Thread: Timber deck + gross weight threshold

Originally Posted by Pogue_Mahone

Hi all... sadly its been a while since I got my hands dirty in any real projects... until now...

We bought a spa .... the area we need to put it is just sand at the moment... it arrives in 4 weeks.

So, I reckon I can knock up a nice deck... I am just worried about the weight. The spa book says any flooring needs to hold up to 4.5 tonnes!

H4 treated pine joists and bearers... or some H3.... with concrete footings

So the plan is <---------- 5.7 m -------->
^
|
3.7 m
|
v

I am intending on 4 x (90 x 45) joists of H4 Treated pine running the 5.7m direction
and 18 x (90 x 45) running he other way.

I plan to use 140 x 25 mm Merbau decking running the 5.7m direction

Do you think this will hold the recommended weight? Are there any hints / tips you can give?

Thanks in advance,

Kev

Similar threads and lots of good advice on this on the sister forum http://www.renovateforum.com/index.php so worth your while to join that and re-ask the question.

You should also buy Allan Staine's book Decks and Pergolas at around $30 at Bunnings and others - it will guide you and also help to ask the right questions. You will almost certainly need approval - so plans etc will need to be drawn and submitted etc. You might be able to owner build, but check with your council on that. Remember too that a spa is a 'pool' so the rules about fencing and restricted access for children etc apply as they do for a swimming pool - and for good reasons. See http://www.dhw.wa.gov.au/193_361.asp and http://www.dhw.wa.gov.au/Files/RulesforPools3-11a.pdf.

As to you deck description there are some issues already apparent. You only need the part of the deck that will carry the spa to hold its weight. Your description indicates that the spa comprises the structure (fibreglass & timber maybe) say 200kgs and around 4300 litres of water? Or is that including a safety factor? That would be a big spa. In any case spas generally come with quite detailed plans of what is needed to support them - or the manufacturer will supply some.

You haven't said how high the deck is to be and that will make a big difference to the structure. For example you might be able to have the spa sit on, or in, or partially in the ground an simply surround it with the deck in which case the deck carries no load of the spa. In Perth and on sand I'd be sitting the spa at least partly in the ground and bringing the deck up around it. That makes the weight issue much easier to manage and I reckon works better anyway.

You need to draw up some plans to scale - use graph paper is easiest - can be done on a computer if you know how and have the SW, but paper is best as it is easier to take to the site and translate measurements and refer to etc while constructing. It seems to me that you need to find a friend who has a bit more experience to help - this can be a DIY job for someone with experience, but an ambitious one for a novice.

In any case your 'joists' running the long way are actually bearers and need to be at least 90x90 and I reckon you need 5 rows not 6. You did not mention the supporting posts under the bearers - I would use 6 under each row. You do not say whether it abuts a wall or not - if so then you would have a ledger on the wall (90x45) and 4 rows.

As I said you only need to have that part of the deck supporting the spa built more strongly, not the whole deck, so work out where the spa will sit (if it is on top) and draw up a separate support sub-frame for that position. Then the other joists can be at 400-450mm centres not the 300mm you suggest.

Summary - you have some work to do before you can get going and you need to do quite a bit more research. You need plans drawn and check with council for approvals. The electrical and plumbing work will need to be thought through and must be done be licensed tradies. Good luck

Sorry for the ambiguity... always the case... fingers don't type everything I am thinking...

The spa will be sitting on top of the decking... it is all self contained. No plumbing required. Power is taken care of. I have a 32amp hardwired lead sitting up there waiting for it.

The book suggests concrete as the best base - not overly attractive though. Our spa will hold 1400L and the book suggests that med - large spas be sat on a floor that can hold up to 4.5 tonnes.. I am guessing there is a fair bit of 'safety factor' in this figure.

Council approval is already in progress for the spa. Approval for the decking is not yet underway, and won't be until I can be sure it will hold the weight.

Originally Posted by Pogue_Mahone

Sorry for the ambiguity... always the case... fingers don't type everything I am thinking...

The spa will be sitting on top of the decking... it is all self contained. No plumbing required. Power is taken care of. I have a 32amp hardwired lead sitting up there waiting for it.

The book suggests concrete as the best base - not overly attractive though. Our spa will hold 1400L and the book suggests that med - large spas be sat on a floor that can hold up to 4.5 tonnes.. I am guessing there is a fair bit of 'safety factor' in this figure.

Council approval is already in progress for the spa. Approval for the decking is not yet underway, and won't be until I can be sure it will hold the weight.

Not quite sure how the council can approve a spa without approving the structure it will sit on? In any case as I said most spa makers will give you engineered drawings for timber that will hold the weight of the specific spa. You should ask if you haven't already. Then all you need to do is to incorporate that structure in the deck design. Not something to be guessed at although a forum member might do some of the table look-ups for you.

To assist you should provide the dimensions of the spa - ie: the load area that the weight sits on.