Freestanding Bath Weight!?

[richo106]

Hi All

 

I am just designing my main bathroom and we have found a freestanding bath we like. One concern I have is the weight of it!

The bath is 180kg, capacity to overflow is 400L and guessing got the allow for 2 adults (worst case) so quite a lot of weight

The bath will definitely site on 4 joists and more likely 5 joists (see attached layout)

They are posi joists at 400 centres, 75x225mm

 

Is this weight spread over those joists anything to worry about?

If so how would i check/confirm this? 

 

Many Thanks

 

[nod]

We installed a very heavy stone bath in our previous build Four of us to carry it up 

I Simple doubled the joists either side of it to take the extra weight 

[JohnMo]

35 minutes ago, richo106 said:

capacity to overflow is 400L and guessing got the allow for 2 adults (worst case)

You only really need allow the 400kg of water (plus bath weight), two adults would displace their volume in water. We are mostly water, so 100kg person should displace around 100l of water.

[richo106]

41 minutes ago, nod said:

We installed a very heavy stone bath in our previous build Four of us to carry it up 

I Simple doubled the joists either side of it to take the extra weight 

My joists are already in place so i can't span them block to block like my current ones. But the ceilings below are not installed so i can anything below to secure/improve them

[nod]

Ours was an after thought and I slipped them in from below 

[BotusBuild]

Add 2x4 to the side of the top and bottom chord of the posi joists from underneath. These are the parts that do the work, generally the top one in compression, bottom one in tension.

 

Some have used a metal strip on the bottom chord instead of extra wood

[ProDave]

Insert an extra strong back, or equivalent through the web close to the bath to help ensure the load is spread

[Nickfromwales]

You can also use T&G pine floorboards for the area under the bath, as bare P5 will cave if you get any kind of moisture under there, and DEFO do not use OSB(3) either. 
 

Are you tiling in here? If so, you’ll be fitting plywood and spreading the point load with the tiled floor, and at that point P5 / ply / tile will be plenty here, with a couple of noggins obvs. 

[markharro]

This is interesting as my other half wants to put in a 160kg bath and I am concerned about the loading. We spoke to MBC who provided the frame and they gave the go ahead but I think I may ask our joiner to beef up the joists below (still exposed) as belt and braces.

[saveasteading]

Assuming the joists are as specified for a bathroom then the bath weight is accounted for. The joists are OK. But you could splice more wood on, or a ply plate on the side if it makes you more comfortable. It doesn't have to reach the supports.

But, as above, the floor boarding is the weakness. 

 

[richo106]

It already has 22mm T & G chipboard floorboard above, all the floor will be tiled and then bath sat on the tiles 

 

I will put some extra noggins and strongback's in like suggested 

 

Thanks again

[Gus Potter]

10 hours ago, richo106 said:

Hi All

 

I am just designing my main bathroom and we have found a freestanding bath we like. One concern I have is the weight of it!

The bath is 180kg, capacity to overflow is 400L and guessing got the allow for 2 adults (worst case) so quite a lot of weight

The bath will definitely site on 4 joists and more likely 5 joists (see attached layout)

They are posi joists at 400 centres, 75x225mm

 

Is this weight spread over those joists anything to worry about?

If so how would i check/confirm this? 

Hello @richo106. Good question. This bath thing crops up on BH from time to time. For a bit of fun I've had a go below to explain in broad terms.. how to have a bathtime without ending up in the sitting room say below.

 

Bit of techy stuff first. I'm going to use loads that don't have an apparent factor of safety explicitly shown as the values I present are based on what is called "permissible" or "safe working loads" that have the factors of safety built in but hidden. I'm also going to refer to the British standards (very much still in use) rather than the Eurocodes... which are not bathtime reading.

 

Let's start with the loads that a domestic floor often is required to carry. BS 6399 part 1 on says:

 

 

Ok the 1.5 kN/m^2 (about 150 kg per square metre of floor)  uniformly distributed load is often seen when you look up any basic joist manufacture's data tables and suppliers of chipboard flooring. But what you rarely see is how much point load say 22mm thick P5 flooring can carry. We want to know this to ensure your bath legs don't punch through the floor in the first instance. The table above requires that the floor can carry a point load of 1.4 kN (about 140 kg). Now say you have a penchant for cowboy boots or high heal shoes. This is a concentrated load which causes punching shear (like bath legs) and we can, if we have the will to live, work this out mathematically to some extent but this would give a very conservative result. For design purposes we want to look at test data. 

 

But to make things a bit easier (broadly speaking) we could find some manufacture's data that tells us what the safe working punching shear capacity of chipboard flooring is! Now as luck would have it Caberfloor have a nifty table as below:

 

 

Start at the top left of the table. For an 18mm thick flooring loaded with a 25 x 25mm contact area the floor can carry 1.9 kN (~190 kg) on glued on joists spaced at 400mm apart... but this is away from the floor edges which are much weaker.

9 hours ago, JohnMo said:

You only really need allow the 400kg of water (plus bath weight), two adults would displace their volume in water. We are mostly water, so 100kg person should displace around 100l of water.

This is true but for the doom merchants (if heard of this) if you are daft enough to fill the bath up to the overflow and then step into it when you are still standing you don't displace that much water. That is when it will fail. Motto is.. keep your pants on until fully submerged to avoid embarresment.

 

For me I would be inclined (if the bath is heavy) to take other posters advice and add a few noggings under the bath leg positions. If tiling with large format tiles I would be double up the joists under the bath leg area to avoid differential deflection that could crack the tiles. Also if the bath leaks a bit then the performance of the flooring drops off dramatically so having some solid timber under the legs of a heavy bath is a good idea.

 

 

 

Edited Friday at 19:20 by Gus Potter