Winter ice on roof: recommendations

I have a problem that occurs each winter. I get ice on my roof. This does not appear to be damaging the roof or causing leaks. My main concern is that when a thaw occurs, the ice comes thundering down and could be very dangerous if it hit somebody. I am looking for recommendations as to how to either keep the ice from forming or keep the ice from falling.
Our roof is rather unique. We have a flat roof (gravel topped) that has has a peaked shingled roof built on top of it. There is a dormer built into the south facing side of the roof, which contains a window that provides ventilation between the two roofs.
Thanks
-di
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Two things combine to cause ice formation on roofs:
1. heat loss 2. inadequate ventilation
The window may not be enough ventilation for your roof. Ideally, you would want ridge and soffit vents to allow the air to flow up the inside of the roof surface. You may also want to look at how much insulation you have in your ceiling and see if you can improve that as well.
A short term fix could be to add heating wires along the first couple of feet of roof near the bottom and plug in the heaters whenever you see ice building up to dangerous levels. It's a little wasteful, but maybe less expensive in the short term and I've seen people use this method effectively to prevent ice-dams from forming and backing water up on the roof.
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I just had an roof constractor give me a multi-thousand dollar estimate for installing heating cables along the edges of the roof (about 300').
Then I looked online and saw DIY heating cables selling for about $1/foot. This seems to be a big price difference. Where is the extra cost (10x) coming from? Is there a significant difference between a DIY kit and the equipment a "professional" would install? Is it all labour? Or am I being taken for a ride?
Thanks
Dave I
louie wrote:

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What exactly is being quoted by the contractor? Are they thermostatically controlled or timed somehow? Is the contractor wiring the cables directly to a junction box somewhere? I think the DIY set is only on when you plug it in and just has a dangling cord that you will need to run an extension cord to. Also, about 300' of cable can be worth at least a day of labor for installation if the contractor takes their time. Plus the contractor needs to make a profit from this so there's a markup on parts most likely. Take a look at the contractor's proposal closely and compare with the DIY kits to see if you're comparing apples-apples.
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Louie has some good advice.
Personally for this winter, I would go for a bunch of soffit vents and gable vents if you can. Gable vents because they have to be quicker than a ridge vent (which would work better). Check out your ceiling insulation as Louie said and the vapour barrier. There should be no way for warm room air to leak into the attic.
Last house I had got ice dams on a roof with a low slope. Cables were already installed on the roof so we decided to use them. They had to be powered ON before the dam formed to work well. Uses lotsa power. It would have been better to fix the cause instead of the symptom.
We learn
Splinter
wrote:

<snip>
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I dsicussed soffits with the contractor. He said to do that would be such a large estimate that he didn't even want to consider it. It would involve ripping out all our existing soffits (wood, unvented) and replacing them. Add to that any complications due to the fact that the interior may require work to accomodate ventilated soffits (given the unique, double roof, construction of the roof).
One thing I am wondering though. The peaked roof over our flat roof has a dormer in it, with a window that is currently closed. Perhaps I can solve this problem just be opening the window?
Thanks
Dave I
snipped-for-privacy@myplace.com wrote:

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I got this done a few years ago by an electrician, not a roofing contractor. I needed about 120 feet of heating cable on the roof, plus 30 feet in the gutters, plus a cable conduit to the breaker panel. If I remember correctly, it cost around $600. Took one guy about 4 hours to do. So, about $150 for the heating cables, and $450 for the remaining hardware and labour. Call an electrician instead of a roofing contractor, maybe you'll get a better price.

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Dave Isaacs wrote:

An old roofer once told me he won't climb a ladder for less than $350 because he learned that's what it costs to offset the risks involved. So start with a base charge of $350 per man per day then add hourly labor and materials plus overhead for both. It can shoot past $2K pretty easily. That said, you can probably get a lower price from the folks south of the border who find their way into the trades up here.
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I just spoke to another roofing contractor, and he suggested yet another solution: install "ice aprons" (essentially flashing) along the edge of the roof. His claim is that the ice cannot hold on the apron, so ice building is not a problem.
Does this make sense? I noticed one of the churches in our neighbourhood has the same solution, except that they applied ice aprons *and* ice railings.
Thanks
Dave I
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