Build up low deck joists?

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Paul1977

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#293310 13-Jan-2022 09:21

I have a sub-frame that the joists aren't all the same height, it looks like some various from when they were milled. I'd like to get them as consistent as possible before laying the decking on top. I know the standard approach is to plane down the high ones (and any other high points), but I don't have a few that are too high - but rather a few that are too low.

Do I need to plane all the others down to match the few lower ones, or is there a material I can put on top of the low ones to raise their height instead?

Thanks

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#2849680 13-Jan-2022 09:38

How much would you be packing them up by? Could you get strips of timber (perhaps H3 cavity battens) and rip these down on a table saw to suit (or fix in place and plane down to suit). You would probably want to apply a preservative like MetalX to the cut edges.

Alternatively, could you disconnect the low joists from the bearers and pack them up? Just need plastic packers and some good fixings e.g. Z nails of CPC cleats?

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Paul1977

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#2849705 13-Jan-2022 10:23

nickb800:

How much would you be packing them up by? Could you get strips of timber (perhaps H3 cavity battens) and rip these down on a table saw to suit (or fix in place and plane down to suit). You would probably want to apply a preservative like MetalX to the cut edges.

Alternatively, could you disconnect the low joists from the bearers and pack them up? Just need plastic packers and some good fixings e.g. Z nails of CPC cleats?

They aren't out enough to rip down timber to lay across the tops, probably just a few mm (which may be within acceptable tolerance?). But I'm laying composite, which I've read shows variations more than other decking materials. Because of diagonal strap bracing that's been installed (recommended by composite suppliers) I don't think it would be an easy task to disconnect any of the joists (at least not easy for me).

I actually had a builder do the subframe for me late last year (I wasn't confident enough to build it myself), the idea being that I'd lay the top myself. He said he'd checked the levels at the time, but it's now been a couple of months (I hurt my back so had to delay laying the top) and I went to double check it before starting last night and noticed the differences.

And I just had another thought... Because of massive supply issues he couldn't get the H3.2 90x45 he usually uses so had to use H4 100x50 rough sawn, and I'm wondering now that perhaps this wasn't sufficiently dry at the time of installation. Could the variations be caused by them drying at different rates from being out under the sun? So if they were level when built, might they even out again after more drying time?

I'll contact the builder, and see what he thinks as well.

duckDecoy

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#2849706 13-Jan-2022 10:24

In the past I have cut and layered up pieces of damp course onto the joist under where each decking board will go to get them level, nailed the pieces to the joist with small nails, and then put the decking timber on top as per normal. You can nail through the damp course when nailing the decking on, and the damp course doesn't compress much.

Was very easy and quick to do.

Jase2985

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#2849715 13-Jan-2022 10:35

couldnt you just use some joist/flashing tape on the lower ones to build them up? or some small plastic shims?

Paul1977

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#2849719 13-Jan-2022 10:41

duckDecoy:

In the past I have cut and layered up pieces of damp course onto the joist under where each decking board will go to get them level, nailed the pieces to the joist with small nails, and then put the decking timber on top as per normal. You can nail through the damp course when nailing the decking on, and the damp course doesn't compress much.

Was very easy and quick to do.

Is that the same as Damp Proof Course?

Bung

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#2849722 13-Jan-2022 10:49

Cavity batten is only H3.1 so even Metalex may not be enough. The best way would have been packing joist/bearer. If using something on top of joist it would have to be compatible with the composite decking. There's a YT video warning that bituminous tape isn't compatible with PVC decking. You can get Macsim plastic shims in a range of thicknesses.

Paul1977

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#2849727 13-Jan-2022 10:55

Jase2985:

couldnt you just use some joist/flashing tape on the lower ones to build them up? or some small plastic shims?

@Jase2985 You mean use the shims on top of the joists? So I wouldn't have to try to disconnect any joists?

Jase2985

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#2849743 13-Jan-2022 11:24

you said there are only a couple that are low? then could you not just use the small plastic shims on top them to level them out?

duckDecoy

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#2849744 13-Jan-2022 11:25

Paul1977:

Is that the same as Damp Proof Course?

Yes. There are various types and some are thicker than others. I have a very thin one that I use so I can increase the shim height by layering.

Plastic shims might be an option too, but I use damp course as you "make your own" to fit the thickness you want simply by layering more on. Its unreactive (the stuff I use is just like a very thin plastic) so shouldn't matter what type of decking you have.

Paul1977

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#2849747 13-Jan-2022 11:27

Jase2985:

you said there are only a couple that are low? then could you not just use the small plastic shims on top them to level them out?

Thanks, just doubling checking what you meant since others had suggested packing the joist from underneath.

Paul1977

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#2849748 13-Jan-2022 11:28

duckDecoy:

Yes. There are various types and some are thicker than others. I have a very thin one that I use so I can increase the shim height by layering.

Plastic shims might be an option too, but I use damp course as you "make your own" to fit the thickness you want simply by layering more on. Its unreactive (the stuff I use is just like a very thin plastic) so shouldn't matter what type of decking you have.

Thanks.

duckDecoy

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#2849749 13-Jan-2022 11:29

Paul1977:

@Jase2985 You mean use the shims on top of the joists? So I wouldn't have to try to disconnect any joists?

Shimming on top of the joist is going to be the easiest way. And even if you take the joist out and get the ends lined up with the bearers and nailed in again you might find inperfections in the joist edge so you have to plane/shim anyway.

I cut mine into thin rectangles slightly less than the width of the decking planks and put it under each plank so you cannot see it in the gaps between decking planks.

chevrolux

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#2849751 13-Jan-2022 11:31

Yea if it's only a few mm, get a bag of plastic shims. Just start laying some boards, and if you start noticing one is lower than the other, just pack a shim in and re-screw.

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

Jase2985

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#2849759 13-Jan-2022 11:44

bunnings sell 3-4 different multi packs of shims, normally ranging from 1mm to 10mm. you can combine different sizes to make the size you need. you can also screw them down to hold them in place.

Paul1977

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#2849768 13-Jan-2022 12:02

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.

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