Ice Damming

How are they going to vent the main roof when they have no soffits due to the fact it’s a mansard roof??

Hey MC Hammer,
Do you leave your Christmas lights up all year???

:lol: :lol: :lol:

No Linas, those were pictures over the holidays.

Well maybe if my wife would not beat on me to take them down, they might stay up all year. ha. ha.

Marcel :slight_smile: :slight_smile: :smiley:

The typical “ice dam” damage that we try to protect against by using eave protection (Grace “Ice & Water Shield” and similar) is water intrusion into the insulated exterior walls and ceilings along these walls. This why the eave protection must extend up the slope far enough so that it is at least 3’ horizontally inside the interior surface of the exterior wall.

As the roof slopes gets steeper, this chances of this type of damage actually becomes much lower to non-existant. The Canadian building code only requires eave protection for roofs up 8/12; above that, it is not required. This is due to the fact that the steepness of the roof does not allow the water to dam and run up under the shingles far enough.

The damage you show may be the result of poorly draining uncleaned / improperly sloped gutters or blocked downspouts. This coupled with the amount and weight of snow that may slide off of this very steep left roof (appears to be at least 12/12 or maybe 14/12) will definitely cause the damage seen. The method of mounting the gutters and the lack of maintenance of these fasteners can also add to this gutter damage.

There may not be a true ice dam here, ice/snow build up …yes. Take the gutter off and see how much or how far ice builds up on the lower slope…probably not much. On the lower slope roofs, a true ice dam with water intrusion can build up even without gutters.

Similar to this Brian;

http://www.countryplans.com/Downloads/Gambrel.pdf

Marcel :slight_smile: :slight_smile:

Doug those are some great pics.

Thanks again this is some good stuff.

Marcel:

Wrote a bit quickly: Should have said “no open soffits and vent spaces in the mansard”. In our codes the gambrel/mansard roofs/walls do not require venting. So how do we cheaply get venting in at the transition from steep roof to main attic?

Here’s the big question that no one in the building inspection department has ever attempted to answer for me.

When do we have a cathedral system (insulated mansard and gambrel walls are, in effect, special cases of the cathedral) that does not require venting and when do we have a cathedral ceiling which requires venting. As I said , mansard and gambrel roofs do not require venting but as the roof slopes drops, when is it required? 16/12, 14/12, 12/12, 10/12…??

Brian; First, I didn’t design this mess. But if you look closely, the biggest heat loss is in fire seperation walls. It appears they were left rather uninsulated resulting in a heat chase to the roof. These row houses were “engineered to fail” because each trade did their minimum requirement. The most important issue here is not in how to ventilate a roof system with no soffits(because you can’t) , but how to eliminate heat loss when heating design temperatures in this climate are based on -29 celcius. This type of roof requires minimum 1 sq.ft. of roof vent per 150 sq. ft. of roof area.

You are correct in code requirements, but those are always only minimums.
The Ontario Building Code for example, contains plenty of information, but at the end of the day it’s up to the Municipal Chief Building Official to interpret the code to suit local conditions.

Doug

Doug:

Right you are!! I’m actually not in favour of all out venting to or beyond codes as it’s an imprecise science and canot be relied on unless your net free vent area is equivalent to 1/2 or 3/4 the roof area or larger!!! (meaning all vents and not much roof/gable ends)

Was just playing Devil’s advocate to those who still want to rely on venting to solve all roof /attic problems from condensation to mould/roof sheathing deterioration to ice damming to high cooling costs and uncomfortable upper floors in the summers to severe premature shingle deterioration. Other practices are now known to reduce or eliminate these problems so that venting can actually be reduced so less chance of snow/rain entry.

You’re singing my song!:smiley:

I think you would agree that increasing ventilation without reducing heat loss can actually be more destructive.

Been saying that for about 30 years. Was glad to see the CMHC “About Your House” brochure Attic Venting, Attic Moisture and Ice Dams in 1997-8 or so as it finally supported what I was saying to people since 1977-8 when I discovered the NRC Building Digests and CMHC’s The Conservation of Energy in Housing.

See: http://www.cmhc-schl.gc.ca/en/co/maho/gemare/gemare_001.cfm