how to calculate wood joist size

Discussion in 'Carpenters' Talk' started by siraz, Apr 28, 2008.

1. sirazNew Member

Please can you help how to work out the joist size ,as the lenght of the span will be 4.20metre

Joist size height, depth. My Joiner said to use 7" X 2" C24.

By the way this is for LOFT CONVERSION

3. sirazNew Member

Dear Bonzo,

I am little confused which size would i need if my room lenght is 4.10mtr by 4.50mtr wide room. Looking from the link i am not sure what to look for.

Joist span i have been told to order 7 x 2 do you think this is right, as my next door neighbour used 9 x 2.

(7")180mm x (2")50mm this will be C24 ROUGH SAWN UNTREATED.

I AM NOT SURE IF HE WILL SPACE 400MM OR 450MM FROM CENTRE OF JOIST. hE TOLD ME TO ORDER 30 LENGHT OF 7 X 2 FOR 2 ROOMS AND PART LANDING.

4. BonzoNew Member

I would use 9"x2" as 4.2m is a big span. Is this span unsupported, is their a wall below at right angles to joists?

5. sirazNew Member

Where i said 4.10mtr this where the joist going to run, so one end will be on the outside wall load bearing wall, which the joist will sit on the innerwall.

The other end of the joist will sit on the middle of the wall.

There is a party wall i presume it will be single wall brick, as i rememeber my neighbour his joist came to our side when we knock the plaster out in our house.

He told me to order 30 joist hanger may wood to wood, as i presume he will knock out few bricks to accomodate to sit the joist on the brickwork.

6. ecmNew Member

Siraz, your joiner has under spec'd those joists the TRADA span tables state that for a 4200mm span @ either 400mm or 450mm centres you need,

C24 220x47 or if height is an issue you could just about get away with C24 170x75.

If you want a copy of the TRADA span tables post your e-mail on here and I'll e-mail them to you.

hope that helps.

7. cosworthNew Member

simple formula to use.

span in feet(divide by 2) then add 2

(span) 14 feet (divide by 2)= 7 (add 2 inches)= 9

so you need a 9x2 minimum at 400 centre.

same formula for mm just add 20mm.

size in feet airs slightly on side of caution but better safe than sorry.

so dont buy 7x2 and go for 9x2 as said.

8. ecmNew Member

simple formula to use.

span in feet(divide by 2) then add 2

(span) 14 feet (divide by 2)= 7 (add 2 inches)= 9

so you need a 9x2 minimum at 400 centre.

same formula for mm just add 20mm.

size in feet airs slightly on side of caution but
better safe than sorry.

so dont buy 7x2 and go for 9x2 as said.

That's unerringly accurate, I'm impressed with that method.

9. cosworthNew Member

on my own extension a few yrs back i used 9x3s which feel very solid on a 14ft span.

10. ecmNew Member

Makes me laugh when builders try to underspec floor joists, especially when the price difference between a piece of C24 9"x2" and C24 9"x3" is approx £1/per m.

11. chip off the blockNew Member

7x3 c24 at 400cts will span 4.5m or might be 4.7m as calclated by my structual engineer. remeber in a loft there is no plaster board on the under side of the joists so the loading is less as pb is 10kg per m2 ish

12. THE BUILDER MANNew Member

a bit of 3 x 2 cls should do the trick or some old pallets

13. cosworthNew Member

a bit of 3 x 2 cls should do the trick or some old
pallets

think that will be over engineered!!!!

roofing batten should easily work on that span

14. lojoNew Member

7x2 max span @ 400c = 3.55m so NO

9x2 gives 4.58m so still near limit

8x3 gives 4.72m

15. lojoNew Member

imo, you dont want to go to near the limit, it may be safe but the bounce is obvious

16. sirazNew Member

Hi ECM,

Thankyou for all your help, i will order 9 x 2 C24 OR C16.

17. ecmNew Member

siraz, I have e-mailed you the TRADA timber design tables. Loads of info in the doc but joist spans are on p16-17. It's a .pdf so you will need Adobe Acrobat Reader to view.

cheers

18. chip off the blockNew Member

look at the tables and a 7x3 wil do as previously said.

19. cosworthNew Member

hello siraz.

just wondering how your joiner took the news??
did he put up a fight for his reasons on 7x2

Dear ALL,