My joists are too short!

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I'm putting some deeper joists in my loft to make a storage area. I have some joists but they're not long enough. I believe it's possible to "trim" across from the 2 adjacent walls (or rather one wall and a convenient steel beam) and hang the joists off hangers to overcome this. I will need to cut the existing ceiling joists to fit the trimmers in and I'm not convinced that standard timber to timber joist hangers will be sufficient to tie the 2 lots of joists together to restore the lateral restraint?

Any advice/suggestions appreciated (including get longer joists, which I'd rather avoid if at all possible - and in any case they would have to sit on my saggy old timber lintel which I doubt has much spare capacity).

Thanks.

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Of course:

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I also own a garage full of surplus building materials and these happened to be in there! I think they cost me £3 each so it's a shame not to use them.
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Obviously they're oversized for the job but I need to trim one for the hatch opening so I won't need to double up on either side as well which saves 2 joists.
 
I can see where you are coming from on this. TBH I think that if you tie your sistered trimmer together with steel bolts (M12 to M16), nuts, penny washers and star washers (in the middle between the two pieces of timber) at, say, 1m centres then you should be OK. Bolting together that way makes for a more rigid "beam". Were this structural I have no doubt that an SE might want three 9 x 2s together, but as it's only for storage calcs aren't really in the equation. Probably unnecessary to say this, but make sure that you nail every hole in the hangers and only use twist nails (you'd be surprised at some of the "solutions" we come across at times)
 
Thanks, that's encouraging. I did consider 3 joists but as you say should be ok for storage and they're TR26 graded. A root around the garage has produced some M12 bolts , spiky washers and some angle brackets that will tie the old joist stubs to the trimmer. Just hangers and twisty nails to get now!

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If at all possible I'd try to use hangers to fix the ends of the old joists onto the new trimmer, even if it means nailing the hangers onto the ends of the old joists first, positioning the new trimmer and then screwing through the hangers into the trimmer using an angle driver. Awkward, I know, but doable. I know that this should be a nailed connection but somehow I doubt that you'd get many nails in with such a tight space unless you took part of the roof off
 
All done thanks. In the end I used angled nailing plates and 6" wood screws through the first trimmer into the old joist ends so they won't be going anywhere. I put 2 rows of noggins in and it's rock solid - probably the strongest thing in the house.

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