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Building a new deck and would like your opinions re cupping

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  1. #1
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    Default Building a new deck and would like your opinions re cupping

    Hi all,

    Deck spec:

    -35 sqm approx.
    -400mm approx. above ground level, attached/butt against back of house, footings required as above lawn (we're excavating to provide 400mm between ground and bottom of decking)
    -Basic rectangular shape
    -135 x 19 mm spotted gum
    -Either 6mm (head size) x 70mm or 80mm Spax screws, these: Multi-purpose stainless steel screws SPAX - Pre-drilled and countersunk
    -Treated pine sub-floor (we're in Melbourne)
    -Verandah will cover 75% approx. of deck
    -8 or 9mm gap between boards

    What do you think the chances of above spec deck cupping over time?
    Are the above screws a good choice (with respect to the cupping dilemma) or would you go for a bugle batten screw?

    Cheers, John



  2. #2
    JB1
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    What guage is your 6mm spax screw equal to?

    I wouldn't use bugle screws are they are pretty ugly.

    I'd use 3x 10g x 50mm screw for a 135mm board wide to prevent cupping.

    However, to be honest, I think the standard 86mm wide boards look better (and also have less issues with cupping), and a 8-9mm gap is too big even for 135mm wide board.

  3. #3
    4K Club Member Marc's Avatar
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    Boards cup for a variety of reasons. Too wide and not thick enough. Too moist on one side and not the other. Poorly sawn, poorly seasoned, poor quality wood, where did the tree grow, what variety and another half a dozen reasons.
    One thing is for sure, if the wood wants to cup there is no screw on earth that will prevent it. Not even a 12mm bolt.
    For 135mm wide, 19mm is marginal. Go for 25mm.
    If you have the right timber you can lay it down the joists with no screws at all and it will not cup.
    Science is never settled,
    it advances one funeral at the time.
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  4. #4
    Je pense, donc METRIX's Avatar
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    Quote Originally Posted by John7 View Post
    Hi all,

    Deck spec:

    -35 sqm approx.
    -400mm approx. above ground level, attached/butt against back of house, footings required as above lawn (we're excavating to provide 400mm between ground and bottom of decking)
    -Basic rectangular shape
    -135 x 19 mm spotted gum
    -Either 6mm (head size) x 70mm or 80mm Spax screws, these: Multi-purpose stainless steel screws SPAX - Pre-drilled and countersunk
    -Treated pine sub-floor (we're in Melbourne)
    -Verandah will cover 75% approx. of deck
    -8 or 9mm gap between boards

    What do you think the chances of above spec deck cupping over time?
    Are the above screws a good choice (with respect to the cupping dilemma) or would you go for a bugle batten screw?

    Cheers, John


    Your gap is too big, aim for 5mm, Bugles are a nono way too BIG, you can go for either TRIM head or standard 10G
    70 / 80mm length screw don't waste your time, they won't do any more than a 50mm screw.

    Oil the backs of the boards before installing, then oil the tops, this will equalize the moisture content in the boards, and help against cupping.
    Agree with Marc, if you can get 25mm boards do this, if you can't then you will have to go with 19mm, three screws in wide boards are a better option, add I would go for TRIM heads as these are more discrete than 10G.

  5. #5
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    Thanks, gents.

    JB1, I think the 6mm spax is somewhere between 12 and 14 gauge.

    "One thing is for sure, if the wood wants to cup there is no screw on earth that will prevent it. Not even a 12mm bolt." - Marc

    "
    I'd use 3x 10g x 50mm screw for a 135mm board wide to prevent cupping." - JB1
    "with 19mm, three screws in wide boards are a better option," - METRIX

    Marc's opinion seems to contradict JB1/METRIX's.

    I'm basically trying to roughly measure the chances of boards cupping over time with my deck spec (as detailed in my first post). I don't want to use 135 x 19mm spotted gum if there's a reasonable chance of it cupping.
    I take it 86 x 19mm spotted gum is much less likely to cup - thoughts?

    I've read (and have told) that merbau rarely cups, even at 140 x 19mm - thoughts?

    How about 135 x 19mm blackbutt? Much the same as spotted gum?

    Cheers



  6. #6
    3K Club Member johnc's Avatar
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    If you are worried about cupping stick to 86mm boards, I personally prefer 86 to 135 anyway, 50mm trimhead screws are all you need. Also Marc has written about the Camo system which I have also used and it provides a tidy result without visible fixings.
    Merbau is very stable, reasonably priced and a long lasting material.

  7. #7
    4K Club Member Marc's Avatar
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    Cupping is one of those contentious issues, however the proof is in the pudding. If you ever go to the Wisemans Ferry Pub, go and see their newish deck. Built 4 years ago and screwed down with 14g bugle head batten screws (just in case) yet cupped anyway. The upward and continuous force the timber can exercise is phenomenal and the screw either gives up or buries its head in the timber. Even the outdoor furniture built in the same way with the same timber is all cupped. Wide and thin board can be prevented from cupping if you cut a groove under the board up to halfway with a circular saw or a V cutter and a router. This will cripple the board and the swelling of the underside will not have any dramatic effect. Floorboards where once offered with a groove or even two grooves under them. This is of course highly impractical for the installer.
    Science is never settled,
    it advances one funeral at the time.
    Max Planck

  8. #8
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    Thanks gents.

    Think I might go the 86 x 19mm spotted gum...
    If I do, what size gap between boards would you go for (I know METRIX has written 5mm)?
    Thinking I'll use 10 gauge (approx.) screw. Should I go for 50mm or 60/65mm?

    Ta

  9. #9
    JB1
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    50mm screws.

    I've installed with a 3mm gap with my 86 x 19mm spotted gum.

    You can go 4, but I wouldn't do 5mm. Metrix said 5mm based on the 135mm board.

  10. #10
    JB1
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    Quote Originally Posted by johnc View Post
    Merbau is very stable, reasonably priced and a long lasting material.
    Fortunately John7 said he is using spotted gum, which I think is a very good timber. And unlike Merbau isn't destroying the environment. Plus, it can't be bad for the Aust economy to use Aust sustainable timber.

  11. #11
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    Quote Originally Posted by JB1 View Post
    50mm screws.

    I've installed with a 3mm gap with my 86 x 19mm spotted gum.

    You can go 4, but I wouldn't do 5mm. Metrix said 5mm based on the 135mm board.
    Ta, JB1.

    Yeah, that's right re Metrix and 5mm - my mistake.

  12. #12
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    Aim for 3-4mm min for narrow boards (these will shrink and open up another .5 - 1.0mm in about 12 months)

    Wider boards means wider gaps, I personally don't like 19mm wide boards, 25mm is preferable, but with Merbau I have seen 19mm wide board decks done and they have been fine after many years, as said earlier Merbau a bit more stable, I could not see an Ironbark 19mm deck lasting long without cupping,.

    There is many reasons for choosing Aussie HW and not Merbau, such as Leeching, rain forest destruction, but mostly spotted gum is a beautiful timber.

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    Cheers Metrix.
    It is indeed a beautiful timber. I considered ironbark 86 x 19mm, but only momentarily because I'm guessing spotted gum is more stable and I think it's slightly more beautiful.

    This is most probably a silly question, but I'll ask in anyway. How important is it that the timber is under a roof prior to purchase?

    Cheers

  14. #14
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    Quote Originally Posted by John7 View Post
    Cheers Metrix.
    It is indeed a beautiful timber. I considered ironbark 86 x 19mm, but only momentarily because I'm guessing spotted gum is more stable and I think it's slightly more beautiful.

    This is most probably a silly question, but I'll ask in anyway. How important is it that the timber is under a roof prior to purchase?

    Cheers
    They are both beautiful timbers, I prefer Spotted gum, it has more colour variations.
    I would prefer timber which has been stored under a roof, or inside, saying that, if it's stacked correctly and not thrown in a heap such as at Bunning's then it should be ok.

    See below, this was an extension to an existing Spotted Gun deck I did, the weathered timber is also Spotted Gum, but just not looked after for a while, the new timber looks fantastic, and the old timber should be recoverable, but it's best to not let it get like that.

    7.jpg

  15. #15
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    That looks brilliant!

  16. #16
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    Does look good. You've posted photos of that deck a couple of times if my memory serves me. What size boards are they Metrix (WxD)?

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