Cold water tank support structure

HuskyDog · Joined: 03 Apr 2005 Posts: 12 Location: United Kingdom

Summary: Is there anything wrong with my plans to move my cold tank? Is there anything I could do better?

The cold tank is located right in the middle of my new loft on what seems to me to be a ridiculously over engineered structure. I propose to move it to one side so as to create more space. My wife has got wind of my plans and is complaining vigorously. She read about that sad case of the baby killed by boiling water and is now convinced that cold water tanks are death traps. I promised that I would seek advice before continuing.

My tank measures 60 x 115 x 64cm and seems to be of a modern design with a lid, insect traps etc. I assume that it was installed when the house was built in 1991.

Its current position is right over the centre of the landing. Huge joists take the load to either side so that its weight is over the landing walls. I propose to build a new structure to support the tank in the V of the trusses, directly over the airing cupboard. The weight of the tank will therefore be spread over 3 joists which will themselves be supported over several walls.

As I see it, the nub of supporting a cold water tank is to have a good solid base (I propose to use the 18mm ply wood that it is currently resting on) and use stout timbers (I propose 100 x 45mm) to spread the load across several joists, preferably with walls underneath. I propose to hold everything together with 12mm bolts. I do not propose to carry any load on the trusses.

Since the tank will be in the V of the trusses, it will need to be raised to about 1m (currently it is about 50cm above ceiling height). This will have the side effect of slightly increasing the pressure in my shower. Purely for convenience, I propose to put the CH header tank on a lower shelf beneath the main tank.

Apart from making more room in the loft, I see two other advantages: The pipes will all be quite a bit shorter and therefore less likely to freeze. The total structure will weigh less (their are currently four 220mm x 35mm x 2.4m joists which must weigh a fair amount on their own).

I have a fair bit of DIY plumbing experience, including installing a cold water tank in my first house. I know about not having insulation below the tank, and propose when I have finished to box in the area below the tank so that the warm air goes straight up to the tank rather than wafting uselessly around the loft.

Are my plans sound? Will it all end in tears, or if my wife is to be believed, death? What could I do better?

Thanks.

sloppytom · Posted: Tue Jan 13, 2009 2:22 am

sounds good to me

Moderator 11 · Posted: Tue Jan 13, 2009 10:24 am Comment:

your question is concerned with strength and loading of roof trusses, so I will move it to the Roof forum.

justlead1 · Posted: Tue Jan 13, 2009 12:38 pm

Hi. If filled to the brim the cistern will have a mass of approx. 360 Kg. Providing you have a sound base, spreed the load over 3 joist and the roof "A" frame you will not have a problem.Good luck

shenks · Posted: Tue Jan 13, 2009 12:51 pm

Personally I'd find another way to do it as I wouldn't be happy with the imposed load on the trusses...

NoPoke · Joined: 04 Jan 2009 Posts: 34 Location: United Kingdom

Modern trusses are complex engineering structures designed to use the absolute minimum amount of wood. I doubt that the supports for the tank are over engineered. Builders don't like putting in any excess money (read wood)

The purpose of the tank supports is to spread the load uniformly amongst the roof trusses. This takes a much more rigid structure than what you would need to support the tank on a solid surface. If your structure is not rigid enough and the water tank load is not shared properly amongst the trusses then you may get a truss failure: you don't want this.

masona · Posted: Tue Jan 13, 2009 4:13 pm

See section 3 and page 51,52,53,and 54

http://www.mii.com/artefact/download.asp?aid=3881

I prefer to come off the bearing walls and not touch the gang nailed roof trusses and if you can get 25mm ply wpb even better

noseall · Posted: Tue Jan 13, 2009 5:08 pm

[quote="NoPoke";p="1108376"

Quote:

Modern trusses are complex engineering structures

no they are not.

Quote:

Builders don't like putting in any excess money (read wood)

yes they do.

details of how to set out tank supports are illustrated in the TRA handbook and give specific instructions to specific tank capacities.

ALL scenarios are detailed within standard fink trusses.

http://www.tra.org.uk/technical.php

HuskyDog · Joined: 03 Apr 2005 Posts: 12 Location: United Kingdom

Thanks to everyone for their suggestions. The document from MII is particularly useful and I have been reading it carefully. I particularly note on page 31 where is says "The standard 230 litre water tank is usually supported over three individual trusses." This more or less reinforces what I had already concluded (i.e. that trusses will support the load of the tank if you spread it out properly).

The figures from page 51 onwards are particularly revealing. My current configuration is similar to figure 68 and what I hope to end up with looks rather like figure 71. I think that I can now see what I need to do and where I will need to install additional supports.

Thanks

xerxes · Posted: Wed Jan 14, 2009 12:28 am

HuskyDog wrote:

She read about that sad case of the baby killed by boiling water and is now convinced that cold water tanks are death traps.

This was indeed a sad case (and not the only one of its kind, if I remember correctly).

The problem occurs where a plastic water cistern acting as an expansion tank for the hot water system, is filled with scalding water, melts and spills the scalding water through the ceiling to the room(s) below. Specifically, it relates to immersion heater thermostats which can fail with the circuit closed, so that the water continues to heat up without any control. Replacement with thermostats which fail safe is the answer.

This is of course quite separate from the question of the load of the relocated cistern.

masona · Posted: Wed Jan 14, 2009 10:36 am

Bring back metal tank?

A thermostat temperature alarm on the tank?

Maybe a plastic tank inside the metal tank as a back up?

Master of None · Posted: Fri Jan 16, 2009 7:19 pm

justlead1 wrote:

Hi. If filled to the brim the cistern will have a mass of approx. 360 Kg. Providing you have a sound base, spreed the load over 3 joist and the roof "A" frame you will not have a problem.Good luck

I calculate 442kg + tank. Unless you have taken into account the empty space for the ballcock and height of overflow.