on the back. Both lean-tos have rolled roofing on them and the high-pitched central portion has interlocking shingles. These shingles and rolled roofing are 20 years old. The front or north side has moss growing on it.

We are considering installing metal roofing on the entire cottage and wonder if it is OK to put the metal roofing right over top of the existing shingles and rolled roofing, which would ultimately cover up the mossy patches on the roof. We are also thinking of installing Styrofoam insulation on the exterior surface of the lean-to portions prior to installing the metal roofing.

Would the fact that we are just covering up the moss ever pose a health hazard or any other type of problem? Would the additional weight of the metal roof pose a structural problem under normal conditions?

— Adam Brown, e-mail

ANSWER — There are several factors to consider before embarking on your proposed upgrades to the roofing on your cottage. While most corrugated metal roofing is very high quality, it may not be appropriate for many styles of roofing. Your roof is a complex one, with at least four distinct areas with different pitches. These different areas may have to be reroofed using different materials and techniques.

Covering over old roofing with new is sometimes acceptable, but generally considered poor practice. In your case, with the moss-covered sections, it would be considered very poor practice. You are correct that this moss will retain moisture and may help promote rot in the roof sheathing if there are worn-out roofing sections under the moss. With 20-year-old roofing, especially low-quality rolled roofing, this is very likely. Removal of all the old material prior to reroofing is a good idea, for several reasons.

Firstly, removal of the old roofing shingles and rolled composition material will allow proper inspection of the roof sheathing below. Especially on the lean-to or shed-style roofs, damaged boards on an 80-year-old building is very probable. Without removing all of the old roof covering, it will be very difficult to inspect and replace the rotten boards, which may be a simple job once exposed. The moss will absorb and retain moisture, preventing proper drying of the old roof boards after they get wet from rain and snow. Also, your plan to install rigid foam insulation will not work over the old roofing and we will deal with that issue shortly.

The next thing to address is the different styles, pitches and the transition areas on your cottage roof. The upper flat section will require special attention, unless you are planning on extending the framing on the lower sloped sections to cover this small area. If that is your plan, which will eliminate the flat portion, then metal roofing may be appropriately installed. Otherwise, good luck sealing the area between the flat roof and the top of the sloped metal roofing. Without very elaborate flashing installation, that area is bound to leak, even if you install a proper membrane on the upper flat section. The transition areas between the steep-pitched main roof and the lower shed roofs may also present a challenge. Upper steep roofs that drain onto lower-pitched sections have a tendency to collect excess snow due to the “bowl” effect of the transition area between the two. This, combined with the difficulty of sealing between vertical sections of metal roofing, may lead to ongoing leakage problems in the years to come.

The next item to address is the installation of rigid foam sheathing on the shed roofs. This type of insulation may be a good idea if the cottage is to be used all year round, but is a waste of money and effort in a seasonal, unheated building. Proper installation of rigid foam insulation on top of the roof sheathing is one method of preventing ice damming and air leakage in difficult shed roofs, but may require several centimetres of insulation to be effective. It also may be fairly costly, depending on the size of these roofs.

The final item to address is the overall idea of putting metal roofing on an 80-year-old building. Metal roofing is very rigid and must be installed on a straight, firm surface to look good and perform properly. I have not seen many roofs of that age that fit these criteria, so modifications will be required to allow for this type of roofing. Extensive strapping and shimming will be required to even out the inconsistencies in the old roof. This will add to the cost and substantially to the labour, but may help address some of the other issues pointed out.

While appearing to be an easily installed, high-quality and long-lasting alternative to asphalt shingles, metal roofing does not live up to the first part of the billing. It is not something typically installed by amateur do-it-yourselfers. Unless the roof is a very straight, simple, gable-end roof, professional installation will be a must to prevent leakage. The roof on your cottage is a very complex arrangement with several different designs and pitches, making it a poor candidate for your personal attempt to install metal roofing.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors-Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com