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Raised bed and decking

AndyEvAndyEv Posts: 1
in Garden design

Hi guys, this is my first post here and I am after some advice. I am about to start a raised bed and decking project as I have been able to get hold of loads of thick gluelam wood for free. My concern is how to waterproof the wood as the bottom and 1 side will be in constant contact with soil. I have searched all over the Web and some people recommend lining the wood with plastic, others say that this actually traps water against the wood and speeds up rot. What are your ideas/experiences? Any advice will be gratefully recieved.

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  • pansyfacepansyface Posts: 22,309

    HELLO ANDREW,

    THANK YOU FOR INTRODUCING ME TO THE TERM "GLUELAM", WHICH I HAD NEVER HEARD OF BEFORE. I WAS VERY INTERESTED TO SE THAT OUR WINTER GARDENS IN SHEFFIELD ARE MADE FROM IT. A VERY ELEGANT BUILDING.

    SO FAR AS I CAN SEE IT IS USUALLY MADE OF SPRUCE OR LARCH. I WOULD GUESS THAT ALTHOUGH IT HAS BEEN TREATED WITH GLUE AND PRESSURE AT HIGH TEMPERATURE THE TIMBER ITSELF HAS NOT BEEN PRESSURE TREATED WITH ANY KIND OF FUNGICIDE. SO IT WILL ROT JUST AS NATURAL TIMEBER WILL ROT UNLESS IT IS TREATED AGAINST WETNESS AND FUNGUS.

    I WOULD SAY THAT IF YOU GAVE IT SEVERAL COATS OF SOMETHING LIKE BLACKJACK IT WOULD HAVE AS GOOD A LIFE AS A PIECE OF NORMAL TIMBER. WHICH ISN'T SAYING VERY MUCH.

    HOW ABOUT USING RAILWAY SLEEPERS FOR THE PARTS WHICH WILL BE BELOW GROUND?

    Apophthegm -  a big word for a small thought.
    If you live in Derbyshire, as I do.
  • raisingirlraisingirl Posts: 6,897

    You can line the raised beds with thick plastic as long as any moisture that gets into the gap between the plastic and the wood can drain away, i.e. don't wrap the plastic around the base of the gluelam. A thick paint of blackjack on the bottom surface that will sit on the soil is a good idea.

    For decking I'd lay a sheet of Damp Proof Membrane plastic over the ground before you start and leave narrow gaps between each board so there's plenty of air movement below, between the DPM and the deck. I'd use bricks or blocks as feet for the decking structure so if water pools on the DPM around those points, the wood itself isn't sitting in it. Again, blackjack on the underside of the timbers - especially where they sit on the blocks - is a good idea.

    Two things to remember: Timber can cope with getting wet as long as it can breathe - get plenty of air around it - which will naturally dry it out. And plastic sheeting will crack if it's exposed to sunlight for any length of time.

    Last edited: 18 March 2017 09:16:44

    “It's still magic even if you know how it's done.” 
  • hogweedhogweed Posts: 4,053

    I assume you intend to make the raised beds on top of the decking? Or are they going on top of soil? If they are going on top of soil they do not need a bottom, only sides. If they are planters on top of decking, they need to be raised up from the deck on legs for drainage and have plenty of holes for drainage in the bottom.

    'Optimism is the faith that leads to achievement' - Helen Keller
  • AHRAHR Posts: 361

    I used slate chippings under my sleepers so the underneath would stay dry and water would soak away. 

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