Decking for Hot Tub - Weight

powerful_Rogue

Afternoon all,

Looking at putting some decking down for the hot tub to go on.

I've seen this 2.4m by 3.0m kit here - https://www.savoytimber.com/2-4x3-0m-deluxe-kit-no-handrail.html

I dropped them a message and they stated ' A wooden decking can hold around 45 pound per square foot.

The hot tub holds 800 litres of water. I used a water calculator and this states the weight of the water would be 1751.67

Am I right in thinking this decking kit would be no good? Struggling to get my head around the math after a long day at work!

Ectophile

I think this perfectly demonstrates why all the people saying we must go back to using imperial measures should be firmly ignored!

45 lb / ft² = 20.4 kg / ft² = 219 kg / m².

2.4 x 3 m = 7.2 m².  Which should be able to hold 1577kg.

800 litres of water is near enough 800 kg.  Add in the weight of the hot tub itself, and the weight of the people in it, and it will probably be OK if the decking is constructed well enough.

Alternatively, 7.2m² = 77.5 ft². 77.5 x 45 = 3487 lb.  Which is rather less than 1751.

powerful_Rogue

Ectophile said:

I think this perfectly demonstrates why all the people saying we must go back to using imperial measures should be firmly ignored!

45 lb / ft² = 20.4 kg / ft² = 219 kg / m².

2.4 x 3 m = 7.2 m².  Which should be able to hold 1577kg.

800 litres of water is near enough 800 kg.  Add in the weight of the hot tub itself, and the weight of the people in it, and it will probably be OK if the decking is constructed well enough.

Alternatively, 7.2m² = 77.5 ft². 77.5 x 45 = 3487 lb.  Which is rather less than 1751.

Very much appreciated, thank you.

rob7475

I'm no structural engineer but I'm not sure if the calcs above are quite right. As stated, the decking could hold a total of 1577kg but that would need to be distributed fairly evenly across the whole span.

What sub base will the decking be sat on? If it's a 4" concrete base, it should take more weight than being sat directly on soil for example.

Those kits probably allow for the joist spacing at 600mm centres. I'd probably look at changing the spacing to 400mm centres. I'd also change the boards to 32mm rather than 28mm too.

powerful_Rogue

rob7475 said:

I'm no structural engineer but I'm not sure if the calcs above are quite right. As stated, the decking could hold a total of 1577kg but that would need to be distributed fairly evenly across the whole span.

What sub base will the decking be sat on? If it's a 4" concrete base, it should take more weight than being sat directly on soil for example.

Those kits probably allow for the joist spacing at 600mm centres. I'd probably look at changing the spacing to 400mm centres. I'd also change the boards to 32mm rather than 28mm too.

Thank you. It will be going on the soil.

Going to go with the one below from eDecks that is designed for Hot Tubs.

https://www.edecks.co.uk/products/46427/233/1383/0/Hot-Tub-Gazebo-With-Decking-DIY-Kit-(2400mm-x-2400mm)

TELLIT01

The claim that the deck will support a hot tub is meaningless if there isn't a solid base for the deck to sit on.  I would suggest that any deck would sink into the soil with the weather we have had if the deck is placed directly onto grass.

weeg

We did a fair bit of research in our (structural engineers) office on this recently, because it's not covered in the loading code. Our conclusion was that for raised decks (and rooftops) the allowance should be 6kN/m2. Which is 600kg/m2. Usual balcony allow is 2.5kN/m2. To be fair, we're probably being a bit conservative. 

But doubling up the number of joists you have for a basic deck is about right.

Nobbie1967

powerful_Rogue said:

rob7475 said:

I'm no structural engineer but I'm not sure if the calcs above are quite right. As stated, the decking could hold a total of 1577kg but that would need to be distributed fairly evenly across the whole span.

What sub base will the decking be sat on? If it's a 4" concrete base, it should take more weight than being sat directly on soil for example.

Those kits probably allow for the joist spacing at 600mm centres. I'd probably look at changing the spacing to 400mm centres. I'd also change the boards to 32mm rather than 28mm too.

Thank you. It will be going on the soil.

Going to go with the one below from eDecks that is designed for Hot Tubs.

https://www.edecks.co.uk/products/46427/233/1383/0/Hot-Tub-Gazebo-With-Decking-DIY-Kit-(2400mm-x-2400mm)

What are the dimensions of the hot tub? If it’s placed in the centre of the deck it should be Ok as the whole deck will just sink a bit into the lawn, but if it’s placed away from the centre then that side of the deck will sink into the soil causing the whole deck to twist. The one you linked to only uses very small joists, 90*40mm which won’t prevent this twisting.

Probably best to look at using ground screws to support the deck. This also has the advantage that you can lift the deck off the ground which will prolong the life of the timber. One at each corner, halfway along each side and then additional ones under where the hot tub will sit should do it.

casper_gutman

Using ground screws might well be a good idea, but would that deck - with such small joists - be up to the job of supporting a hot tub if it had to span between point supports instead of just sitting on the ground? On the other hand, standard treated timber won't like being in direct contact with damp soil and will not last long. One way or another, the total lack of any information on the eDecks site about suitable foundations/bases for the product seems like a pretty glaring omission!

Albermarle

I suppose the ideal solution would be to lay a concrete base. An easier way could be to dig down a few cms and lay some gravel. It might still sink a little after heavy rain, but at least would let some air around the decking base so less likely to rot. 

Murmansk

When I installed some decking on soil I dug holes and hammered a fence spike into the bottom of the holes then poured concrete into a mould made of Contiboard. I ended up with 4 nice solid platforms effectively bonded to a nice "pile" (the fence spike) which I reckon gave the concrete a much better chance of not moving than concrete alone.