floor joist size for loft conversion (+rsj) Re-generate an old or tired space and give it a fresh breath of life. Click here for answers, information and tips

[windy55]

Hello
I'm about to embark on a loft conversion (mid victorian terrace)
the span is 5.8m and the designer has spec'd 250 x 50 c24 joists at 400 centres
Height is really tight so even slinging them between the existing joists
i'll still lose about 7 inches
can i double up 150 x 50's or 175 x 50's to give 6 x 4's or 7 x 4's
or reduce the centres to 300 or even 200mm
(designer is away at the mo so i can't ask him)
(sorry for using metric and imperial, the design is in metric but my head still thinks inches
Also the width of the house is 4.75m What size RSJ's do you think the structural engineer would spec?- just getting my head round the weight issues

cheers
Windy

[stoneyboy]

windy55,
Even for the lowest possible loading of 0.25kN/m2 the specified joist size is close to the limit. I assume your designer has calculated the load and joist sizes.
Is there any way you can provide structural support part way across the 5.8m span - this would reduce the joist size.
Cannot help on RSJ size - insuffient detail.
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[windy55]

Thanks Stoneyboy for your speedy reply
There is a wall supporting wall mid span but it has a through lounge cut into it on the ground floor so i think the designer was trying to avoid using it
what i was trying to find out is assuming 10 x 2 joists at 400 centres were ok to use, what would be the equivalent, but with a lower profile, perhaps 8 x 3
or 7 x 4 or even 6 x 4 at 300 centres, I can't find span tables that go closer than 400 centres or thicker than 75mm

Cheers

Windy

[stoneyboy]

windy55,
Use the Mid-span wall to reduce the timber size otherwise you will spend a fortune on the joists.
Your designer should be able to do the load calcs for the lower profile timbers.
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