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After this recent episode of ice and snow I noticed a wet spot...
18 February 2006


Q. When we had the roof replaced about seven years ago they removed the old roof before putting on the new one. After this recent episode of ice and snow I noticed a wet spot on the ceiling near the front wall by a large picture window. I called the roof er who put the roof on and said I had a leak. He told me that it was from the snow melting and his roof job wasn't the fault. I don't understand. What’s happening and what can I do?

A. What's happening is a phenomenon known as "ice damming" or "back-freezing". It's such a common occurrence up north that they build their houses with roofs that anticipate it. Many homes built above Boston have flashing as roofing for the first two fe et up the roof from the eaves and no gutters. The gutters just get torn off each year by the weight of the ice. But here in Maryland I have seen ice damming occur only a few times in the last 30 years so area roofers tend not to prepare for it. It take s enough snow to stay on the roof and a series of day/night thaw/freeze cycles.

Usually, when Maryland gets a snowstorm, the sun comes out the next day and it soon melts away. Our recent storm did not cooperate in that manner and the snow combined with freezing temperatures at night set up conditions that guaranteed ice damming on some area roofs.

Ice damming is usually worse with older homes whose attics are not particularly well insulated or ventilated but I've seen it happen in newer homes too.

The problem stems from heat leaking straight up from the house which in turn warms the underside of the roof, melting the snow that lies directly upon the roof shingles. As the snow melts it runs under the snow and ice blanket down the roof surface unti l it reaches the building line. That's an imaginary line that extends up on a plane even with the outside wall of the house.

The area of roof that is past the building line comprises the overhang-- sometimes called the eaves or soffits-- and the roof there will be the same temperature as the outside air. And this week that temperature was below freezing almost every night. T he water then freezes when it meets the building line creating a "dam" of ice that prevents the melted snow water from naturally draining from the roof.

Shingled roofs are designed to shed water on an incline and are not watertight like boat bottoms. Water begins to build behind the ice dam and backs up the roof getting behind fascias and under shingles, finding any opening, crevice or seam and pours in to the soffits and wall cavities and finally into the house soaking drywall or worse. The most likely location of ice damming leakage entry into a house is above windows where wooden framing headers act like a water conduit as you've seen in your house. I've seen leaking window heads that looked like someone turned a faucet on inside the wall with clear ice water cascading into the house. Seeing icicles forming behind the gutter and between the fascia board to which the gutter is attached is a signal that ice damming is forming even if you haven't yet noticed leaking in the house.

Once a dam has established itself at your roof's edge it will inhibit the draining of the rest of main roof's snowmelt and you've got trouble until we get a warm sunny day to completely thaw things out. There isn't a whole lot that can be done about it while it's happening. Jumping up on the roof with an ice-chopper can spell greater damage. I've heard of roofers being hired to chip openings in dams to relieve some of the pressure but they need to be careful not to damage the roof. Some refuse to do it. Homeowners who attempt doing something like that should be extra careful. Falling off a ladder or a ladder slipping out from under you on ice or snow is a sure trip to the emergency room.

One approach I have seen that worked was attaching a hose to a threaded hot water faucet at a utility sink and by playing a stream of hot water in one location melting a section of ice dam from the roof edge allowing the water behind the ice dam to drain from the opening. It worked. If you don't' have a utility sink you can attach the hose to the drain faucet at the bottom of your water heater and get hot water into a hose that way. One more tip: bring any hoses you intend to use into the house to thaw any frozen water trapped in the hose when you rolled it up last fall and stored it away in a unheated shed or garage.

I don't like the idea of using electric heat tape on roof edges but some use it to melt overhang ice. A product called IceViper (www.iceviper.net) is an ice melting tube device that you lay across t he ice dam every 10 to 15 feet along the roof edge to drain the ice dams. The website says Home Depot has it. Rock salt or calcium chloride works but again you get into using ladders. You can make a dam-buster out of a nylon stocking filled with rock- salt that you lay across the ice dam to melt a channel through it.

When the ice is gone and the drywall, insulation, paint and flooring damage have been repaired, it's time to plan for the future. Step one is proper attic insulation and the sealing of heat loss sources that vent into the attic. Hatches, drop stairs, b ath fans that only vent to the attic and ceiling recessed lighting fixtures are prime offenders. Look at your roof after a frost or light snow. If you see the shadow of the rafters in the frost or see snow spots that melt faster than others—you h ave a heat loss to the attic.

Talk to your roofer about preparing the first three feet of your roof for ice damming. A combination of flashing and a W. R. Grace product called "Ice and water Shield" or its equivalent can be used to prevent future disasters. For the marginal additio nal cost, it's worth the money. Even when it's 95 degrees and the thought of ice damming is is distant, ice damming shouldn’t be forgotten.

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