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Roofing felt bubbled up because of cold weather?

  1. Apr 20, 2008 #1
    Recently I was helping a friend of mine nail sheets of felt on a roof, and we finished applying the felt in the afternoon when it was warm. We came back the next morning to nail the shingles on the roof, and the felt was bubbled up. It about 20 degrees colder that morning than it was the afternoon of the day before, when we nailed the felt on the roof.

    He told me that the felt bubbled up because it is cold outside. I know that most materials shrink when they get cold. However, if the shrinkage was caused by the felt's contracting due to the cold weather, I don't understand why it would bubble up. Let's assume that there is a sheet of felt that is 10 feet long by 10 feet wide. Why wouldn't the felt just shrink to become, say, 9.99 feet long by 9.99 feet wide square, but still remain flush with the roof surface without bubbles?

    If the length and width of a sheet of felt becomes smaller, but the extra length and width just become bubbles, the felt isn't really getting smaller per se. It's just being crumpled up.

    Assuming that felt shrinks, as opposed to becoming crumpled up, wouldn't that just make it have a smaller length and width without bubbling up?
  2. jcsd
  3. Apr 20, 2008 #2


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    Perhaps the side exposed to the air shrinks in the cold and this bends the material so it ripples.
  4. Apr 20, 2008 #3
    I'm still confused.

    Yeah; that's what my friend told me.

    But you never addressed the crux of my question. I know that cold weather is not really going to shrink felt a whole foot, but for the sake of clarity, let's say cold air shrinks the felt by 1 foot. We will assume that the felt is originally 10 feet long by 10 feet wide. The cold weather shrinks the felt to 9 feet long by 9 feet wide. If the felt is actually shrinking, I would expect that the 1 foot that has disappeared on both ends would not become a ripple. If the felt is actually shrinking, the 1 foot should just disappear, rather than show up as a ripple.

    Example #1 Original 10 feet of felt = _____

    Example #2 Felt that has ACTUALLY shrunk to 9 feet = ____

    Example #3 Felt in which the LENGTH is = __/\__
    9 feet, but the there is a foot
    of felt in a bubble

    In example #3, the length of the felt from one end to the when there is a bubble is only 9 feet, but if the felt was stretched out, there would be a full ten feet of length.

    Wouldn't cold weather make felt example #2, rather than example #3?
  5. Apr 20, 2008 #4


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    The top surface has shrunk to 9.9ft while the bottom is 10ft, that has bent the material - stretching the bottom surface. The extended bottom surface has then popped-up off the roof.
  6. Apr 20, 2008 #5
    probably the sun was hitting it at a more direct angle, and even though it was 'colder', the blackness of the felt paper absorded more sunlight and heated up (expanding) in those morning hours
  7. Apr 20, 2008 #6
    Do you have roofing boards or plywood covering? If roofing boards, are the bubbles more like long rinkles aligned in the direction of the roofing boards?
  8. Apr 21, 2008 #7
    Maybe. I would guess that the temperature of the air would affect both the top surface and the bottom surface almost exactly equally.

    Another poster thought that the heat from sunlight on the top surface is what's making the difference, and that sounds more plausible to me.
  9. Apr 21, 2008 #8
    It was roofing boards, but I don't remember how the bubbles were aligned. This all happened three weeks ago.
  10. Apr 21, 2008 #9
    Thinking about this more, I think that you people are right on track. I think that the bubbling up has something to do with the top side and bottom side of the felt being different temperatures and contracting at different rates (as opposed to both sides of the felt contracting at the same rate).

    I don't entirely understand what's going on here, but I think I have a vague idea of it.
  11. Apr 21, 2008 #10
    I'd disagree. Bubble are a likely result of trapped gasses.

    No one has yet considered the thermal contraction that could occur in the substrate. For that matter, we don't know if the substrate is older, cured tar-felt or wood.

    Does tar expand on curing as presumed?

    But more likely is to focus on the mechanical nature of the fibre composite. How do the quazi-aligned fibres in the felt react to applied heat, and subsequent cooling. They will relax toward their shape before being pressed into felt. The felt is probable manufactured by pressing between a pair of rollers under tension. In relaxation, the pad would get thicker, contract along the dirction of greatest fibre alignment (the roll length), accompanied by possible contraction in the remaining direction (the roll width).

    Most of us have seen the effect in a piece of paper that has gotten wet then dried, where wringles often occur in the direction of the long axis of the paper. Paper consists of fibres maintained within a binder. How do the wrinles happen? It seems to be caused by tension along one axis.
    Last edited: Apr 21, 2008
  12. Apr 21, 2008 #11
    The substrate was wooden roofing boards. Doesn't that rule out the possibility of the bubbles being the result of trapped gases?

    I don't think that the felt got wet that night.
  13. Apr 21, 2008 #12
    I'll add some more to my 'thoughts'---

    When the roof felting was put down, it was probably done in the shade (no thermal effect). If the felting was in the sun the next morning, the temp on the felt rose dramatically, being black, and expanded dramatically also.
  14. Apr 21, 2008 #13
    Not really. I was referring to gas as a result of outgassing.
    What you call felt is tar coated felt, like they make hats out of. Wetting paper allows the binder to release from the cellulous fibers that make up the paper. In like manner heating the tar allows the fibres to move around changing the shape of the tarred felt. These fibres naturally want to move due to their resilence like little springs that have been bent from their relaxed position during the manufacturing process.
  15. Apr 22, 2008 #14

    I'm a tile setter, not a physicist. What is outgassing?
  16. Apr 22, 2008 #15
    Hi to you, rewebster.

    I must say, it's an amazing thing discussing bubbling roof felt to such an extent.

    Outgassing would be, for instance, from the volatile components of the tar, like methane and butane and stuff coming out of the tar. I'm just speculating on all of this like everybody else. Now I'm beginning to wonder if moisture in the air could expand the top surface of the tar, and cause it to arch where it wasn't put down securely.

    Did you put the tar down with a propane torch?
  17. Apr 22, 2008 #16
    But it's still somewhat interesting, at least to me.

    No; the felt was nailed down with a hammer. We never used a propane torch at all.
  18. Apr 23, 2008 #17
    No problem. After all, in the relativity section their hot on the trail of rolling grapefruit at the speed of light.
    Well... I'm not only baffled, but have been on the wrong track. This changes everything. I've been watching too much of the home improvement channel.
  19. Apr 23, 2008 #18
    I've heard of propane torches being used when laying down roll roofing, but I've never heard or seen propane torches used to lay down roofing felt. On the other hand, I'm not a professional roofer.

    I believe that tar is one of the ingredients of roofing felt. Could outgassing be what caused the bubbles? If not, what do you think caused the bubbles now?
  20. Apr 23, 2008 #19
    Sorry, I'd been taking this as a theoretical excercise.

    If your primary interest is to get a good roof down, where the reason for the bubbles is secondary, type 'roofing felt bubbles' into your search engine. I got a few hits.

    From what I've read there is some skill involved in laying out felt to avoid wringles and bubbles. You're supposed to brush it down with a broom. It takes some doing to get it to conform to a surface. You can imagine that any temperature variations causing it to tension in one direction will cause oblong bubbles or wrinkles to align in that direction. Anyway, in ten minutes of reading, you'll know more than I.
    Last edited: Apr 23, 2008
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