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Attic ventilation needed for a 100-year-old slate roof in a coldish climate?

slateandall | Posted in Energy Efficiency and Durability on

Hi all,

First time owners looking at 100 year old house in NE US (climate zone 7).

Beautiful slate roof, very steep (making the attic much smaller than 1300 sq foot footprint of house).

Attic is totally not vented.

Inspector said to look into venting it to help roof last longer.

My question is, it’s lasted this long (I’m sure it’s had repairs along the way) is it safer to leave as is or look into venting?

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  1. user-2310254 | | #1

    A couple of questions:

    - Can you tell us where the house is located?
    - Is there any insulation in the attic?
    - What do you mean that the attic is "totally not vented?" Can you elaborate?

  2. slateandall | | #2

    -Southern New York, not far from ocean.

    -There is insulation in floor of attic.

    - there is no ridge vent or gable vent or any visible vent.

    -the attic is easily accessible from the 3rd floor through a normal wood door. I don’t know if that counts as venting

  3. GBA Editor
    Martin Holladay | | #3

    If the slate roofing was installed the traditional way -- over skip sheathing -- then the roof is air-permeable and vapor-permeable. No venting is necessary for this type of roof, because everything evaporates readily to the exterior (especially when the sun is shining).

    If your roof has plywood or OSB roof sheathing covered by 30-pound asphalt felt, my answer would be different.

  4. slateandall | | #4

    Understood. How do I get information on the sheathing. I assume the issue is not what was done in 1915, but rather on subsequent repairs?

    Can a roofer tell me, or do I need to ask roofers who did repairs?

    Btw, for what it’s worth roof slopes east and west.

  5. GBA Editor
    Martin Holladay | | #5

    Go into the attic with a flashlight. Look up at the rafters. The boards between the rafters are called sheathing.

    If you can't tell what you are looking at, take a photo and post it here.

  6. user-2310254 | | #6

    Just as an FYI, you are not in Zone 7. You can identify your climate zone by clicking on the map located on the main Q&A page.

  7. slateandall | | #7

    Thank you for pointing out the zone error! I am actually climate zone 4A or 5A.

  8. Expert Member
    Dana Dorsett | | #8

    Find your county on this map:

    Yellow is zone 4, green is zone 5, blue is zone 6.

    If you're less than a half-hour to salt water it's almost certainly zone 4.

    Skip sheathing for slate shingles or cedar shakes looks like this:

    Depending on age may or may not have asphalted felt between the shingles & sheathing as seen in the pictures, but asphalted felt is not a vapor barrier, and still allows the sheathing to dry into the air gaps in the slate. Without the felt you will be looking at slate between the sheathing planks.

    If you're looking at OSB or plywood between the skip sheathing, or if it's ship-lap sheathing without gaps it will take a bit more investigation to figure out the assembly stack up. There are many 100 year old homes with the original slate and roof sheathing, but also many that have been re-done, sometimes with a layer of OSB or plywood.

  9. slateandall | | #9

    Just looked at inspector report. It says that sheathing is “tongue and groove “. The very blurry picture in the report seems to back this up—I don’t see any gap between the boards. What now? Thanks for all your help!

  10. Expert Member
    Dana Dorsett | | #10

    If it's really t & g or shiplap sheathing adding some sort of venting may be worth considering, but ONLY if there is apparent high humidity in winter or visible mold/moisture damage on the rafters, etc.

    If the plank sheathing has gone unvented for 100 years without causing moisture issues in the attic, let's not steal a defeat from the jaws of victory just because we would build it with venting if building it new today. If it looks pretty good, smells pretty good, with no punky rafters or decking, leaving it alone is a good option.

    Air sealing the attic floor/upper-floor ceiling to prevent wintertime convection transported moisture would still be a good idea though. A round of blower-door and infra-red imaging guided air sealing would probably be cost effective on almost any 100 year old house if it hasn't been done already, and that will also mitigate against future moisture issues in the attic. Venting the attic without having done that initial air sealing can create higher stack effect draws of conditioned space air into the attic, potentially wetting attic-floor insulation near the air leaks.

  11. GBA Editor
    Martin Holladay | | #11

    Dana gave you good advice. If you don't see any signs of moisture problems -- black stains, mold, or dripping nail points -- you don't have to make any changes to your attic.

  12. slateandall | | #12

    Thanks guys! Not worried about the roof any more BUT:

    Wondering if attic ventilation would materially help to cool the house.

  13. GBA Editor
    Martin Holladay | | #13

    Q. "Would attic ventilation materially help to cool the house?'

    A. No. For the full story, see "All About Attic Venting."

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