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Roof furring questions

jackofalltrades777 | Posted in Green Building Techniques on

When it comes to furring a roof, are 1×3 or 2×4 furring strips better to use? I assume the latter because they are stronger/wider and give a better grip area to the sheathing above. Spacing should be what: 12″ or 16″ on center?

My other question is with the sheathing above. I read that plywood is better than OSB although there has been a lot of problems at the lumber yards with plywood cupping. The lumber yard attendants said that the QC on plywood is really bad because it is so rarely used. I assume 5/8″ sheathing is ideal for a metal roof?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Peter, What's the purpose of the furring under your sheathing? Are you using it for cross ventilation between stud bays?

  2. jackofalltrades777 | | #2

    Malcolm,

    I am pricing out doing a furred out roof above a SIP roof.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Peter, the choice as to whether to use 1x or 2x for furring is probably best decided by how deep a ventilation cavity you need. Both have stronger holding for fasteners than the SIPS sheathing.

    If you are using 5/8" sheathing above you can go to 24"oc furring without using H clips.

    Something is wrong if the lumberyard's 5/8" plywood is cupping. Something not inherent to the material.

    Advantech is a better choice than OSB, although it might be an idea to find what the SIPs are sheathed with before making a decision. If they are OSB, then why not the roof sheathing?

  4. mtr7982 | | #4

    In our area 2x4 is far cheaper than 1x3 / 1x4 so we use 2x4 for most furring roof/wall

  5. GBA Editor
    Martin Holladay | | #5

    Peter,
    I agree with the earlier comments. It sounds like the purpose of what you call "roof furring" is to create ventilation channels. I wouldn't use anything less than 2" thick (nominal) lumber for that purpose, because if your ventilation channel is less than 1 1/2" deep, it isn't doing much.

    For more information on this type of ventilation channel, see How to Install Rigid Foam On Top of Roof Sheathing.

    Plywood is better than OSB, but I would buy my plywood from a different lumberyard. I don't think that 5/8" plywood is ideal, but it will work. Of course 3/4" plywood (or solid boards) would be better, but the builders who used to install solid board sheathing on roofs are no longer with us, unfortunately.

  6. Expert Member
    MALCOLM TAYLOR | | #6

    Given that by most codes you can span 24" with 1/2" roof sheathing, what governs the thickness of the plywood is the roofing manufacturer's specs. All the snap-lock roofing I've seen asks for 1/2" plywood too, so 5/8" is already an upgrade. 3/4" would be wonderful but my back and my wallet would object.

  7. GBA Editor
    Martin Holladay | | #7

    Malcolm,
    I'm sure you're right. I'm just an old-time Vermont builder who is used to rough 1-inch-thick board roof sheathing. That's why 5/8-inch plywood always seems flimsy to me. But I belong to a lost generation, I guess.

  8. jackofalltrades777 | | #8

    Yes, the roof furring is being used to create ventilation channels for the tops of the SIPs. The SIPs use 7/16" OSB as their skins and 8" polyurethane cores.

    Malcolm you mentioned the AdvanTech sheathing and I read up on it and there is a lot of positive reviews. It is supposedly even better than plywood. They have some documented tests showing AdvanTech sitting in water and it never took on bulk water unlike the standard OSB and plywood that did. Lowe's sells AdvanTech on special order for $25 per 4x8 sheet. It is made by HUBER and uses moisture resistant/waterproof glue to hold the strands together. Some interesting tests show that it outperformed plywood which cupped and took on water:
    http://mobilehomerepair.com/phpbb/viewtopic.php?f=6&t=6670

    So 2x4's running vertically from ridge at 16" oc. Do they make ridge & fascia vents that would resist fire ember intrusion?

  9. Expert Member
    MALCOLM TAYLOR | | #9

    Peter, Being in the PNW I don't give wildfires much thought. Vents are probably the Achilles heel of a roof assembly where you are. You would think someone in the Southwest would have come up with an ember resistant vent, but I've never heard of one.

  10. jackofalltrades777 | | #10

    Interesting: I learned today that AdvanTech OSB is made by Huber who also makes Zip Sheathing. The AdvanTech OSB they make outperforms plywood and they have the tests to back that up. Pretty cool and innovative stuff. Looks like the finally make an OSB product that doesn't swell, cup and rot as quickly as the mainstream OSB does now.

  11. LucyF | | #11

    Peter,
    I think most contractors use AdvanTech for subfloor (at least in my area). We built our house during a monsoon season - a summer where we had an inch of rain almost every day for months. I had to sweep the floor every day to get rid of the rain. We even had to drill some holes in the floor to let the puddles drip out. I was so worried about the subfloor warping, but it never did. Here are some photos of the wet subfloor. I sealed the seams between each panel with Prosoco Joint and Seam Sealer, but I should have done that later in the building process.

  12. jackofalltrades777 | | #12

    Is it possible to just have the dip edge eave vent and NOT have a ridge vent?

    Ideally I know having a ridge vent would be best but with a 2x4 furring strips and a 1.5" eave vent, there should be adequate airflow getting inside of that area to allow for any drying.

    I just can't see water penetrating the metal roof, then the peel & stick membrane underneath, penetrating the 5/8" OSB sheathing and then getting onto the SIP sheathing. That's 3 layers of water proofing. Even so, airflow will still be happening in that 1.5" vent channel between the SIP and sheathing above it.

  13. jackofalltrades777 | | #13

    Lucy,

    Thank you for the photos. Yes, I am sold on AdvanTech OSB sheathing. I read up on all the studies and it blows away plywood in regards to waterproofing and in preventing cupping/warping. Plywood will take on more water and warp more quickly than the AdvanTech would. It comes with a 500 day exposure guarantee and a Lifetime Warranty on the panel itself after it is covered.

  14. Expert Member
    MALCOLM TAYLOR | | #14

    Martin, We build mostly with junk. I've just got used to it.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Peter wrote: "Is it possible to just have the dip edge eave vent and NOT have a ridge vent?"

    Unfortunately not. Unlike a rain screen wall, the cavity relies on air movement, not diffusion, for drying. It needs an inlet and an exit. It also needs a differential in height to create this movement. That's why vented low slope roofs are problematic, and why building codes generally specify what percentage of the vents must be at the eaves, and how much at the ridge.

  16. jackofalltrades777 | | #16

    Malcolm,

    If at the ridge peak the vent channel is open and then drops down the other side of the roof/rake, wouldn't that create a continuous vent channel from eave to eave and create an inlet and exit for the air? Plus at a 5:12 pitch from eave to ridge and ridge to eave, that should suffice on the height differential, correct?

  17. GBA Editor
    Martin Holladay | | #17

    Peter,
    I agree with Malcolm. If you are going to the trouble and expense of installing ventilation channels above your SIP roof, it makes sense to get the details right. Include a ridge vent.

    Why are you reluctant to include a ridge vent?

  18. jackofalltrades777 | | #18

    Martin,

    The area is a wildfire risk area. If it was the big city or a suburban area, I would have no problems putting in a standard ridge vent. Unfortunately in fire areas ridge vents or any attic vents for that matter is the Achilles heel of a fire resistant home. Thousands of embers can and will enter the vents just like light blowing snow would enter a gable, ridge, soffit, etc, vent but unlike light fluffy snow that will just melt, these embers will burn your house down. Studies show that 80% of all homes that burn in a wildfire are NOT caused by the flame approach but by drifting embers from miles away that make their way into attics.

    It takes a different building approach when building in fire country. There is nothing green about building a home only to have it burn down to the ground. If there is a ridge vent that will prevent embers from showering through them, I would like to know. Otherwise I have to resort to not having a ridge vent and just using the eave vent to channel air.

  19. GBA Editor
    Martin Holladay | | #19

    Peter,
    Using the search function, I find that you are an expert on the topic. In January you posted this comment:

    "Embers make the same journey that light blowing snow does. The snow won't do much harm but fiery embers will burn your house down. In a wildfire thousands of embers will take this journey through the soffit vents and into attics and land inside the attic. Then it's goodbye home. ...In fire country I believe that unvented roofs are the wise way to go."

    So perhaps you should take your own advice and build an unvented roof.

  20. jackofalltrades777 | | #20

    Martin,

    Yes, unvented roofs are the easiest way to go in fire country but a "cold roof" or a vented channel for a SIP is said to be best practice by some. So the debate rages. Creating a vented channel for a SIP with a fire resistant vent would be ideal. After more research I think I found a solution. They make a product called Vulcan vents which resist fire embers and flames from entering via roof vents.

    In California new building codes in some areas mandate fire resistant vents on a home since so many homes burn to the ground from wildfires. This Vulcan vent might be the solution I have been looking for. I hope to resolve this conflict once and for all.

    Going back to my original questions. They have been answered. 2x4 furring is better and 5/8" Advantech OSB is top notch sheathing for a roof. Only the fire resistant vent issue remains but I think I may have found the answer.

    I do appreciate all the tips and advice. They have been very helpful.

  21. JanaRPruitt | | #21

    I really appreciate the given idea.

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