Plinth for Relocating Water Tank in Loft

[middleman]

I currently have a 200 litre cold water storage tank for a gravity fed system in my loft as pictured below and my plumber is replacing it with an unvented 210 litre cylinder at the end of the loft against the wall.


I am going to prepare the plinth for the tank to sit on along with the walkway up to it for maintenance etc. Looking some advice on the structure of it on the joists to take the weight whilst accommodating the extra height of the cylinder.


I have included a diagram of what is there at present (excuse the drawing, didn't have my crayons handy ;-0).

I am looking for advice on how much will be need to keep to spread the weight of the new cylinder across the joists. There is 37cm between the joists.


There are no accessible solid walls to put it above before it is mentioned but the current tank isn't above one either.

Thanks

 

[middleman]

Am I overthinking this and should I just floor across the joists in ply?

The existing tank is 200 litres and plastic.
I am going to put in a 210 litre cylinder.

 

[Newboy]

No - you're not over-thinking it!

That's a quarter of a tonne of tank and water spread the load as you suggest in your drawing. Don't put the ply directly onto the trusses, they cannot take that amount of imposed load.

Although I'm not sure where you plan to source 150 mm ply!!

 

[middleman]

No - you're not over-thinking it!

That's a quarter of a tonne of tank and water spread the load as you suggest in your drawing. Don't put the ply directly onto the trusses, they cannot take that amount of imposed load.

Although I'm not sure where you plan to source 150 mm ply!!

That wasn't my plan, thats what the existing cold water tank is sitting on at the moment but I assume some of the height is to account for it being gravity fed to give pressure. I need to lose some of the above in my new plinth to allow for the extra height of the unvented cylinder. My plan would be to lay 4 x 2's across the joists to spread the load and I have some 1/2" ply to go over the top for the tank to sit on. That is 1/2" ply not 150mm as in the dodgy diagram.

When the existing tank is removed, I will then make a walkway to assist in maintenance later. Just wondering if by using the 4 x 2's to spread the weight, will that be enough? Don't want the tank landing in the bedroom some night.

 

[Newboy]

I'd be inclined to use 200 x 50 onto the trusses and a WBP ply deck on top - ensure you get it level and that the deck is fully supported

 

[middleman]

I'd be inclined to use 200 x 50 onto the trusses and a WBP ply deck on top - ensure you get it level and that the deck is fully supported

Attach the 200 x 50 to the diagonals instead of laying onto the ceiling joists?

 

[Newboy]

No - lay them on the ceiling joists (excuse my lazy use of terminology, your roof is made up of trusses and technically the ceiling joist is the bottom chord of the truss).

If there is a stud wall below any of the 'joists' then try to lay your timber over it/them (every little helps!)

 

[middleman]

No - lay them on the ceiling joists (excuse my lazy use of terminology, your roof is made up of trusses and technically the ceiling joist is the bottom chord of the truss).

If there is a stud wall below any of the 'joists' then try to lay your timber over it/them (every little helps!)

That's what i thought. Have seen a raised platform on the trusses before for a coffin tank. There is a stud wall in roughly the centre of the house there so that would work. Sorry for all the questions. The 200 side downwards I assume?

Not sure of the height required by the plumber for the install but I'm trying to give them as much as I can.

 

[Newboy]

50 mm face on the truss/joist - 200 mm face vertical

 

[middleman]

50 mm face on the truss/joist - 200 mm face vertical

I'll get some 200 x 50 sorted out then. I have 1/2" WBP, 3/4" probably be better but I'll put extra 200 x 50 in to compensate for that and above the stud as suggested. I'm sure if the plumbers not happy he'll let me know. Thanks for your help.

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