I've built many retaining walls and garden beds with class 2 hardwood sleepers (2.4m x 200mm x 50mm). A few weeks after installing them in a vertical position with no gap between sleepers, a significant (over 10mm) gap develops between the sleepers.

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As shown in the photo, the sleepers are screwed into the steel posts, so it doesn't seem likely that they are moving away from each other.

I assume that they are shrinking? If so, why? These aren't new sleepers - sometimes they've been lying around for months before use.

  • 1
    Strongly related.
    – FreeMan
    Jan 1, 2021 at 15:50
  • It's worth saying that those dinky screws would not stop movement due to warpage in those timbers.
    – isherwood
    Jan 1, 2021 at 16:29
  • 1
    the lumber is shrinking. Purely natural event. When there's high moisture (rain, humidity) in the climate the joint will probably shrink (close).
    – ojait
    Jan 1, 2021 at 19:27
  • @isherwood What screws would you recommend? Those screws are Buildex Climaseal 12-11 x 45. I don't think I could use anything longer given the thickness of sleeper?
    – lithic
    Jan 1, 2021 at 20:10
  • There's nothing wrong with the screws you have there, but they're intended to hold position with respect to the ground, not prevent small seasonal movement in the wood grain. I wouldn't attempt that.
    – isherwood
    Jan 2, 2021 at 0:25

1 Answer 1


What you are showing is bare wood apparently without any treatment or coating. In that case the wood will shrink and swell due to humidity and, since this appears to be outside, rain, snow, etc.

Wood changes dimensionally the most at right angles to the grain. In your case the grain is going horizontally and so the dimensional changes will happen most in the vertical direction. That, of course, is exactly what you are seeing.

If you don't want this to happen, there are two options that I can think of:

  1. Treat the wood (usually this means primer, paint) with a product that will seal it against humidity and moisture. This will likely need to be redone periodically.
  2. Replace the wood with a product that is not subject to moisture.

Otherwise, what you have is entirely normal. You could just live with it as it is.

  • 1
    Absolutely correct. As noted many times at Woodworking, there really are very few products that can totally seal wood against moisture, so planning and allowing for some wood movement is just the thing you do when working with wood.
    – FreeMan
    Jan 1, 2021 at 15:52
  • Thanks for the answers. What seems strange, then, is why the sleepers are always in their non-shrunk state when installed and they shrink afterwards?
    – lithic
    Jan 1, 2021 at 20:02
  • 1
    UK/British English 'railway sleepers' are what Americans call 'railroad ties'. Used ones sold by the railway would be decades old and treated with something like creosote and would surely have done all the shrinking they were going to do? Jan 1, 2021 at 21:05

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