Floor Joist Span If you want to know how your DIY projects could be affected by building and planning regulations, click here to post questions and view answers

[samackroyd]

Hi,
I want to conver tmy loft into a useful space. I want to lay a floor onto a load bearing wall and hang it off a large timber that supports the roof. the distance between the load bearing wall and roof timber is 2m.

Please could I get advise on what dimensions I need for the floor joists at 400mm centres and 2m span, I would prefer to er on the side of caution and go too big.

cheers,
Sam

[Freeman]

I'm guessing that the 'large timber supporting your roof' is actually a perling.
Hanging a floor off it is not advisable, you could end up bending your roof, or worst splitting the perling.

Depending on the use you have in mind for this new loft space, legally you might have to meet building regulations.

[eljaybee]

If the large timber beam is at ceiling level then it is a ceiling binder (sometimes called summer beam) rather than the purlin that supports the rafters.

I would advise that you do not support any timbers off this beam unless yopur get the size of it checked out by a structural engineer as by simply adding the dead load of the new floor you may over load the beam.

And as said if you plan on using this space for anything other than a loft space or storage then you must get building regulations approval. (for the area to be classed as storage then you must not plaster out the rafter line of the roof.