On 19 Oct, 20:08, Martin Carroll wrote:
I am putting a megaflo HW cylinder in the renovation and need to
construct the floor of the airing cupboard (which is above the
staircase).

The weight when full of the particular model will be 228Kg and I was
wondering how I ensure that the floor it is standing on is strong enough
for that weight.

Is their an easy way to calculate the joist size I need to use?

joist sizes used to be specified in tables to the Building Regs,
unfortunately my copy is old & probably out of date: if you are lucky
someone will be along soon with chapter & verse. If not, I will look
out the info for you tomorrow.

Apart from the load (228Kg), you need to know the span of the joist &
the distance between joist centres to lookup (or calculate) joist
sizes.

The other point you should watch is that you mention a staircase.
Simple Bldg Reg joist sizes are mainly for joists fully supported at
both ends, eg by walls. If the floor is otherwise supported -
suspended, held up by a Newel, on a cantilever, part of a truss, etc -
the whole structure should be reassessed for strength. If in doubt
please repost with more info.

228Kg is not that much of a weight - only 3 average adults & a floor
to BReg stads should support that. However if left as a point load
for a prolonged time it could be enough to cause a floor to sag.

HTH