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HOUSE WORKS: Skylight coolings and metal roofing over old shingles


Metal roofing like this can be applied directly over old asphalt shingles as long as the roof deck is free of sagging and waviness.
Metal roofing like this can be applied directly over old asphalt shingles as long as the roof deck is free of sagging and waviness. - Steve Maxwell

Q and A with Steve Maxwell

Metal roofing over old shingles?

Q: Can I fasten metal roofing directly on top of old asphalt shingles? I have a utility shed with a roof that needs replacing.

A: Yes, it’s fine to secure metal roofing directly to a roof with old shingles still in place with no strapping.

Anchor sheets of roofing with special roof screws that go right through the old shingles and into the underlying roof sheathing.

When it comes to the job of roofing, strapping is wooden strips applied horizontally over the roof, creating surfaces in which roofing screws can grip and hold.

 Metal roofs are held down with waterproof screws like these. The domed metal cap, the neoprene washer and the hex head make these screws easy to install and very reliable. - Steve Maxwell
Metal roofs are held down with waterproof screws like these. The domed metal cap, the neoprene washer and the hex head make these screws easy to install and very reliable. - Steve Maxwell

The thing is, strapping is only necessary under metal roofing if your roof structure is not completely flat.

Metal roofing offers many advantages, but one of the drawbacks is the need for a very flat roof structure.

Metal roofing is fairly rigid and doesn’t like being forced to follow undulations. Strapping can be shimmed more or less as it goes down on a wonky roof, creating a nice, flat surface for metal to rest on.

A shed is a pretty small building, so chances are good that there are no waves or sags to deal with.

My only complaint about metal roofing is that I’ve never seen it keep looking good for more than 20 years. Even at 15 years, the paint is often dull, faded and sometimes peeling.

Cooling down upstairs bedrooms

Q: How deep should attic insulation be? I’ve got a two-storey rental property and the upstairs is hot in the summer, even with the air conditioning going strong. Is insufficient attic insulation the problem? Blocked attic vents?

A: Poor attic insulation and blocked vents could be involved, but there’s probably more at work than that. Here in Canada, even properly insulated two-storey homes often still get too hot upstairs during summer.

The issue is all about trapped hot air rising in your houses, preventing cooled air from moving upwards. Central air conditioning systems all use ducts that are primarily designed to deliver heat not cold. You may be able to help balance cooling action by closing downstairs vents and opening upstairs ones, but this won’t completely solve the problem.

Have you looked into your attic yet? See how much insulation you’ve got and bring it up to about 22 inches of depth. Batts or loose fill both work about the same.

What sort of attic vents are in place now? Increasing attic vent area may help keep your attic cooler and in-turn the rooms below, but there’s a more effective solution.

A few openable skylights in upstairs rooms do an amazing job of making the entire house more comfortable in summer. Leave these skylights open just an inch or two all the time and air conditioning will work much better everywhere in your house.

The best skylights for this purpose close automatically at the first drops of rain, too. Visit baileylineroad.com/beat-the-heat/ to see how skylights make homes cooler.

Building a garden shed base

 Openable skylights like these are highly effective at boosting ventilation that cools the whole house. This is especially true with two-storey homes that get too hot upstairs. - VELUX
Openable skylights like these are highly effective at boosting ventilation that cools the whole house. This is especially true with two-storey homes that get too hot upstairs. - VELUX

Q: Can I put down pressure-treated plywood for the base of a new garden shed? I just took down an old metal shed that was built on a platform of pressure-treated 1x6 boards with 3 /4--inch spaces. Can I screw down half-inch pressure-treated plywood over the 1x6s to use as a base?

A: What condition are these boards in? If they’re solid you could certainly put pressure-treated plywood over top. If I were you, I’d pay the extra and get plywood that’s made especially for below-ground foundations.

Yes, they make foundations entirely out of wood sometimes and it doesn’t rot or delaminate, even in contact with the earth.

You’ll find 5 / 8-inch pressure treated plywood made especially for this application, and it’s what I’d recommend for your floor.

Steve Maxwell is a syndicated home improvement and woodworking columnist who has shared his DIY tips, how-to videos and product reviews since 1988.

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