Sounds like you need more Ice & Water Shield. My parents had a similar problem- we ripped off the shingles and put I&W up the entire slop (around 12′), just to be safe. Haven’t had a problem since (12 years).
As a note, typically you want the I&W up at least 3′ past the wall of the house. If you’ve got deep overhangs, the 3′ of I&W that you have isn’t enough.
Bob
Fine Homebuilding Recommended Products
FastenMaster Screw Bolt Fastening System
These solid, time-saving bolts don't require pre-drilling and come in very handy when it comes to guard rail posts.
MicroFoam Nitrile Coated Work Gloves
These gloves are comfortable and have a good grip, making for splinter-free work.
Angel Guard Deck Demon
Demo has never been easier with the deck demon. The tool keeps the worker in an upright position and uses leverage to remove decking from framing, a must have tool for demo.
Portable Wall Jack
Raise walls safely with the Guardian wall jack — we’ve personally seen small teams raise large walls with these devices, and they work great.
Wirefy Crimping Tool
This crimping tool from Wirefy is high-quality and reliable.
Replies
Ice dams are frequently a symptom of inadequate insulation, or uninsulated heat ducts in the attic. That's what I'd check first.
Andy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Code around here (SE PA) tends to be 24" past the inside wall. That's measured on the level so when transfered to the roof slop plus any overhang a single run of I&W often won't cut it. A good roofer can pull out the eight rows of shingles or so and lay in I&W as needed. Depends on the age of the shingles of course as any color matching would be difficult at best.
Thank you for your input. The dams do form above where I rake, but I am hopeful that I can find a solution by more venting of the attic.
Regards,
Bruce
My attic floor is well insulated, but some of the other replys make me lean toward incresing the ventilation at the end of the attic where the dams seem to be forming. I added an addition at that end, and lost one of the end vents, and I also fiddled with some of the insulation at that end in the center. That combination of event might just be at the root of the problem. Thank you for your reply.
Regards,
Bruce
Focusing on the roof itself is treating the symptoms, not the root cause.
Figure out how warm air is getting into the attic space. If the attic space is cold, the ice dams will go away.
I will also urge you to consider the attic insulation. In addition and not secondary, you need to be sure the attic is well vented so that any warm air that enters will leave without melting the snow.Jeff
Thank you for the input. Although my attic floor is well insulated I did lose an end vent when I put on a new addition recently, and the dams seem to form near that end. In addition there are some insulation gaps at the center of that end of the attic that may be contributing to some attic warming. I will vent that end more extensively and work on the insulation gaps and keep my fingers crossed.
Regards,
Bruce
I like the way you think. Other inputs suggest the same solution, and in thinking about it that might just be at the root of the problem. as the dams started after I put on an adddition that reduced the venting at one end. Thanks for for the input.
Regards,
Bruce
Assuming that you don't have any "messy" areas like complicated valleys, the ice dam forms as follows:
Snow melts up on the roof, then the meltwater flows down to the roof edge. The edge (where the eaves overhang) is colder, so the water freezes again. This creates a dam, causing water to build up behind.
The basic cause of this is having the roof at two different temperatures: Warm near the top and cooler towards the bottom. In general (ignoring some of the "messy situations") this temperature difference is due to heat coming from inside the house. The eave area is cooler because it's not over the house or (in some cases, especially finished attics) because less heat escapes near the edges of the roof.
The solution to this problem is twofold: First, improve the insulation in the house so that less heat escapes into the attic area, and second improve attic ventillation so that heat that gets into the attic can escape without warming the roof so much.
The main "messy situation" that can also lead to ice damming is the case of a complicated roof pattern where one part is in the sun and another part is shaded for a significant part of the day.
"Ice and water shield" (rubber membrane applied under the shingles) will provide extra insurance, and may be necessary for the "messy" cases. It generally shouldn't be your first line of defense, though.
One thing that will help you understand the situation is to observe the pattern of snow melt on your roof. On a well-insulated roof the edge areas will generally melt first, but on on a poorly-insulate, poorly-ventillated one the center will melt before the edges. With a finished attic ("half story") you may notice a line of melting right along the middle, where the corners of the rooms come in close contact with the roof, preventing ventillation and limiting the amount of insulation that can be installed.
Thank you. Your reply was helpful. The floor of my attic is well insulated, and it is vented pretty well, but when I added an addition recently I lost one of the end vents at the addition wall. The dams seem to form near the end that lost the vent. I think maybe if I open up the soffits more at that end it may help. Worth a try. Thanks.
Regards,
Bruce
In addition, check especially where the two sections join to make sure there are no air leaks into the attic. These can be around pipes and wires, etc, but a biggie is when there is a difference in ceiling height, resulting in part of the stud cavity being open to the attic.
Good input. I have not finished insulation the common wall between the attic and the addition side. The addition side wall is heated and could well be a source of attic warming.
Regards,
Bruce
Bruce, it might be worth your while to buy a couple of old FHB articles from the main site. Two that I'd consider are Fixing a Cold and Drafty House, and Fixing a Damp and Moldy House. They're related. Also search on Mark Rosenblum - he's got a good article on air sealing. Air sealing is at least as important as insulation or ventilation.Andy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
One solution that has worked for me in my prior home was an attic exhaust fan wired so that I could control it with a manual switch in addition to the thermostatically driven one. When ice dams threatened I could run the fan and keep the attic cold, thus no melting. My neighbor's house, built by the same builder would have ice dams when I didn't. Not as efficient as sealing all the leaks, but a lot quicker.
Quicker, yes. But every attic ventilation scheme carries an energy penalty, power venting most of all. Particularly in the winter, the highest pressure in the building envelope will be at the upper ceiling, due to the stack effect (think chimneys and warm air rising. The greater the temperature differential, the greater the drive from warm to cold). A vented attic increases the pressure differential across the ceiling plane, and facilitates air leakage from inside to the attic. This warm air wreaks all sorts of havoc, from ice dams to attic condensation to global warming. OK, that's not all your fault <G>. Your fan may suck enough cold air in to the attic to prevent ice dams, but the smart money says that it's also sucking a lot of your heating dollars out into the wild blue.Andy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Thansks for the input. I am hopeful that I can solve the problem without a shingle redo.
Regards,
Bruce