1930s Roof Joists and Building Regs

[Stu70]

Hi,

I am new to the forum and would appreciate some input in working out how the floor joists in our 1936 attic are so strong. They are 4"x2" at 350mm centres and span nearly 4m. According to the Building Regs joist span table these joists should only be spaning about 2m yet they feel solid.

I have had floor boards up to investigate and can find no intermediate support along the span. There are roof binders running above the joists and coincident with the purlins above at approx mid span but I can't see that these contibute to the floor strength.

The attic space exists and we want to convert it into habitable accomodation but Building Control are likely to say that we have to insert deeper joists to meet the span - we don't want the associated cost, disruption and reduction in floor to ceiling height.

Any guidance or explanation on why these joists feel so strong and how we could justify the existing joists as being adequate would be appreciated.
Thanks

 

[^woody^]

Two things really

The timber from that time was grown slow, cut form old trees and seasoned and dried slow and so it was a great deal stronger than todays soft timber cut from young trees and speed dried and wrapped and sent to the merchants with a speed that birds-eye would be proud of

Secondly, the span tables are very conservative and if you calculated a joist, or timbers in a load sharing situation like a ceiling, then smaller sections would work out fine

I've just mentioned in another thread that the 1000's of houses built with 3x2 rafters (and ceiling joists) in the past should be falling down according to today's span tables

 

[Stu70]

Woody,

Thanks for your reply and shedding some light on this. The big question is whether the existing structure can be justified to Building Control or whether we will have to lump it and insert deeper joists. If I get you right, Building Control might accept calculations based upon load sharing?

Alternatively, given that the attic space already exists I wonder whether the attic floor could be excluded from the BRegs application for the work on our house.... or am I dreaming?!
Kind regards

Stuart

 

[big-all]

no you need to comply with the regs at time off alteration as in now

 

[freddiemercurystwin]

Woody,

Thanks for your reply and shedding some light on this. The big question is whether the existing structure can be justified to Building Control or whether we will have to lump it and insert deeper joists. If I get you right, Building Control might accept calculations based upon load sharing?

Alternatively, given that the attic space already exists I wonder whether the attic floor could be excluded from the BRegs application for the work on our house.... or am I dreaming?!
Kind regards

Stuart

Its possible but pretty unlikely your existing structure will be up to it. But you will need to employ a structural engineer to find out. I've never done a loft conversion that didn't need additional floor structure.

Of course you cannot exclude the floor from a Building Control Application.

 

[Stu70]

Woody,

Many thanks for your advice and time - I'd better bit the bullet and upgrade the joists!
Best wishes

Stuart