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I am building decking 310 by 330 to support an inflatable hot tub and a gazebo. the ground slopes away from the house by roughly 10 cm/meter so will need to raise the decking about 30-40 cm at the end furthest from the house.

I have had one builder round to have a look but was also considering doing it myself as I am pretty good at DIY around the house and looking online I had figured that I could do it myself

However the Builder who has been to see about the job insists that, for the amount of weight I will be having with the hot tub I have a brick wall built on 3 of the 4 sides instead of just using posts.

Is he correct, or can I just use properly foundationed posts into the ground (700mm deep is what I have read). If I do just use wodden posts how many shuould a deck of this size and carrying this weight use?

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  • How far off the ground it the deck? I assume since this is an inflatable hot tub it is sitting on the surface of the deck and the deck is not build around the hot tub? Brick sounds odd, concrete block would be quicker and better. The biggest concern I'd have with using wood beams to support the tub would be the wood eventually failing due to weathering. – Fresh Codemonger Apr 6 '19 at 15:28
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Metric... Lol, the easy part. 1 liter of water is 1kg so you need to consider that inflatable hot tubs are 800-1200 ish liters, so up to around 1200 kilograms. That's 2645 pounds. That's how much a compact car weighs. Plus people, plus your gazebo. Decks are very easy to build but you need more than a deck you need support and lots of it. I don't know your skill level and I don't ever want to discourage anyone that wants to get in there and get dirty but there are valid safety concerns. That being said I don't think you need a brick wall but I don't know the conditions of the ground your deck would be on. Here in Ontario we dig holes in the ground at least 120cm deep 25cm around and fill them with concrete as a support pads for the structure. A brick wall seems like an expensive option but it would be very strong. If you live in a cold climate and experience freezing temperatures, that brick wall has to extend below the frost line or it will heave.

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  • Thanks I’m in the UK so weather is not as extreme as it can get in the US. My plan was the same thing dig down place a concrete block in the bottom of the hole for the post to sit on and then postcrete round it. – Richard C Feb 24 '19 at 10:15
  • The use of wood posts vs brick or block is really misleading. Neither will fail in compression from weight from a deck, even with a hot tub. What could fail would be the support settling into the ground. So as rightly pointed out the contact area with the ground is the crucial thing. Typical soil can support say 2000 lbs / sqft (~10000 kg/m2). This can vary a by soil type. Anyway with a large enough footing its easy to see how the post can support a lot of weight. I know a lot of building in the UK is done with brick/block but I think that is more by convention than for structural reasons. – StayOnTarget Apr 6 '19 at 0:46

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