Live in New England; typical Colonial.
Every few years it seems we get water damage due to "ice dams"
a. exactly what is the mechanism that causes these ?
What conditionsmust be present ?
e.g., temp's at night or during the day, etc. ?
b. We have gutters.
Would it help minimize the possibility of having, if we removed them ?
Any thoughts on this would be most appreciated.
Inadequate attic insulation and/or ventilation causes them. The
underside of the roof needs to stay below freezing and if there is not
enough ventilation or insulation the heat from the house intermittently
melts the ice on the roof.
Heat from below melts some snow, and then freezes again. More snow
melts but it cannot run off, so it backs up behind the ice (dam) and
seeps under the shingles. Refreezing under the shingles can also cause
more of a gap, allowing more to run under the roofing. Adequate
ventilation and insulation should keep the attic
cold enough that the bottom layer of snow does not melt. Got soffit
and ridge vents? Do a google on "formula attic ventilation" - I think
it is 1 sq. ft. of vent on top for each 300 sq ft of attic floor area
(not roof area).
Gutters that are plugged up can also cause backup or backsplash - had
that in florida, which caused a little wetting of wall inside.
I haven't checked these out but a quick Google search turned up these
They make a product just for this problem. It's called a water & ice
shield. It's a rubberized membrane that is 3' wide and very sticky and
goes on the decking under the shingles along the eaves. It self seals
around the roofing nails and prevents any water that gets under the
shingles from penetrating into the attic and causing damage. A bit
pricey but if you have continuing problems, I'd have it installed the
next time you have repairs done.
Cheap insurance, and SOP on re-roofs these days around here. Some places
even have it required by code on new work. I insisted on it for my new
roof a couple years ago, bottom edges and valleys. Hope it helps- I have
one inside valley on lee side of roof, that always gets a huge snow
cornice and had leakage problems for years for previous owner.
(Stupid-ass roof design on addition, with new roof dying into old roof a
foot above old gutter line, creating a low-pressure dead spot. If I was
gonna stay in the house, I would have spent the extra money to extend
soffit on the addition side, and bring that roof down to the same level
as the old roof, which also would have given me more shade on deck.)
The membrane gets installed before the shingles are put on. If the
roof is in otherwise good condition, it may be a long time before the
membrane can be cost-effectively installed.
In the meantime, consider use ice melt cables. I've been using them
for many, many years and they have virtually eliminated the ice dams.
I used to have water runing inside the walls, now I barely gey any
icicles at all.
This in no way implies that you shouldn't look into getting the attic
ventilation and insulation up to the proper standards.
If your attic is uninsulated the house heat rises up and melts the snow
which flows out to the overhang on the roof which is unheated and the
water freezes. So water keeps flowing down and hits the previously
frozen ice at the bottom and pools. If it gets deep enough it back flows
under the shingles and leaks in.
Prevent it by raking off the snow 3 feet above the over hang, insulate
the attic put heat coils on the eves or when you reshingle the roof put
'ice and water' paper under the shingles half way up the roof. It's
sticky paper that self seals at the edges and sticks to the roofing
material. It water proofs the roof against back flow.
Where do you get "half way up the roof"? That can be overkill and it
could be insufficient.
"How much Grace Ice & Water Shield do I need for a specific roof pitch?
Grace recommends that the Grace Ice & Water Shield is extended a min. of
24"" beyond the inside face of the exterior wall line of a building and
should extend from the eave above the highest expected ice damn. Check
local codes and regulations as they may vary. If installing full
coverage of Grace Ice & Water Shield, use same roof area calculation for
finished roof covering."
On Sat, 31 Jan 2009 09:06:16 -0500, Van Chocstraw
I had some trouble with that: a new ice dam developed where the snow
cover was removed. The water ran down under the snow, then froze where
I'd raked it clear.
When I re-did the roof, I put membrane 3 courses up from the edge.
Re the general matter of venting: there was an article in 'Fine
Homebuilding' some years ago about this. They reviewed quite a lot of
research. One thing that really struck me was an (apparently thorough)
Canadian study, which concluded (paraphrasing), "if you're getting ice
dams and your vents are open, plug them. If you're getting ice dams and
your vents are closed, open them."
Ice dams can form even when the roof is well ventilated and adequately
insulated. When you have snow on the roof and a warm day comes along
it melts and then freezes at the eaves...voila...ice dam. I don't
clear the snow off my roof, it's enough work just keeping my driveway
usable. I use a 10-foot length of iron pipe wrapped with heating tape
to relieve these temporary dams. It works kind of like a bangalore
torpedo. I have a low pitched rambler so it's fairly easy and safe to
move around. I wouldn't use it on a steep or high roof as it might
Removing the gutters won't help. That will only coat the ground below
with some very slippery ice, which can be a big problem if it is a
walk or drive way.
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