Suz wrote:
"Ian Stirling" wrote in message
...
Suz wrote:
We want to specify heavier than normal joists in the extension. What
size
should we ask for?

Why?
What's the span between supports, and what's the load you want to put on
it?

Doubling the width, or doubling the frequency of joists doubles the
stiffness of the floor.
Doubling the height of the joists quadruples the stiffness.

Oh why is nothing ever simple?

Because it's real life :/

The idea is to have real strudy floor to minimise any bend and noise
transference. The longest one will be in the roofspace and as the kids will
be there I want to reduce the elephantitis noise from above.

The other problem.
8" joists - compared to 6" will in fact moderately reduce noise.

What will reduce it a lot more is to completely float the floor.
You basically put in joists like:

______________
# # #
# # # # # #
# # # # # #
# # #
____________

Where there are two sets of joists, interspersed.
The bottom ones are maybe slightly smaller, as they only support the
plasterboard, the top ones support the floor above, and they only are
connected at the side walls, being completely free to move in relation
to each other.
Adding rockwool in the space in between helps too.