Complicated roof being converted to metal

Q. My home is in central Ontario, Canada. We get extreme winter and extreme summer heat, also. The house is a century home from 1881 with a 100-year-old addition and another 20-year-old addition.

The entire upper level in each portion of the house has vaulted ceilings with smaller attic spaces above. There is currently no air flow from the eaves anywhere in the house; no gable vents, only static and whirlybird vents. The house gets very hot upstairs. There have been no mold problems in the attic in our 20 years here. We are not running air conditioning.

I have a few upcoming projects where I need some expertise.

• A steel roof will be installed over the middle and newer portion of the house. The oldest part has steel already. My idea to help with the lack of venting, summer heat issues and winter ice damming, was to put a radiant barrier film under double strapping underneath the steel roof panels.

Would you recommend this approach?

• Would double strapping the roof combined with a ridge vent short circuit what little ventilation I have?

• What would be the best options for venting? I would prefer a ridge vent with the steel but there is still no proper air flow into the attic.

• Given A/C not being used, would a powered gable fan be preferable?

• Do you have a preferred ridge vent product for steel with external baffle?

• If installing steel over shingles, is a membrane necessary?

• When butting steel roofing into vertical vinyl siding, what is the best practice? Cutting into the siding and lifting the bottom channel seems very risky and messy for cracking the siding. I was planning to leave the existing flashing below as a secondary protection for water penetration, cut the panel to push the flimsy vinyl tight to the wall and use Mulco supra caulk. Some type of butyl stick on moldable flashing would be nice over top but I haven't found a product like that that can be exposed to the elements. Intersecting vinyl and steel has been my biggest head scratcher.

• Is it always necessary to caulk under steel roof seams with conventional profile panels? I don't have any shallow pitch roof angles.

• Is a 26 gauge going to be sufficient for most applications? I know you recommend 24-guage steel roofs.

If this is out of the scope of the amount of info you can provide? Do you offer short consultations?

A. Thank you for the photos. Your roof has some complicated intersecting lines, and I can see the difficulties in venting all these attics.

Your asphalt shingles look like they are about at the end of their useful life, so it's a good time to think about a new roof covering, and a metal roof is a very good option, especially the way you plan to install it. An ambitious project, but one that should solve the ice damming and significantly reduce the overly hot upstairs. However, it will be quite complicated to do in light of the many different planes of the roof.

Your plan to double-strap over the existing shingles and install metal panels tells me you are not considering a standing seam roof and plan to do the work yourself. Otherwise, double-strapping would not be necessary, but a plywood deck over single strapping from eaves to ridge would be required.

Installing a radiant barrier will be quite difficult, and it will only be effective until it gets covered with the inevitable dust from air currents flowing over it. Is it really worth the cost and trouble because the ventilation you will be providing should do a very good job of cooling the roof - the best solution to solving the ice dam problem and reducing excessive heat.

You do not need a membrane over the existing shingles; they are sufficient protection against any leakage under the metal roof.

Natural ventilation will take place from the eaves to the ridge vent, but be sure to install screening at both locations to prevent undesirable critters from nesting under the metal panels. Be sure that any exposed rakes are sealed for the same reason.

Venting the ridge is challenging with metal roof installation, but it can be done. You can use externally baffled ridge vents such as Shinglevent II, but you will have to get the right rubber strips to seal the profile of the metal roof panels. Most roof panel manufacturers should offer that option. The ridge vents will be set on top of these rubber fillers.

I do not recommend power venting attics, as it ends up robbing energy from the living spaces. Because you do not use central air conditioning, you obviously can keep most of your home comfortable by opening windows.

The fact that you have no mold in what are technically unvented attics (whirlybird and other static vents are of little value) suggests that there is little or no convection of conditioned air from the living spaces below the attics.

After removing these practically useless vents you might consider screening their holes so that some ventilation, however minor, may still occur, encouraged by the air movement generated by the new cold roof - the Bernoulli Principle.

Do not alter the existing flashing where the shingles meet the walls; leave the existing roof as is. Looking at your photos and the type of vinyl siding you have, it will be a challenge to flash properly at the joint with walls. Enough of the siding should be removed to do a professional job, and you may need the services of a siding contractor to do this and reinstall the altered siding once the flashing is done.

So here is a simple suggestion: Install the new metal roof as if it were a fly or tarp over a tent and continue to rely on the existing roof as your main protection. Protected from the elements, the shingles should last for a very long time.

But if you want the new metal roof to become the primary roof, the best advice I can give you is to have it installed by professionals. And consider a standing seam roof. Too much is at stake.

As to caulking the metal panel overlaps, if special vinyl strips are not furnished by the metal roof manufacturer, these overlaps should be caulked with polyurethane to prevent water penetration from creeping in from snow melt, wind pressure and surface tension.

The gauge of the metal panels is determined by the manufacturer. I recommend 24-gauge for standing seam roof, as it is more rigid than 26-gauge and reduces oil-canning in strong wind.

• Henri de Marne, a former remodeling contractor turned columnist and consultant, is the author of "About the House with Henri de Marne" (Upper Access Publishing). He continues to take questions from readers for this column and his website, www.henridemarne.com. Email questions to aboutthehouse@gmavt.net.