Joist stregth question

[gg1000]

I have a loft space with joists that are 2" x 4", spaced about 450mm apart (yes I know I'm mixing units!). The span in question is 3.2m and I am wondering if it is strong enough to support a couple of water tanks. I want to install 2 25-gallon tanks (the coffin type) and they will sit side-by-side straddling 4 joists each will sit on its own chipboard platform. (See diagram below).

+------------+ +------------+ | TTTTTTTTTT | | TTTTTTTTTT | ====| TTTTTTTTTT |=======| TTTTTTTTTT |==== | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | ====| TTTTTTTTTT |=======| TTTTTTTTTT |==== | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | ====| TTTTTTTTTT |=======| TTTTTTTTTT |==== | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | | TTTTTTTTTT | ====| TTTTTTTTTT |=======| TTTTTTTTTT |==== | TTTTTTTTTT | | TTTTTTTTTT | +------------+ +------------+

===== Joists

+---+ Chipboard TTTTT Water tank

What I need to know is: are these joist strong enough, or do I need to suppliment them? There will be no significant space left above or around the tanks when installed so there will be no chance of anything else being stored in the same area (adding more weight).

I have found some figures for floor joists, but these don't seem to apply as this is not a floor.

Any ideas?

[Christian McArdle]

IANASE.

According to Superbeam, I make it is needing 50x170mm joists.

Assumptions:

1. Cisterns themselves weigh little.
2. Cisterns are rectangular.
3. Weight is evenly distributed amongst joists.
4. Weight of cistern is evenly distributed from 30cm from each end to 1.3m from each end.
5. Allowance for 100kg people standing between the tanks.

If you allow for standard floor loading as well, it increases to 50x195mm, although I doubt this would be necessary.

Christian.

[gg1000]

So, as the current joists are in place (and have been for about 50 years) what should I do to strengthen them? I might be able to add a couple of joists between the existing ones, or maybe bolt on to the side of them.

[Christian McArdle]

A quick calculation shows that just sticking new 100mm joists between won't strengthen it enough.

You can bolt new ones on the side of the old ones as you suggest.

Alternatively, if you have access to the end wall plates, or can hang new hangers, then install new joists between, but spaced a cm or 2 above to avoid touching the existing ceiling. This solution is better as it insulates the ceiling from flexing (and hence cracking) every time someone has a bath.

Christian.

[gg1000]

If I go this route what size should the extras be, and what spacing for the bolts? I would far, far rather go for extra width rather than extra height as the tanks are going to be so close to the rafters that there will only just be space to fit a thin insulating jacket (at the tank ends and corners - the top and sides will have a little more space for some extra fibreglass).

There is very little space to do anything at the joist ends - If I were to attach new joists I don't know how I would nail the ends!

[Set Square]

The problem is that joists get their bending stiffness from their *height* - not from their width. For example, an 8 x 1 is *far* stiffer than a 4 x 2 even though it's the same amount of wood.

Are you *sure* about the span. Is it *really* unsupported over that distance - or are there any supporting walls underneath. If you can contrive to put your tanks directly above supporting walls, 4 x 2s will be fine - as mine are!

[Christian McArdle]

Also, another alternative is to not have any tanks at all. Only possible if you have good mains pressure and flow rate.

Christian.

[Set Square]

Ageed - as long as you're talking about a mains-pressure stored hot water system rather than a combi boiler!

[gg1000]

But not much cop with ~1 bar water pressure!

[Tom]

Are water supply byelaws still applicable these days? Byelaw 31 is deemed to be satisfied where a storage cistern has an unobstructed space above it of not less than 350mm.

[Christian McArdle]

1 bar is fine provided that it doesn't drop when in use. It is flow rate at the tap that is the essential feature. Admittedly with 1 bar, the flow rate will only be good if you have a nice wide bore pipe to a wide main.

Christian.

[gg1000]

It drops if you so much as look at it funny. We are near the end of a very long narrow pipe from a tiny local resevoir. When I spoke to the water board they said "Oh, there! You'll need a large storage tank".

[Christian McArdle]

An alternative would be a large accumulator, if you find it easier to find space in the basement rather than the loft. This still uses mains pressure, but stores as well.

main advantages over the big loft tank idea are:

1. No need to strengthen loft structure.
2. Can store in a basement where freezing is more unlikely.
3. Silent whilst filling (no noisy ball valve).
4. Silent whilst showering (no noisy shower pump).

Which is best probably depends on budget and available space.

Christian.

[gg1000]

Rats!

That leaves me with only one option - a single 25 gallon tank placed centrally in the space. Does this now mean that the joists are strong enough as they are?

I guess that this does rule out any chance of a pumped shower though!

[Tom]

Don't rule it out yet, try and get a second opinion, I'm retired and out of it these days so I'm not sure whether it's still applicable. Regards Tom

[gg1000]

It does:

[Christian McArdle]

Do you have any space downstairs? Possibly under the floorboards, understairs cupboard, or in a custom "shed" outside next to the kitchen?

If so, an accumulator beckons.

Christian.

[Christian McArdle]

you think they apply to accumulators too? Perhaps one could be loft mounted without the need for headroom? Getting it up there might be another matter, though!

Christian.

[gg1000]

Do you have any useful links or info on size and cost?

[Jim Hatfield]

Until recently I had a round 50-gallon tank sitting on ONE joist. OK the chipboard underneath it spanned three, but it had turned into cardboard. A surveyor who saw it advised me to fix it asap else I'd come home one day and find it in my kitchen. The joist was well bowed. The room below (bathroom) has a pine ceiling stuck over the plasterboard so the bending wasn't visible.

I now have a 50-gallon coffin tank sat on a nice raft of 18mm ply strengthened by three lengths of 4x2. I think it spans four joists.