Sistering joists for new deck build

Apr 30, 2019
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Hi all,

I am planning to build a new wooden deck here in sunny Finland this summer. After much thought I have a structural plan that involves three layers. The primary layer will be sistered 2x6" joists (at about 1700 c/c) that span the new concrete foundations (also placed at about 1700mm c/c, making a roughly square foundation grid). The second layer will be 2x6" joists @ 400mm c/c which will support the tertiary layer that is, of course, ~1x4(/5)" decking boards. The deck is, in total, about 7000x5300mm in size, with the deck boards spanning in the longer direction.

I want to sister the primary joists because: 1) it saves me buying different section sizes, 2) it saves me much work digging, since I have to lower the ground level by hand for this job, 3) the steel fixings I will support the primarys off are about 100mm wide, I can't find any (cheap) 50mm ones.

My question is this: What's the best way to sister primaries? I have read online that best way is to use carriage bolts, but I am having a real struggle finding galv or stainless carriage bolts in Finland >8mm in diameter and >100mm long. Can I use galv coach screws (like these), instead? If I use those, do I need tooth plates between the joists at the fixing points? I would guess not, since the screws should pull the joists together very tightly, right?

Anyway, thanks for any help. Feel free to comment on the general plan I detailed above, I am pretty new to DIY. I am an engineer by trade, but I don't usually deal with timber and this stuff is usually best done with jobbing experience rather than theoretical knowledge! Looking forward to learning from the forum.



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