Ice Dams

[mrwilsonthomas]

Hello, I live in Philadelphia and had some problems with ice dams in the back (north) side of my house after a big storm last winter. I have a split-level home with a pretty simple "A" roof (no valleys or dormer windows).

I had a new roof installed in the spring and had the entire roof surface covered with ice and water shield (I know that ice/water shield won't prevent dams from forming, but hope that it will minimize or prevent any leaking to occur inside the house in the event dams do form).

I also had the roofer extend the eaves in the back of my house out to about 1 foot, allowing for a continuous soffit vent to be installed underneath the eave. A gable vent and two cap vents were previously installed in the roof as well.

Finally, I am having my existing attic insulation upgraded (not much there right now at all).

Should these measures be sufficient in order to prevent damage from ice dams in the future, or are there other things that I should be doing as well?

[louie]

Make sure that there is a path for the air to follow between the ridge and soffit vents. The soffit vents don't do any good if they're blocked by insulation. As you add insulation, staple some of the styrofoam vents along the roof near the edge of your attic to keep the insulation from closing off the space entirely.

[Dick]

You can always fill an old tube sock or two with ice melt and throw them up on the roof just above the ice dam. This will punch holes through the dam and help prevent it moving up the roof.

[mrwilsonthomas]

Thanks...the insulation contractor said he will use baffles to make sure the soffit vents stay clear.

Interestingly enough, another contractor said that soffit vents were not necessary in this area and would have covered them with insulation.

[Joseph Meehan]

I'm glad you did not choose the other contractor. :-)

You should be free of the problem except for some very unusual situations.

[robson]

You got that right. Tried it last year in a pinch and it saved me some major damage.

[Richard J Kinch]

Ice dams are like floods. Eventually a year will come along when the snow is so thick, you will have them; better construction just raises that thickness.

[Chris Lewis]

On the contrary. Ice dams have _nothing_ to do with snow thickness, and indeed, most ice dams occur with less snow rather than more.

If the eaves/attic is cold like it's supposed to be, you don't get ice dams.

[Richard J Kinch]

Nonsense.

The interior of the structure is above freezing, and assuming the outside is below freezing, then somewhere in or around the structure, is a boundary line between the two regions. Call this the freeze-thaw boundary.

Whenever the freeze-thaw boundary moves out to the snow on the roof, you get melting on the roof. Anywhere the melt runoff crosses the boundary again, such as at an eave, refreezing will occur, and a dam will form.

Snow thickness definitely affects ice dam formation, because it affects the motion of the freeze-thaw boundary, by virtue of its action as a very effective insulator.

Do not confuse temperature with heat. Sub-freezing snow can very effectively insulate heat that results in above-freezing temperatures.

No passive construction can avoid ice dams. Certain conditions will always cause them. People are confused about this, because (1) they don't know thermodynamics, (2) it depends on the weather pattern and this varies, and (3) it depends on the type of construction which also varies. Bad construction makes for bad ice dams, but that doesn't mean the best construction can always avoid them.

Your error is in assuming the attic is always "cold like it's supposed to be". Attics may be prematurely warm from poor insulation, but eventually they all get warm from exterior heat. Eventually the weather turns warm, and the freeze-thaw boundary moves out of the house and up to the roof. When and how often this happens depends on the weather and the quality of construction, but it does inevitably happen.

The only guarantee against ice dams is active removal, by heating or mechanically or even chemically, which are all expensive and/or impractical.

If you think ice dam control is simply a matter of proper construction, then consider the Space Shuttle. $Billions and lives lost to a similar type of ice formation, despite the concerted effort of the best minds to avoid it.

[RobertPatrick]

Richard J Kinch wrote in news:Xns972494212FE93someconundrum@216.196.97.131:

That's just about what I've found. I have a one story home and I get out the roof rake before the snow gets a chance to turn into ice. Around the garage and heated rooms on the end of the house I don't have to use the rake. It's only on the valleys and the rooms in the middle of the house. Rob

[Chris Lewis]

Right. But, that's only if the freeze-thaw boundary _ever_ gets on the roof.

On a properly ventilated roof, it never does (except during overall thaw) - the attic is just as cold as the outside.

When it gets warm outside, if the roof has a uniform coverage and has relatively uniform temperature across the whole thing (which is what proper venting _does_), the eaves warm up slightly before the rest, and the snow cover erodes from the perimeter in. More to the point, proper venting ensures that the eaves are never colder than the outside or the attic proper. Which means you don't get ice dams.

Certainly, there can be very unusual weather conditions that might trigger some ice dam formation (rapid temperature excursions). But, with our house, for example, we get uniform coverage up to 3 feet of snow by winter's end, without a hint of ice damming or icicles throughout the whole time, including throughout spring thaw.

Properly constructed/ventilated, the only time people around here consider snow removal is if the overall weight gets too high (our codes specify 70 PSF snow load).

That's a rather different situation - we're not dealing with liquified gases venting at absurdly low temperatures into very humid climate.

[Richard J Kinch]

A perfectly vented and insulated attic would still make ice dams under certain conditions. The roof can thaw while the eaves freeze, for reasons other than heat loss from the house.

[Chris Lewis]

During a rapid temperature drop (above to below freezing), there can be a short period where water that's already melted off the main roof and slowly flowing out will get caught by a quickly freezing bare eave. But that's pretty short duration.

Remember, (assuming proper ventilation) while the roof & eaves are snow-covered they can't go above freezing until the snow cover comes off.

On a properly ventilated roof, the melting starts at the roof edges and progresses inwards, hence, the eaves clear first.

[Richard J Kinch]

It melts wherever there is enough heat, whatever the temperature. Snow can melt in a below-freezing ambient from heat from solar radiation. That's one way you get ice dams on a perfectly insulated structure.

It all has to do with that 32 deg F isotherm envelope I mentioned. You can never guarantee it will enter and exit the house uniformly, and then you will have a potential melt/refreeze situation.

[Chris Lewis]

That tends to sublimate (solid -> vapor) rather than melt, and with an even snow cover, the melt water tends to stay where it is (ie: uniform ice buildup).

In practise, however, a properly ventilated roof will almost never build up enough of ice on the perimeter to do anything bad. Even in the worst conditions, our roof has never build up anything noticable, meanwhile, buildings with poor ventilation build absolutely amazing ice dams and icicle formations.

[The only time we've ever had icicles is when the ceiling vapor barrier got badly damaged in two places in the detached garage. Major ice buildup in the adjacent eaves. Went away when I fixed the plastic.]

[Banty]

In the latter part of the winter here in the northeast that can happen every 24 hours, though. Which is when we (not me so far in 11 years, thank all dieties) get ice dams.

Banty

[mrwilsonthomas]

Thanks for the helpful responses. Here's another question...if I remove most of the snow from the eaves (as far up as I can reach) with a roof rake or other similar device, will that help alleviate the potential for ice dams to form?

[mrwilsonthomas]

Thanks for the helpful responses. Here's another question...if I remove most of the snow from the eaves (as far up as I can reach) with a roof rake or other similar device, will that help alleviate the potential for ice dams to form?

[Rescate]

[e-mail] wrote in news: [e-mail]:

Yes, that's what I use.

[Rick]