Moving CWST into loft - will the joists take the weight?

[theomegamale]

Hi again

I am reassessing whether to try installing a CWST in the loft rather than leaving it on the top floor. See previous post here if it's of any use.

I've had plumbers in who have suggested a £2k approx bill to reinforce joists, re-site a new CWST to the loft, commission shower etc etc, so I had discounted it as an option and was going to keep my tanks on the top floor and do-it-myself. However there are some cans of worms that might be best left unopened by going that route, so I'm rethinking the CWST relocation.

What I've noted about my loft:
My joist dimensions are 35mm x 120mm and spacing is 600m except for at the end where the final two joists are 400mm apart.
I could site the CWST in the loft partially above a concrete block partition on the top floor to provide additional support.

So looking at what I have in the attic, I'm not sure if reinforcement is really required. My thoughts:
a) if I'm pumping the supply to the shower (i.e. need a large flow from the tank) @ 9 litres p/m, a 50 gal tank i.e. 227L should be sufficient. I might even drop to 40 gal.
b) 227L weighs 227kg + there is some small additional weight of the plastic tank.
c) A coffin tank would spread the load and could be laid perpendicular to the joists (I was thinking on 18mm plywood) and partially over the concrete partition on the floor below for further support. I was thinking about this one for example, which could span 3 joists

Does anyone have any thoughts on the above? What are the conventional methods for figuring out whether additional support is required and how to go about providing it? As long as 200 litres won't crash through the ceiling, I'm happy.

 

[doitall]

The normal procedure is to fix two long sole plates 100 x 50 will do, over 4 or more joists, if you have a wall under then so much the better.

Ply the top, a minimum 20mm ideal 25mm, and stand the tank on the ply, if you need to raise the tank for better pressure you build a stand up from the plates.

50 gallons is not enough storage if you intend to install a pump, should be a minimum 75gallon, and ideally 100gallons. Of course it also depends on the recovery rate from the mains as well.

 

[theomegamale]

Thanks for the response. So given my existing joist dimensions (I don't know whether 35mmx120mm is "standard" and is designed to carry these sorts of loads), if I follow this proposal and lay down sole plates across 4 joists + 25mm ply I can get away with 75 or 100 gal?

Recovery rate is poor, flow actually goes to zero if you turn on the kitchen cold tap. That's on the 3rd floor (it's a town house). In the loft (i.e. 4 floors up, so to speak) it'll be even worse. I haven't measured flow at the expansion tank in the loft to check yet.

The calculation I made is: 227 litres pumped @ 9 litres per minute = 25 minutes. But I guess increasing that to 75 gal/ 340 litres is safer and reduces the risk of burning out the pump if we flush loos etc while showering.

 

[doitall]

The longer the sole plates the more the weight is distributed, plus you have the wall below to give extra strength. you will be fine.