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My son is making a backyard deck of about 6 x 6 metres for my daughter. I am too old and doddery to help much, but am advising. Now we find that some of the planks (400 x 23 cm composite) are curved by up to 2mm, that is like a banana lying on its side.

The standard plank spacing is 5mm, and we have invisible clips for this. But if two adjacent planks happen to be curved the wrong way the gap could be up to 9mm at the centre of the planks, which will look horrible.

It seems that we must select planks as we lay them to minimise this effect, but it's not trivial to sort and reverse 4 metre planks, weighing 10+ kilos, to find the best fit. Most of the time my son will be working alone.

Does anyone have any tricks or advice to make life easier?

Update 14 July: It turned out that we can (un)bend the planks with just a few kilos of force, applied with the knee. Thanks everyone for thinking along.

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    @isherwood I misread my notes, they are 21cm wide. Here is a link. Sorry it's in dutch gadero.nl/…
    – NL_Derek
    Commented Jul 9 at 8:24
  • Thanks. Dutch isn't a problem in the modern web. Instant translation. That width will make it more stiff, but 2mm should be manageable.
    – isherwood
    Commented Jul 9 at 12:39
  • I have been building a deck and was boggled by the crown in some of the boards since they are composite and I presumed would be perfectly straight. Unfortunately, they are not. Then I found a device called a Bow Wrench from Cepco Tool. Amazon and the box stores have them. I do not own stock in the company. This device has made my installation MUCH easier and the gaps in the boards uniform. It is worth the money even for one deck. Good Luck!
    – ArchonOSX
    Commented Jul 14 at 23:00

4 Answers 4

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Put them down, wedge them over with a pry bar, and screw them in.

Decks don't need gap tolerances like Japanese automobile panel fits.

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2mm (~1/8") isn't a bend in the carpentry world. I'll be surprised if you find any that aren't curved at least that much just due to milling (manufacturing) tolerances. It's your job as the carpenter to straighten them to your satisfaction.

Tiger Guy's right. Here are some tips to make that easier:

  • Fasten one end using multiple screws (on the first two joists or whatever).

  • Skip 2-3 joists and set the board there. It's easier to bend a board over a slightly longer distance, and you won't see any remaining curve in that span.

  • Fasten the far end before you get too close to it. End positions are more critical than the middle, so leave enough free length so you can move it to the correct position without a struggle and so it doesn't affect the next board.

  • Don't use spacers. They simply transfer error from board to board. Instead, eyeball from one end or use a string line to verify straight every couple runs.

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  • tape measure works pretty well too for verifying that you have them straight, as long as you measure from the same reference points for every board so you don't have the same creeping error problem you do with the spacers.
    – Perkins
    Commented Jul 11 at 19:58
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Get a board bender. You tack one end of the board down and the move down the board bending it into place with the tool and tacking it at each joist.Board Bender (https://www.deckwise.com/deck-board-straightener.html)

Board Bender (https://www.amazon.com/CAMO-Lever-Board-Bending-Locking/dp/B084VW3NVY)

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  • A ratchet strap can help pull things into line too. Not a friction-locking motorbike strap, a good truck/trailer tiedown with a clicky handle. And if that's unavailable, look at a spanish windlass made with a rope and any two lengths of timber/pipe. I've seen a dead car pulled out of a river with a rope and two tree branches, time and muscle power only.
    – Criggie
    Commented Jul 9 at 22:33
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    A new hand tool I've never seen before!
    – nigel222
    Commented Jul 10 at 9:12
  • :) You should see my shop. Ill buy something if I think it will help me on a job, and then hang it up so I can remember that I have it next time. I have too many specialized had tools.
    – Mike Vonn
    Commented Jul 10 at 14:33
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    Me too, and not just hand tools. I value my time. It doesn't take many hours saved to make a (cheap but good enough) power tool worth buying.
    – nigel222
    Commented Jul 11 at 9:09
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    Not to be confused with a board stretcher!
    – Matthew Leingang
    Commented Jul 11 at 18:34
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Rotate them all 90 degrees about their long axis so they are on edge. Line them up. Order them from the highest to the lowest center deviation. Install in that order, spaced evenly at the ends. You’ll see minor deviations at about the 1/4 and 3/4 lengths, but the ends and centers should be near perfect, and the deviations will have a nice symmetry about the center, with some aesthetic randomness evident. (If you really want to be neurotic, tweak the results by flipping them about any of their three axes and reorder as needed so the the 1/4 and 3/4 deviations are as minimal as possible. But you may find that you get it so good that the few places where gaps are off are noticeable and distracting. Perfect can translate to no character.)

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    (If there is an “up” or top side you have to use, it may limit the fineness of variation).
    – Nalacram
    Commented Jul 9 at 4:44
  • this is exactly my problem, 36 square metres is a lot of big, heavy planks and we don't have the room (or energy) to sort them like that.
    – NL_Derek
    Commented Jul 9 at 8:37
  • In which case you can measure the center deviation of each plank one at a time, write it on the center with an arrow, then lay them one by one in order from low to high or high to low. With such a small variance, a digital caliper might help, they aren’t expensive. You’ve gotten a couple independently viable solutions here, from different sources. Only you can provide the energy.
    – Nalacram
    Commented Jul 10 at 3:45

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