new roof creating ice dams where old roof did not

Reply

  #1  
Old 02-10-11, 10:33 AM
Member In Good Standing
Thread Starter
Join Date: Jan 2008
Location: usa
Posts: 159
Received 0 Votes on 0 Posts
new roof creating ice dams where old roof did not

This is my third winter of battling an ice dam issue on the north side. My 1977 home previously had cedar shakes. These were replaced with traditional fiberglass Certainteed landmark triple-layer shingles along with the felt and the ridge vent. At the same time the house was sided with James Hardie siding, fascia, and their soffit with vents. I later added a 4" round louvered soffit vent to each rafter bay. I did this because the Hardie soffit vents were not installed properly. They just installed the new soffits directly over the old ones and the venting did not line up with the old vents... When I complained they drilled holes through the new vents and then through the old soffit above it. I was not comfortable this was an effective solution, and added the round vents.

Anyway, the old roof always had significant icicles at the eves, but never had any ice dams. I am trying to figure why I have them now. Is it because the new shingles lay flat on the roof and simply transfer the escaping heat better, thus melting the snow and allowing the resulting water to run underneath the snow build up; and the shakes allowed better air circulation and just a bit of insulation? Or, do you think it is because the roof is not being vented as well as before?

I am not able to add any more insulation to the roof as it is mostly cathedral and no reasonable way to increase the insulation. I was able to add some insulation to the area where the worst of the ice dams form. It did not make any difference. This leads me back to thinking the cold roof venting system is the problem.

The venting set up was never completely effective as I have always had the icicles. It is the ice dams I am worried about.

Any suggestions? Thanks
 
Sponsored Links
  #2  
Old 02-11-11, 05:56 PM
Temporarily Suspended
Join Date: Jul 2008
Location: NY
Posts: 10,986
Received 0 Votes on 0 Posts
I'm in a house built in the 20s which has no insulation, no soffit or ridge vents & the same shingles that you have now. There is some ice build but nothing that bad. Based on that, my guess is that the venting has nothing to do with it.
 
  #3  
Old 02-11-11, 07:08 PM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,947
Received 0 Votes on 0 Posts
I am certainly not an expert in this area, but everything I've been reading about ice dams sayd they are caused by warm roofs. Typically a result of not enough insulation and/or poor venting. I think you are on the right track with what you think the cause is. I would add this to your hypothesis: the new asphalt shingles you put on get much hotter from sunlight than the cedar shingles did. I think that is what is causing the the snow and ice on your roof to melt during the day and refreeze at night.
 
  #4  
Old 02-12-11, 12:14 PM
Member In Good Standing
Thread Starter
Join Date: Jan 2008
Location: usa
Posts: 159
Received 0 Votes on 0 Posts
drooplug, I agree the sun is the major culprit. No doubt there is some heat leakage/penetration/ex-filtration through the insulation and roof. However, nothing really changed except the swap form shakes to shingles and possibly the ventilation. We get a lot of intense direct sun and I think the darker shingles cause most of my problems.

I am trying to figure out how to resolve as I really cannot add insulation. While I can increase the eave or soffit ventilation, it would be at a loss of aesthetics and require a fair amount of work. I am willing to do it if it solves the problem. I just don't want to do it just to see if it works. I am somewhat doubtful ventilation will solve the issue. I think I am going to have the problem until I either run a bunch of ugly roof heat tape or grow too old to care.

Can anyone say fairly conclusively that increasing the soffit vent size will indeed make a significant improvement?
 
  #5  
Old 02-12-11, 12:53 PM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,947
Received 0 Votes on 0 Posts
It's more than just adding soffit ventilation. You need to have enough flow and the right kind of flow from the soffits and out of the top of the roof. Keeping your roof cold will solve your ice dam problem. I think buildingscience.com will have some papers on ice dams. It's been awhile since I poked around on there.
 
  #6  
Old 02-12-11, 03:44 PM
Member In Good Standing
Thread Starter
Join Date: Jan 2008
Location: usa
Posts: 159
Received 0 Votes on 0 Posts
I recognize there is a need for flow of air from the eave to the ridge and out. I once knew all the ratios of the # of sq inches per foot of intake and outlet etc. The set up I have has more than the necessary at the eave, on paper. But, and this is a major but, it is dependent on there not being any obstructions along the pathway. Since I did not personally build the house I cannot know for certain whether the pathways are continuous. I once was able to see a part of the pathway. I installed some Velux roof windows and removed the interior sheetrock, insulation and could see from the eave to the ridge. The system was done correctly in those specific rafter bays that I was able to inspect. However, that does not mean the whole roof system was done correctly.

I also saw that the original roof was done incorrectly. I had hail damage and needed to replace the shakes. When they were removed I saw that the original felt actually went over the ridge venting. Meaning that the roof sheathing was correctly stopped about 2" from the ridge on each side. This is to allow the air to exit to the ridge vent system. However, the felt was incorrectly run all the way to the top, right over this gap, thus covering the air exit path at the top. When the roofers redid the felt for the new shake roof I made certain they did it right and left the airway open at the top. Then they installed a proper ridge vent.

Then later, when the shake roof was replaced a few years ago with the new Certainteed shingles I was very specific in my instructions to the roofers about leaving the vent space open at the top. And they installed the new ridge vent system. However, I did not actually inspect their work. So, once again, I do not know for certain the ridge vent was done correctly. This coming summer I will climb the roof and pull off some of the ridge cap vent system and see if it was done correctly.

I am not sure what else to do. Is there another way to verify the vent system was done correctly???
 
  #7  
Old 02-12-11, 04:36 PM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,947
Received 0 Votes on 0 Posts
I'm not sure I have any ideas for you. I have noticed that after a fresh snow fall, the areas around vents are the first to melt on a roof. That could be a good starting point, but it may not indicate that you are getting full flow through the vents. It sounds like your attic is finished so there isn't much you can do to beef up you insulation without tearing down your drywall. Seeing how your roof is new, it will be some time before it gets ripped off again so you won't be adding anything on that side either.

I'm really surprised that you are on your third roof now. I would have thought you would have gotten 20 years out of each one.
 
  #8  
Old 02-13-11, 08:00 AM
Member In Good Standing
Thread Starter
Join Date: Jan 2008
Location: usa
Posts: 159
Received 0 Votes on 0 Posts
The first shake roof was about 12 years before hail got it. The second shake roof had to come off when we did an addition. They no longer allow shakes due to the number of forest fires in the area. Did not want shingles on the new and shakes on the old.

When we get bigger storms the ridge vent is covered. It is always the last part of the roof to get covered. May be several days to a week or two before it melts off the ridge. Depends on temperature, winds, and sun. If the venting is working then I wonder whether it is shut down during this time, or if it works its way up through the snow. Might be why the ridge is always the first part of the roof to show itself, from the warm air exiting and melting the snow. I always assumed it was the sun and wind. I will have to re-think this.
 
  #9  
Old 02-13-11, 08:23 AM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,947
Received 0 Votes on 0 Posts
It probably is best to get up there and try and see if it was installed correctly as you suggested before.

I have an old house so I've been looking at people's roofs and watching how the snow melts off of them compared to mine and others around. It's interesting to see who has more insulation than who just by the way it melts. You can even see where the rafters are on poorly insulated homes.
 
  #10  
Old 02-13-11, 10:42 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,429
Received 26 Votes on 24 Posts
Hi jeweler, sounds like you are having a lot of fun so decided to join in.

A couple of details to expand on, those 4" round vents represent about 12 sq in of area. However, their effective ventilation area (EVA) is around 20% of that number. It seems air does not like to flow through small holes as easily as it does through bigger ones. So when those vents and your new soffits become a bunch of little holes, there is a reduction factor that needs to be applied. And, if I read the thread correctly, you have two layers of holes for those soffits. Bottom line, you may not have as much area as you thought.

For a ball park, you are looking for 1 sq ft vent area for each 150 sq ft of attic floor or in your case, the ceiling below, half high and half low. Although the low can be the larger of the two. On well air sealed homes with proper vapor barriers in place, these numbers can be cut in half, but since you are dealing with a problem issue, I would hesitate to use the lower limit.

Next is the actual air flow. The paradox with natural ventilation is, it is supposed to remove the heat, but you must have heat for it to work. What that is saying is, there will necessarily be some temperature increase remaining, even in a working system. Fan forced systems can exhaust all of the heat, a possible option.

Add to all of the above, the cathedral ceiling with its narrower air path, possibly longer chutes, and the air pressure you need to accomplish venting will be larger.

So, it all adds up to basically what you are thinking. The new roof and the new soffits have reduced your air flow and are increasing the solar snow melt. The performance of the ridge vent is still to be determined, but from the lack of melting snow over it, something is probably not working correctly.

Question: Have you undertaken any specific air sealing measures. Leaking air will transport much more energy (heat) than what otherwise conducts through your insulation. Can lights are a common source.

Since spring will be a much better time for any work, consider removing the current snow and keeping it clear for the rest of this winter. Do any testing needed while it is cold to check air flow, like smoke testing those soffits to see how strong the pull is.

Good luck
Bud
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: