Build up low deck joists?

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duckDecoy

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#2849773 13-Jan-2022 12:06

chevrolux:

I'd highly doubt you'd actually notice though if it's a matter of a few mm.

This is always true, but only AFTER you've done a job. DURING a job its impossible to think these things can be left to slide.

mdf

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#2849843 13-Jan-2022 12:39

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.

mdf

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#2849846 13-Jan-2022 12:41

Found the video:

https://youtu.be/6l_uj20v7c0

He talks about testing tape earlier in the deck series and laying the composite later in the series.

nickb800

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#2849848 13-Jan-2022 12:43

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

Jase2985

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#2850065 13-Jan-2022 14:26

Paul1977:

Thanks for the feedback guys.

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?

Paul1977

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#2850168 13-Jan-2022 15:03

Jase2985:

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?

When I thought about it some more you're right, it doesn't complicate it.

Using Outdure ResortDeck, bottom not scalloped - just different textures on each side.

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Paul1977

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#2850172 13-Jan-2022 15:11

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?

mdf

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#2850233 13-Jan-2022 15:48

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.

mdooher

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#2850237 13-Jan-2022 16:17

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?

Matthew

mdf

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#2850277 13-Jan-2022 18:52

mdooher:
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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