On Jun 23, 8:03*pm, "Dave Liquorice"
wrote:
On Thu, 23 Jun 2011 19:43:13 +0200, David in Normandy wrote:
Someone has asked me to help put in an upper floor in a farm
outbuilding. The room dimensions are 7 metres by 5 metres constructed
with a single thickness of breeze blocks. He is planning having 7 metre
long joists and fixing down traditional tongue and groove floorboards on
top.

Our 2nd floor (22mm chip on top, plasterboard underneath) is
supported by 7 x 2 joists at normal spacing and it is, erm, "bouncy".
These joists are
only 4.5m long...

8 x 2 (250*50mm) would be to code for 4m span, 400mm spacing - I
think, for domestic floor loads.

Gut feeling is that 7m is going to need some seriously chunky bits of
timber, 8 x 4, 8 x 6? they won't come cheap in 7m lengths.

Depth is what counts. Strength is proportional to square of the depth,
Stiffness is cube of the depth.

Why doesn't he use the 5m dimension for the joists?

--
Cheers
Dave.