Replies

1. 
   EricPaulson | Sep 10, 2009 01:35am | #1

   You would do better to determine what is causing the ice dams and proceed from there.

   There is likely a lot of heat escaping that is warming the underside of her roof. I may be wrong.

    

    

   "When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896

2. 
   seeyou | Sep 10, 2009 01:58am | #2

   My question is, do the older homes with slate roofs have ridge vents?

   Typically not, unless they've been retrofitted. But, many homes with slate roofs have gable or static vents. And a good slater can add a ridge vent if desired.

   Would there be any benefit to installing soffit vents if there is no ridge vent?

   For a venting system to work properly, it must consist of inlet and outlet.

   I am no roofer and certainly don't want to mess with the slate.

   Then find someone who knows what they're doing.

   What determines how many vents are needed. I assume more is better?

   There are lots of charts available, but that always has to be tempered with appearance and roof/cornice design considerations. Placement might be more important than "more".

    

    

   copper p0rn

3. 
   popawheelie | Sep 10, 2009 01:58am | #3

   I would want to vent every rafter bay. And I would want to use rafter chutes.

   http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/96/961110.html

   It gives the air a chute to follow and you can insulate below it.

   The problem with some roofs is the area above the top plate is to close to the bottom of the roof.

   Can you vent out the top at a gable end instead of at the ridge?

   "There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves."
   Will Rogers
4. 
   WayneL5 | Sep 10, 2009 03:30am | #4

   Ice damming is caused when the roof is warm, melting fallen snow.  The water runs down the roof then encounters a colder portion of the roof (the overhang) and freezes, usually under the accumulated snow, forming an ice dam.  The solution, therefore, is to keep the main roof as cold as possible so the snow does not melt any faster over the heated building than it does at the eaves.

   So to accomplish that, more soffit vents would be better.  Then the air has to circulate, so at a minimum gable vents should be provided.  If that does not work then a specialist could install ridge venting the following summer.

   There must be free air circulation, so insulation at the wall edges must not touch the underside of the roof deck, blocking the air flow.  Two inches minimum is required and can be achieved with barriers that are easy to install.

   Also important, possibly more so, is to reduce heat entering the attic.  So there must be adequate insulation thickness throughout the attic.  Plus, no holes that could allow warm air through, such as around pipes and wires.  Lighting fixtures, however, must not be covered unless they are IC (insulation contact) rated.  No heating ducts should be in the attic.  If there is a chimney the space around it must be blocked, but anything within 2" must be non-combustible.  So flashing with sheet metal and caulking with appropriate caulk would be the way to go.

   An expensive solution would be to spray foam insulation on the underside of the roof deck.  In some cases this would be the better solution.

5. 
   joeh | Sep 10, 2009 03:41am | #5

   First thing to look at is the attic floor.

   Soffit vents to get rid of the heated air escaping from the living space are not the answer.

   And where is the air from the soffit vents going to exit? Ridge vents too?

   Joe H

6. 
   DanH | Sep 10, 2009 04:24am | #6

   Generally, the rule of thumb is one square inch of vent for every 500 square feet of roof, divided more or less evenly between soffit and rooftop. But whatever you can do is usually better than nothing.

   It should be noted that the typical slate roof (especially an old one) is going to be naturally less airtight than most other roof types, and may supply enough rooftop ventilation without added ridge or rooftop vents. (Soffit venting is still likely to be needed, however.)

   The choice between rectangular and circular is one of appearance and effort. You need several large circular vents to equal one rectangular one in area, but with the circular vents you can spread out the vent space more evenly (and having the space more evenly spread is good). Every third rafter bay is the bare minimum.

   As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
1. 
   fingersandtoes | Sep 10, 2009 06:17am | #7

   "the rule of thumb is one square inch of vent for every 500 square feet of roof"

   Shouldn't that be inches and inches or feet and feet? Even at that it's very low. Our code has 1/300 as the minimum.

1. 
   DanH | Sep 10, 2009 06:34am | #8

   Yeah, you're right -- it should be inches and inches or feet and feet. And the rule has varied over time and with locality, so 300 is likely code in some areas.1/300 would work out to about one square inch vent for every two square feet roof, which is at least an easy amount to figure. A standard 8x16 rectangular vent has somewhere around 100 square inches of clear area -- ie, it can handle about two "squares" of roof.Should also mention here that some attention must be paid to the possibility of insulation blocking the vents. Depending on construction techniques and the type of insulation used there may be insulation in the eave area behind where the vents would go, blocking vents unless properly dealt with.
   As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz

7. 
   hammer1 | Sep 10, 2009 03:45pm | #9

   In the Northeast, most prefer continuous soffit and ridge venting. The venting is only one part of the "system". You need to stop the heat loss which is the job of the insulation. One without the other doesn't do the trick.

   Beat it to fit / Paint it to match

1. 
   DanH | Sep 10, 2009 03:55pm | #10

   More important than the insulation is sealing air leaks through the ceiling. Especially likely in older homes, before drywall.
   As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz

1. 
   hammer1 | Sep 10, 2009 03:58pm | #11

   That goes without saying.Beat it to fit / Paint it to match

1. 
   DanH | Sep 10, 2009 07:17pm | #12

   Except that it's important that it be said.
   As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz