Fixing Centers for Sister Joists.

30 Oct 2008
Reaction score
United Kingdom
I am sure there is a BS table for this operation I need to find - but I can't find it... :)

I have a 3.7m span floor at 320mm centers, spanned with 50 X 175 timber. It's already over engineered.

I just need to level the thing out since it's crowned as the walls dropped over the years and the middle crowned round a supporting post.

I wonder what size sisters I should use?? The thought I have is that the new joists will sit about 1/12" above the old and need to overlap enough to join sensibly... So I thought 47 X 150 mm ?

Next part is I am going to use M12 bolts and toothed timber connectors. What sort of fixing pattern is appropriate? All in the center - or staggered? What centers?

Sponsored Links
For anyone else searching this post - if your not adding sisters for structural strength reasons (as I am not doing) - then the thing you need are "Timber Drive Screws" I used 4" screws at 600mm centres. Any old timber with a reasonable overlap is sufficient. I used 150 X 47.

As advised by a structural engineer I am using for consultation on this project.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links