Extending a joist

[Dave]

Here's another one for the assembled experts ... I've got one joist that's been cut short and then attached to a trimmer in order to get round a soil stack - but I want to remove the trimmer as part of some building work. I plan to screw & glue the extension timber to the side of the existing joist, are there any rules on how much overlap, etc? The joists are about 195x48, it's the joist nearest the wall, it'll be about 40mm too short when the trimmer is removed.

[Set Square]

Is it worth it for 40mm? Can't you leave the trimmer in place?

[Dave]

Unfortunately not. I want to drop the loo soil pipe (and the shower waste) down between the joists - the trimmer will have to go!

[Dave]

... that should be 400mm!

[in2minds]

overlap should be minimum 26", use 2 suitably long coach bolts with washers (I use the square type) and put star washers (not sure of the technical term but the builders merchant knew what I was on about) between the 2 joists, one bolt 4" in from either end

this order... bolt, washer, joist, star washer, joist, washer, nut

LJ

[Dave]

Thanks, where did you find the info? I couldn't find anything in the BRegs or www.

[Simon Stroud]

I think those "star washers" are called timber connectors, like this:

(that was the first site I could find with a picture).

When our loft conversion was done they were supposed to fit those in the several places where multiple joints were bolted next to each other. They didn't bother and by the time I noticed and complained it was too late to take everyting out (or maybe they just lied and said they'd fitted them - I can't exactly/would rather not remember). And they didn't do the nuts up more than finger-tight either. And being coach bolts with only a certian amount of thread they had to fit big piles of washers under the head and nut so that they didn't run out of thread when "tightening" them - think they only had one length of coach bolts and couldn't be bothered to get correct-length ones.

Anyway it hasn't all fallen down yet. Hopefully if you do the job better than that then yours will be fine.

Regards, Simon.

[rrh]

I did some a few months ago and planned to use the star-washers-or-whatever-they're-called. But they kept the joists apart however much I tightened the coach bolts. I thought the joints were more solid without them so didn't use them in the end.

[Nick Atty]

That's exactly how our builders have just done it when they had to re-organise joists to create the hole for a new staircase (and to fill the hole where the staircase used to be, for that matter).

I think they used hefty (M10 I'd guess) studding rather than coach bolts.

[Mike]

I assumed the gap was why you did use the star washers so that the joists can expand and contract symmetrically.

[Roger]

The message from "rrh" contains these words:

I did a loft conversion a few years ago and the BCO suggested I use them on the doubled up joists below the transverse stud walls (that he insisted on) but he didn't object when I didn't bother.

[John Rumm]

12mm is the standard size for these and plate washers...

[in2minds]

from the BCO (building control officer)... he knows everything (well thinks he does (c; )

LJ

[rrh]

No I don't think so; I believe that ideally the star washers are supposed to bite deeply into the timber on both sides, leaving little or no gap. The joists wouldn't need to expand much side to side anyway. My joists might well have been strong enough if I'd used the washers, but I was happier with the timbers tight up against one another. Maybe I have unusually hard timber for my joists...