Decking expansion gaps

Discussion in 'Carpenters' Talk' started by masonst0rm, Aug 3, 2006.

  1. Micky D.

    Micky D. New Member

    I put my decking down 9 years ago using a treated soft wood once down I used a Cuprinol decking seal and have done so every spring, I was advised not to have a 10 mm gap as coins and other slim objects can be lost through the gap so I opted for a 4 mm gap. during the winter my boards expand every year the gaps close and the boards buckle. short of dismantling the deck and staring again, 2 years ago I used a skill saw to open the gaps when they were most swollen this helped but when small stones get wedged in the gaps the boards still buckle.
    I built my decking in such away that to dismantle it completely to realign the boards was going to be difficult and to long a job. Unfortunately this year I discovered that some of the support framework has rotted and I am forced to uplift parts to repair the supporting frame.
    The reason I am on this Forum is because having reached this point I thought I would enquire as to what the right gap would be and not one of the above suggestions has been helpful, intention now is to have a 5mm gap
     
  2. Micky D.

    Micky D. New Member

    Chippie 244 hi.


    I think it's a wind up for Handyandy, why pay extra for fluted boards.
     
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    The problem is they don't think too much. Above is an example. Top of board is likely to dry first, then cup up at the edges. Nice little cup to stop water from running off the side(even if it is just 50mm away).

    Anyway, I'm sure I mentioned somewhere here that when firsat laying decking boards, you should estimate the expansion by considering the weather and the present size(width) of the boards. Example, new boards freshly tanalised may be near their maximum size already(wet) - fit with little or no gap if fitting in cool weather.
    If already dry when fitting in warm weather they may be at their smallest, so fit with 6mm gap.
    Measure the width of the boards yoy have bought against what size(nominal) they are sold at. Example, you buy some 140mm width boards, when they arrive freshly tanalised they measured 145mm width. You will expect them to shrink when dried - fit tight or no gap. Etc
    Different conditions mean you can't suggest a gap for a 'fit all' situation.

    Mr. HandyAndy - Really
     
  4. chippie244

    chippie244 Super Member

    Wood cups with the rings.
     
  5. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Wood cups also when top dries and bottom is wet.

    I'll tell you this. You take a cupped deck board up, turn it over in the sun - it straightens out! Drying side shrinks - fact.

    Mr. HandyAndy - Really
     
  6. chippie244

    chippie244 Super Member

    If you've got the grooves on the bottom it won't be wet.
     
  7. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    It will. Because the top dries and cups and keeps the water in.

    Mr. HandyAndy - Really
     
  8. chippie244

    chippie244 Super Member

    What water, you have just said the top is dry?
     
  9. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    No I didn't. I said 'the top dries out'.

    When it dries out, it cups. When it rains, the rain fills the cup. It can't run off the sides.
    The cup won't flatten until both top and bottom are of equal humidity/wetness/dryness.

    Mr. HandyAndy - Really
     
  10. chippie244

    chippie244 Super Member

    That makes no sense Andy, according to you the wood cups when the top is dry but won't un-cup until the bottom is wet.
     
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I never said any such thing.

    Mr. HandyAndy - Really
     
  12. chippie244

    chippie244 Super Member

    What did you mean by "When the top and bottom are equally wet"?
     
  13. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    If the board has been wet throughout for a while(like in the winter never really dries properly) the boards may even out and lose any cupping.
    Wnter time (as long as the boards have ample expansion room) you will find they are as flat as flat can be. Equal humidity.
    But as I said, if you turn a board over, as the bottom dries(to match the top) the cup will even out and the board will flatten(equal dryness top and bottom).

    I hope this is the end. I am quite tired of stating the bleedin' obvious.

    Mr. HandyAndy - Really
     
  14. chippie244

    chippie244 Super Member

    So the boards aren't cupped when wet and they aren't cupped when dry?
    Who turns a decking board over?
    Your argument is an utter fallacy.
     
  15. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Nobody turns a decking board over. I'm just telling you why the boards cup(drying,shrinking), and to prove it, a board turned over will flatten because the wetter side will then dry. Nothing to do with any rings. Just wetness in one plain or the other.
    There's no fallacy there, it is fact.

    Mr. HandyAndy - Really
     
  16. chippie244

    chippie244 Super Member

    So are you saying that wood doesn't cup according to the rings?
    I was saying that on a board laid with the grooves on the bottom the bottom won't be wet because the water will reach the first groove and drop off, the same as it does when it reaches the drip on a cill.
    Have you pulled up a board to see what it does and why?
     
  17. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Yes i have. That's why I'm telling you facts. A board turned over and dried will flatten!

    And your theory of water dropping off the first 'drip' is balderdash. Wood soaks through with water. If you have a cup, the middle of the board will hold water, some will evaporate, some will soak down through to the underside. The underside will stay wetter for longer, hence cupping. In the winter it may be thoroughly wet all the way through, so evening out the cups.
    Middle and edge cuts will have some effect on how boards cup, but I think moisture is greater.

    Come on man. Give it up.

    Mr. HandyAndy - Really
     
  18. chippie244

    chippie244 Super Member

    What do wooden barrels, wooden ships, wood shingles, log cabins, dug out canoes and canadian kayaks have in common?
    You said the wood cups when it dries not when it gets wet, if it cups and then gets wet it will un cup. According to your "logic" if it's dry on top but wet underneath it will cup and hold water so then it is wet on the top and bottom and will un cup so it's only cupped when it is dry when it doesn't matter and when it's wet it isn't cupped so it doesn't matter.
    Surely if it was cupped it also would be more like a gutter so you wouldn't need the grooves?
     
  19. wiggy

    wiggy Screwfix Select

    I worked for a decking company for years and after laying 1000s of m2 of decking in every possible situation, in climates from Ireland to Africa, from softwood to hardwood to composite decking, using various screws, hidden fixings, plugging etc
    I think I have good authority to say groove side down every time, for all the reason stated in the above posts by chippie, only people that buy timber from wickes put it groove side up and if you're buying timber from them you don't have a clue anyway so it doesn't really matter what way you put it down.

    As for gap, we always went with a standard 4mm but these days I tend to take it on the particular job and the state of the deck boards.
    But 9 times out of ten I use whatever spacers I have, normally 6mm ply.
    The gap isn't just about water run off but also air flow.

    The grooves in decking where originally kerfs to stop the boards cupping and over the years those kerfs have developed in to the grooves we now know.
     
  20. chippie244

    chippie244 Super Member

    Thank you Wiggy. :cool::cool:
     

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