I built a new log home over the past two years, during this winter I had some ice build up in a valley and the ice backed up under the metal roof , followed the ice and water shield to the fascia down to the soffit , back along the soffit to the wall and through the top plate to the interior, i went up to the roof nop easy task 30′ and pounded out the ice almost 2′ thick at the end of the valley, when I framed the roof I used 16″ I joist with a 16″ glum lam valley rafter and have 12″ on insulation with a vented soffit and vented ridge , my question is does anyone know the solution to this problem, ex whirly bird venting to increase cool air flow, heat cable to melt ice as building, or do I remove metal , place 2″ of ridgid sytro over sheathing then strap with 2x 6 on edge to increas air flow the reapply sheathing and metal roof with expandede fascia, expensive and more work?
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Replies
Dude, like chit, ice dams happen. People can talk all the theory that they want, but so long as there is melting snow, the water will freeze at the edge of the snow. The best way to prevent the water entry is to start watching the roof when there is snow on it. I personally use heat lamps to melt the ice away. On my old roof I started to scrape the snow off. I found significant damage to the shingles and it only moved the ice dam up to the edge of the snow. By the end of the winter I had 5 ice dams marching up my roof.
Ice dams only happen when the roof over the attic is above freezing and the roof over the soffit is below freezing. That only happens when you're losing too much heat from the living space into the attic and don't have enough ventilation to remove the heated air from the attic. The right combination of insulation, leak sealing, and attic ventilation will prevent ice dams.
Negative, Unc..
When the outside temperature approaches freezing--especially with a metal roof--ice will begin to form at the eaves as the melt off runs down the roof. At about 30 degrees is when I'd start to see the water pouring in. Then it would refreeze, of course, further building up the dam, making the next 30 degree day even worse. I assure you, I had adequate ventilation and plenty of insulation in the attic space at the valley (the same as the rest of the roof, yet only the valley had ice).
Also, the valley will hold the most snow, as it slides from the roof above onto the existing snow load below. Hence, the most melt water.
>> When the outside temperature approaches freezing ...
Do you mean warming up to freezing or cooling down to freezing? If you're saying the melting starts when the temperature gets up to 30, where's the heat coming from? Air at 30F won't melt snow.
I used to agree with you. Used to. You see, my attic was unvented and the rafters were insulated. When the roofer was contracted to put the roof on, he wrote in that the mfr. would be voiding the waranty because it wasn't vented. So we over sheathed the roof with 3/4" batten strips between. This thing ice dams up just as bad as before. And I don't even have gutters on!
The last year before we tore the roof off, I really went after the snow. I scraped it off when ever I saw an ice dam forming. The problem is that the snow is a good insulator. The heat bleeds through the insulation (R-21 in my case) from my unheated attic and melts the snow on the underside. The water travels down until it gets to the edge of the snow and refreezes. As I scraped the roof, that snow edge kept on moving up, and so did the ice dams. And yes, even the uninsulated roof over the unheated garage damed up just as bad.
Well, you guys clearly know more about ice dams than I do. I've never seen one, never seen anything that I recognized as evidence of one. My brothers in the building trades have never said anything to me about techniques for preventing ice dams, or anything about repairing damage caused by one. I've never heard a home owner complain about water damage from one.
Still, I'm fairly sure that if the temperature of the air under the roof is the same as the temperature of the air above the roof, you'll never see any freezing at the margins of the roof while the rest of it is still melting, except maybe very briefly at the end of a warm day. If you're getting ice dams, it means the upper part of the roof is warmer than the lower part of the roof for extended periods of time, and since the sun shines pretty much the same on the upper and lower parts of the roof, the extra heat must be coming from inside the house.
Maybe it's a regional thing. I very seldom see any ice on the ground from snowmelt, either. If we get a warm day after a snowstorm (which we very often do), as it starts to cool off in the afternoon the melting slows and then stops, but the already melted water keeps draining, and by the time the ground or sidewalk or street freezes, most of the water is already gone.
It definitely is a regional problem. Yes, good insulation, ventilation, cold roofs, ice and water barrier, standing seam roofs are all good, but there is no cure all. We have had an ice dam problem on an unheated building where water by the sun from an upper roof fell to a lower roof and created an ice dam there. When the temperature rose water filled the dam and flowed into the house. We had to pull the shingles and siding in this area and run ice and water barrier under both. Haven't heard a call back yet but wouldn't rule it out.
[QUOTE]
Still, I'm fairly sure that if the temperature of the air under the roof is the same as the temperature of the air above the roof, you'll never see any freezing at the margins of the roof while the rest of it is still melting, except maybe very briefly at the end of a warm day. If you're getting ice dams, it means the upper part of the roof is warmer than the lower part of the roof for extended periods of time, and since the sun shines pretty much the same on the upper and lower parts of the roof, the extra heat must be coming from inside the house.
That's the basic theory of insulating/ventillating to avoid ice dams, and it's about 95% correct, with the slight addition that the "extra heat" can also be due to sun shining on the other (likely bare) side of the roof.
However, you can also have problems on a sunny day when the temp is around 28, and part of the roof is shaded (either by trees or due to a dormer, etc). In such a situation the sun can melt snow up high on the roof (without the roof itself being overly warm) and it can run down into the shade and freeze. The freezing by itself isn't a problem, of course, but the ice can then cause water to pool behind it and that water then runs through the shingles and into the structure.
You're right, I neglected to consider either sunshine on the other side of the roof or partial shade. I'll try to keep that in mind the next time I'm tempted to make sweeping, dogmatic statements on subjects outside my expertise. :o)
You mentioned that your Insulation is R-21, in snow country.
This topic has been tossed around here quite a bit. the two methods for dealing with Ice-dams are of two varietys.
A cold roof IE: stop all air exfiltration from the heated space going into the attic. This is done a number of different ways. I personally lean toward foaming every possable air gap in the attic, and then over filling with cellouse, lots of cellouse. and have lots of ventalation, keeping the temp of the attice as close to the outside air as possable.
or for a lower pitch roof a membrane covering the complete roof under the tile.
or both. Belt AND suspenders...
It would seem that any "fix" for this would include upgrading your insulation in the attic.
links to other threads covering this exact topic....
http://forums.taunton.com/tp-breaktime/messages?msg=39738.12
http://forums.taunton.com/tp-breaktime/messages?msg=14569.1
http://forums.taunton.com/tp-breaktime/messages?msg=3515.1
there are others
Well, that would be true for the portion of my attic that's insulated. Unfortinatly, R-21 is the most you can fit into a 2x6. Don't forget, this attic is unheated and the floor-ceiling is also insulated (albet poorly). Now, my garage which also ice dams is totally unheated and uninsulated. There is no heat here. The snow must be melting from so other source (at least for the garage). The garage and the house both seem to ice dam just as bad. Naturally, because the water is concentrated in the valley, the ice dams are the worst in the valleys. I've also notice that the ice dams are worse on the north side of the roof. They are proably twice the height of the south facing ones.
Theory-the sun is somehow heating the shingles through the snow layer causing the snow on a warm day (28) to start melting from underneith. The snow on the south side melts fast enough that the ice dams really don't build up. On the north side though, the snow hangs around slowly melting from underneith and bulding up.
By the time I notice the ice dam on the north side, it was about 6" high. The south roof also had dams, but the snow had melted off so I really wasn't concerned about them. The north side though, still had 6" of snow on it.
Thanks for your feed back
I placed 12" of insulation in between the I joist, which is R 40 code in northern ontario, I also was very conscientious about the vapour barrier and sealing all potential air leaks, I have vented soffits and a vented ridge, the problem I believe is a combination of heat transfer through the valley rafter and the sun shining on the above dormer for most of the day, there is a photo of my house at http://www.northerncomfortloghomes.com under projects , my house is the princess and the valleys are seen in the front view, I think the easiest solution for now is to install some heat cable partitially up the valley and turn it on occassionally, the valley is a combination 4/12 with a 10/12 dormer that runs into another dormer with thje same slope.
Edited 2/27/2004 1:20:02 PM ET by Captain sawdust
View Image
This is the view I was trying to show you, the valley on the south east side ( left of the middle) is the area that gives me the most grief
Edited 2/27/2004 1:30:32 PM ET by Captain sawdust
Looking at the website, it appears that there is a T&G ceiling above a two-story open foyer where you're experiencing the ice dams. Is that accurate for your house? If so, is there an air barrier behind the T&G? Probably poly? I'm asking because there could be a pretty high pressure differential there from the stack effect. The T&G doesn't stop much air flow, and if the air barrier behind it is punctured, you could be leaking a lot of hot air there. I'd be interested to see what a smoke pencil might reveal.Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Dunno about your garage, but lots of garages are open to the rafters, so there is no "attic" to speak of. These generally aren't ventillated, and so any heat load generated by the sun shining on the walls or roof builds up. Even if the ceiling is closed, there's generally no insulation, and minimal venting.
It's pretty typical for ice dams to be worse on the north side, since it's more in the shade. Since heat rises, the warmer air will collect near the top of the attic area, and it will be cooler towards the eaves. All it takes is the right combination of sun and air temp to get the snow melting rapidly near the peak but still freezing near the eaves.
Re the valleys that have ice dam problems, I'd bet that the majority have a southeast exposure -- the snow on the upper east-west part will be getting the sun in the afternoon, while the lower part of the valley is in the shade.
Valleys can be a PITA with regard to ice.
It would be interesting to know the exposures. Even if you did everything right, you can have problems when, eg, a valley is shaded in the late afternoon while the adjacent roof is in the sun. In such a case a heat cable down the valley is probably the most reasonable solution.
Eerie...
This is exactly what happened to me this winter. Built a new addition last summer which intersected my existing metal roof causing a steep valley (8/12 pitch meeting a 23/12 pitch).
We had huge snows this winter that just kept accumulating on the roof. Ice dams backed up melt water under the metal, up over the grace ice/water shield, down the sheathing, into the soffits, over the plates, and exited under the wall onto my subfloor (fortunately--no floor coverings yet!) Eventually, my ice dams actually ripped the new metal roofing panels to shreds at the base of the valleys.
My thinking also is that this may be unavoidable unless the snow is removed during the winter. I'm opting for a heat cable in the spring after the repairs are made.
Big mistake using a metal roof in a valley. This is true I swear. I worked on a crew doing insurance repair in this area for over 10 years. I live in heavy snow country and almost always a roof repair is on a metal roof and usually involves a valley or a chimney at the eave. It doesn't matter how well your roof is insulated or vented. Lots of snow will build up in a valley and will not be able to shed. This results in crushing and damaging the metal, shearing off screws and often causing ice dams. Around here we always use ice membrane up a valley at least 5' up each side. Only greenhorns that hired an out of town architect and builder use metal roofs. If you are already considering replacing your roof then jet the metal and go with comp. Otherwise you will end up replacing your metal every other year depending on how it makes it through the winter.
I have 50 year comp on my own roof. The framing is engineered to hold a 100 year storm. The shingles hold up well under deep snow. I don't plan on going up there for another fifty years.
So are you saying the metal is bad because the snow slides easier and gets to greater depths in the valleys? I didn't do metal on my own roof because, at 12:12, there's a hazard zone below the eaves with a metal roof. Each year up here a few kids are hurt (and sometimes killed) when a big snow load slides. I couldn't see plantings / fencing off 6' wide x 130' perimeter around the whole house.
So I went with arch-grade comp shingles and the snow stays put. Regardless of outside temps, it stays on the 12:12 until it melts in the spring (i.e. April-May). And I never have icicles hanging off the eaves, which I take as a marker for roof problems. But my ceiling is R-70 and I was very careful that the venting was continuous, thorough and with generous openings. I spent a fair amount of time staring at each bay and visualizing the flow of venting air. And if I didn't feel good about a particularly circuitous path, I'd drill a few more holes.David Thomas Overlooking Cook Inlet in Kenai, Alaska