floor joists - which grade

M

marsaday

i am working out the sizes of joists to use in my loft conversion.

Is c24 grade the best to go for. i would rather over spec you see and dont mind paying the extra.

also my floor will need to be a different size in a main place as the span is quite big. 3/4's of the floor area is to be in 6x2" joists as the span is about 3.15m. The other section has to span 5.2m. So here i am going to use 9x3" joists.

Does this sound ok?
 
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M

marsaday

how is that helpful?

it is pretty easy to make the floor one level. or is it.

does anyone have any practical advice rather than taking the ****
 
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Probably, it sounds like you've no engineer? Are you sizing the joists yourself? In which case BC will need to approve it so double check with them. I'd hazzard a guess that your 9x3's may possibly be a little undersized.
 
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M

marsaday

the joists have been sized by the independant BCO.

just spoke to BCO in the council and 9 x 3 is fine.
 
M

marsaday

more research.

a c16 timber 9x3 will span 511cm.

i need to span 518cm, but cannot find a table for c24 timber. i am sure this will be ok though at 9x3.

any sources for tables?
 
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The tables http://www.planningportal.gov.uk/Pp...portal.gov.uk/uploads/br/BR_PDF_AD_A_1992.pdf page 45 table A2 were written for use in the old building regs but AFAIK grade C24 is a newer replacement grade for SC4 you should be governed by the SC4 tables. In which case using 75x220 joists @ 400 centres should be able to span up to 5.25m but don't take my word for it as that assumes a floor loading that only an engineer would be able to confirm (or is qualified to anyway).

BTW the tables were removed from the regs because BC were uncomfortable with self builders and the like determining structural member sizes themselves. :idea: I guess there were a few failures! there can be more to sizing timber sizes than looking in a table especially when you (as in this case) are getting up to the limits. Can't believe there'd be a catastrophic failure tough tbh.
 
M

marsaday

This will cover my span, but in the builders merchant c24 can span up to 5.4m.

The loading are minimal as the room is an attic bedroom. a bed, cupboard etc. Plasterboard walls. We have 2 solid walls to build off as well. it is just the span our bedroom at the moment is big 5.15x4 room.

i think these timber sizes will be fine. more research though to be doing.
 
M

marsaday

thanks for the link. this is useful.

can you clarify what load a bedroom floor is likley to be taking. i assume it is light.

in the table there are 3 weights a floor can take.

using the c24 timber 9x3:

not taking more than 0.25 kn/m2 = 5.42 span.

0.25-0.5 kn/m2 = 5.25 span (my span is 5.18 so this is ok)

0.5-1.25 kn/m2 = 4.88 span (this will be undersized in this case).

i would have thought the c24 timber will be ok for our use, but does anyone know what these kn weights actually are in reality?
 
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thanks for the link. this is useful.

can you clarify what load a bedroom floor is likley to be taking. i assume it is light.

in the table there are 3 weights a floor can take.

using the c24 timber 9x3:

not taking more than 0.25 kn/m2 = 5.42 span.

0.25-0.5 kn/m2 = 5.25 span (my span is 5.18 so this is ok)

0.5-1.25 kn/m2 = 4.88 span (this will be undersized in this case).

i would have thought the c24 timber will be ok for our use, but does anyone know what these kn weights actually are in reality?

They are loadings not weights, not quite the same and only an engineer can confirm exactly what the loads are. However I usually use the centre column when in doubt but again this is an assumption and may be risky if you are near the limits!

What does your independent Building inspector say, is he inspecting the works and issuing you with the approval? I thought you said your local BC were happy with 9x3's? :confused:
 
M

marsaday

i rang bc and they said it sounds ok.

the private inspector is the ex head of BC. We havent started yet, i am just doing the research. the B inspector did my plans last yr. we've been delayed due to the C Crunch and so i am gearing up to start next spring.

Nothing like a bit of planning though !

i am pretty sure the mid table figure will be fine, but i may speak with an engineer.
 

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