Shimming on top is possible, but you will end up with a lot of shims. I'd usually aim to shim between bearer and joist. If it is only a few mm, I would whack a bar in to loosen the nails then slide a shim in afterwards to bring it up a couple of mm (don't remove any fastenings).
If that's not possible, joist tape would be another good option but someone mentioned tape reacting to composite? I haven't come across that before but probably worth testing on an offcut. I'm sure I've seen at least one YouTube video with composite decking and joist tape - probably Robin Clevett. But would depend on exact composition of tape and decking.
Shims under the deck plank/joist interface - sounds like a lot of work! Especially if planning/cutting so they don't show between planks, but probably the way to go.
You could just hold them in place with masking tape or spray-on adhesive as they'll be screwed through when you screw the planks
The plastic shims sound like the easiest solution. The composite decking uses a propriety hidden fixing system which slightly complicates it, but I'm sure I can make it work.
Thanks again.
Im not sure how it makes it complicated? most screw down between the boards
Timbertech make joist tape for their composite decking
What brand are you using? is it scalloped on the bottom?
mdf: Shimming on top is possible, but you will end up with a lot of shims. I'd usually aim to shim between bearer and joist. If it is only a few mm, I would whack a bar in to loosen the nails then slide a shim in afterwards to bring it up a couple of mm (don't remove any fastenings).
If that's not possible, joist tape would be another good option but someone mentioned tape reacting to composite? I haven't come across that before but probably worth testing on an offcut. I'm sure I've seen at least one YouTube video with composite decking and joist tape - probably Robin Clevett. But would depend on exact composition of tape and decking.
Thanks, I'll have a look and see if that's a possibility as well. I need to have a more thorough look in the weekend with a nice long straight-edge. The only thing with tape is that it's usually pretty thin, so would surely need a lot of layers?
Yeah, you're not getting 5 mm out of tape, but joist tape has a bit of thickness to it. It's sort of rubberised, I think probably butanyl or similar? A couple of strips would add up to a mm I think. Similar to DPC but self adhesive and its (supposed to at least) "self heal" around the fastener holes.
2-3 mm ish is awkward territory. Probably too much for tape, but you'll be hard pressed to cut a packer to run along the joist that thin without it splitting. But also enough of gap that you probably do want to fix it to avoid waves in the deck.
I would still be aiming to raise the joists if you can manage it, though might not be possible.
why not just attach a new piece along the side of the existing joist but a bit higher... the same way you would correct rafter sag if putting in new purlins?
why not just attach a new piece along the side of the existing joist but a bit higher... the same way you would correct rafter sag if putting in new purlins?
Deck joists usually have dwangs or nogs to help stop twisting and bowing. Though there is probably a different name for them if installed between joists instead of studs...
Might also affect the screw spacing symmetry but sounds like the OP is using hidden fasteners.
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