Snow Causing Roof Leak

DEAR TIM: I have an unusual narrow roof valley against a dormer where snow builds up, turns to ice and then leaks into my house. The shingles look fine, so I’m wondering if the water is coming in through some wood siding that forms one side of this roof canyon. That’s what I call it. The snow can get up to 3 feet deep here in the dead of winter. How can I determine where the leak is coming from? What’s the best way to stop all water from entering the house? Trying to remove the snow is very problematic, so please don’t tell me that I must do that. Victor C., Ontario, Canada

DEAR VICTOR: Guess what? I know you live in Canada where the cold winter winds blow but the advice I’m about to offer you will help someone in sunny Florida or any other place where wind-driven howling rainstorms cause roof leaks. This is a rare Spandex one-size-fits-all answer about roof leaks.

The photo you sent tells the entire story. Photos allow me to really see what’s going on and give you the best advice. I have to say, you sure do have a ‘roof canyon’ that will cause all sorts of leaks if the roofing is not installed by a real professional familiar with the weather conditions in your area.

Snow builds up in this roof canyon and causes leaks in the winter. Photo Credit: Victor Cheung

Here’s an interesting fact many forget. I was lucky to start my building career in an old city with some of the houses built in the late 1800’s. What I can tell you is that I rarely would find a roof detail like yours. I feel this is because roofers back then knew that it was quite hard to prevent leaks like your having. Part of this was because they didn’t have access to the modern underlayments we have and the solution they did have most regular people couldn’t afford.

When roofers of old faced very challenging roof details like yours where a wall and two roof planes all come together much like a kitchen funnel, they’d use sheets of copper soldered together to create a waterproof roof covering in this area.

If copper was too expensive, flat locked tin-plated steel was a great substitute. The only issue with the tin is that it required periodic painting to keep it in perfect condition. No matter what material was used, it was a very labor intensive process and only the wealthy could afford it.

You don’t have to go this route because you can purchase remarkable peel-and-stick roof underlayments that contain rubberized asphalt. The brand I prefer and just used on my entire roof this past summer is Grace Ice and Water Shield.

Here’s what’s going on and why your roof is leaking. Heat leaking up from your home is melting the snowpack in the tight roof canyon. The cold weather turns the super-chilled water to ice and the ice pack begins to get thicker and thicker creating a dam quite like Hoover Dam, but a much smaller.

Most roofing materials are designed to work under the influence of gravity. The materials are typically layered with each successive course overlapping the one below it as the materials march up the roof. The ice dam causes water to build up under each layer and it doesn’t take long for it to overflow the top of a layer, find gaps in the wood sheathing and pour into your home.

The underlayment I used on my house sticks to the wood sheathing and it sticks to itself. When installed according to the manufacturer’s written installation instructions, it forms a waterproof barrier under the finished roofing that covers it. My membrane is so good that it seals to the nail shafts that are used to secure the finished roofing to the sheathing. It’s an amazing product.

To fix your leaks permanently, you need to strip off your existing roofing down to the wood sheathing. You need to remove the finished siding on the sides of the dormer. The membrane needs to be installed as they say in the instructions and it needs to lap up at least a foot or more on the sides of the dormer to ensure you never have a leak along that wall. Consider putting covering the entire side wall of the dormer to the deepest depth snow can build up in this area of the roof.

I urge you to get the written installation instructions of the Grace product, read them and understand them. If you can’t inspect the work as the roofer is doing it, ask him to take lots of photos as he installs each piece of the membrane. Remember, this membrane needs to start low and each successive piece overlaps the one below it.

You want to make sure the membrane is perfect before it gets hidden by the finished roofing. If the roofer wants a few extra dollars for the effort of taking the photos, it’s well worth it. You want to approve the membrane before the roofer proceeds with the final roofing.

Be very careful about accepting a substitute membrane no matter what the roofer says. Not all waterproofing membranes are the same! Some do not stick well to themselves. Some do not seal around the nail shafts of the finished roofing that penetrate them. Trying to save a few hundred dollars here is being penny wise and pound foolish.

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