I created a cheap but reasonably attractive bench using cinder blocks and pressure treated 2x8 boards. One of the boards is curling severely. I was wondering if anyone else has had this problem using this type of bench before? I’m guessing I will have to just replace the board. I’m wondering if the ends need treated after cutting?
I will make nicer looking wooden ones eventually, but I’m not sure where exactly they will live permanently so I’m testing locations using these benches for now. I also thought they could be dismantled and made into a cold frame rather easily this winter.
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If you can find Kiln Dried After Treatment (KDAT) lumber this will be less likely to happen.
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I don’t like using treated wood for bench tops because of all the caustic chemicals used on them to inhibit rotting. I like using Redwood boards, or the cheaper option of Cedar 6’x5.5” fence planks.
If you’d like to avoid cupping, try to sort through the lumber for something as close to quartersawn (QS) as possible. Avoid any pieces with the center pith though, it can be very unstable as you have found. Depending on your lumberyard, you’ll likely find some QS cuts mixed in with all of the other more common flatsawn lumber.
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Green treated pine or spruce or fir will curl. If you place them one block down with cinder block on top it can minimize the Warp or twist of the board.
Also would put more space between trees and make viewing the two levels easier. 
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Yeah, also purchase from a quality lumberyard who sells quality dried lumber. Most of the stuff sold at big stores like Home Depot is dripping wet and itching to warp as it dries.
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Most woods are not sufficiently dried any more. + having them in full sun significantly increases the issue. I had 9cm thick maple cup within 2 hrs. And Aaron’s graphic is spot on.
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@imager993 I just started bonsai in November. I needed quick and easy so I went to Home Depot and figured this wood would be less likely to do exactly this. I’m not a fan of chemicals either. Thank you for the information. I will build proper benches later but I’m really trying to not kill and make trees at this point in time as well as see what parts of my yard the trees like sunlight/wind wise and I can move these benches easily.
I would screw the 2 boards with a slat. Every 50cm one should improve the actual situation and prevent from further crawling.
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I would have done that too, batten screwed on the underside to lock them together. Can still keep the few cm gap between the boards. Might help a bit.
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Pressure treated is usually green and subject to curl. What I have done in the past to correct for the curl is to scew ( not nail) the warped boards to a 4x4, or 6x6 section to force them in to some kind of straightness.
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You can also use more narrower boards including ripping the current ones into narrower potions and the the curl (twist) will cause fewer issues. One problem with screwing (or nailing) the boards to a batten or other cross-wise piece is that the tendency is to use 2 or more screws at each location. This inhibits the ability of the board to expand and contract across the grain with changing moisture content and will often speed up cracking. It is better to use a single screw at each location - perhaps driven on the side of the board that is high to pull some of the twist out. Another option from furniture making is to use slots that are perpendicular to the grain to allow for expansion and contraction.
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On the topic, does anyone know where the video of Ryan making benches is? I can’t seem to find it on YouTube. It was one of the first videos I remember seeing of Ryan and Mirai.
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Looks Great, My boards have curled as well, over time they seem flatten back out, just have to put some heavy trees on there and get some weight on both boards like you have on the top/back bench
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The garden design video has lots of images of monkey poles and benches. Not actual construction, but good views of the structures.
I have also used a video by Bjorn that shows him making his garden.
video is about 9 minutes long.
Have fun making your garden unique.
@SatoriBonsai thank you. I might do that and add in dropping a level like bonsaibob recommended also.
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I have used the cuts to the underside of a board with some success. The problem usually occurs because of your wood source. They tell you they have kiln dries treated lumber but I have taken home some really green wood that warped in days to become rocking chair feet material. The best method I have found is looking at all the wood stored in the lumber yard and see if it is laying flat in storage. If they are selling green wood to turn a quick buck the yard will be filled with twisted stock. Then it’s time to look for a new source.
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