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New low deck - joists different sizes

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  1. #1
    Apprentice (new member)
    Join Date
    Jul 2018

    Default New low deck - joists different sizes


    First, thanks for this forum - I’ve been lurking for a while now and have learned a ton.

    I have a specific question about joists but also want to share what I did so far in case you spot any concerns.

    I am midway through building a ramp. There’s not much height available - around 200mm - so I’m following the “Residential Timber Decks On or Close to the Ground” guide. I can’t dig it out any further since there’s a bunch of sewerage and stormwater pipes in the area.

    I laid a weedmat and then about 80mm of drainage gravel. I am using 100x75 bearers (the 100 horizontal/75 vertical). I painted the bearers with a waterproof membrane since they sit directly on the gravel, except for one that acts as a ledger at the top of the ramp (this is bolted in with 6 AnkaScrews). The bearers are spaced around 1800mm apart.

    The joists are 100x50 F14s spaced 300mm between each other. The ramp is about 9m long so the joists join at either of the two centre bearers (alternating).

    I used stakes to hold the bearers steady while I assemble it but will probably take them out when I am done.

    The final decking is 90x19 Merbau, screwed down. I will probably put some protectadeck on the joists.

    Now to my question: the joists are different thicknesses. I ordered 100x50 hardwoods. Some of them are 100x50 or slightly over, some are more like 95x46. I guess this is down to shrinkage - the smaller ones tend to look “dryer”. I didn’t check this before I started.

    This causes two problems:

    1 - If I put a straightedge across the joists (perpendicular to the joists) there are differences in heights of around 5mm.

    2 - Where the joists butt against each other at the joins, there’s a height difference of again around 5mm.

    What would you do in this case?

    Ideas I came up with:

    - Use an electric planer to plane them down and align everything. I think the end result will be best, but what happens if the wetter joists eventually shrink - will I be back at the same problem?

    - Make wedges/shims to adjust the joists where they meet the bearers to get them roughly level. Downside: not sure how durable this will be.

    - Just leave it for a while (how long?) so the timber can all shrink to the same size, then plane as needed. Downside: I’m in a hurry )


    Here’s some photos:

    (Please use this site's image uploader)



    Also open to any other suggestions on what I should have done differently. I’m doing all this as a learning opportunity.

  2. #2
    2K Club Member
    Join Date
    Oct 2005


    What I’d do. - Plastic spacers on the bearers to get them all equal. Screw down your decking so if worst comes to worst in a year or 2 , you remove a few deck boards and add/remove packers

  3. #3
    4K Club Member Marc's Avatar
    Join Date
    Jul 2003


    Packing is one choice, however depending on how much material you have, you could use the smaller joist all in the same place so all is level and the next section you use the bigger joist and plane a little ramp so it is not noticeable. 5mm is not that much. Even better return the one you have the least and replace.
    Real knowledge is to know the extent of one's ignorance

  4. #4
    2K Club Member
    Join Date
    Aug 2005
    brisbane . australia


    Thats the reason why I and most builders wont touch hardwood anymore....just rubbish..Pack it and plane it..make sure you predrill all screw holes or most screws will just snap.

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