Will this hold for a hot tub?

[Martin Östlund]

Hello,

I'm in the process of building a deck "directly on the ground" with pavers that the joists & beams rest on.
The spa manual states:
"Ensure the surface/deck can support the spa load with bathers of approximately 650 kg/m2"

The total dimensions for the deck are 4300x5300mm
The spa measures 2130x2130mm and will be placed at one end of the deck.

I was thinking of constructing it like this...
Dig out 30cm,
Lay down landscape fabric
Fill with 15cm of gravel 8-16
Compact the whole area
Fill with 10cm of gravel 4-8 on top of gravel 8-16
Compact the whole area
Then beams (outer frame) 45x145
Screw joists in the same dimension 45x145 CC 60
Screw joists in the same dimension 45x145 CC 30 under the area where the spa will be
Decking boards 28x120
Beams + joists will rest on regular paving stones on top of the base layer.

Do you think the construction in 45x145 and 28x120 is sufficient to handle the weight stated in the manual? I'm not quite sure how to calculate or check this?

 

[justusandersson]

You can disregard the dimensions of the decking boards as long as the center-to-center distance between the joists they rest on is no more than 600 mm (for 28x120). The load-bearing capacity and strength of the joists entirely depend on their spacing and span width. In your case, the critical point is primarily the load-bearing beams. If the distance between the foundation posts and the center-to-center distance is too large, the load (shear force) on each post becomes too great. I can tell you right away that single 45x145 beams as load-bearing beams lead to very small foundation post and center-to-center distances. It's better to use double screwed-together (no need for gluing in this case) 45x145.

There is no single correct solution; rather, it can be seen as a zero-sum game. Example: Double screwed-together 45x145 as load-bearing beams with a 1 meter post distance (i.e., slab distance) and 1.4 meter center-to-center distance. Then you can use 45x120 at center-to-center 600 as joists.

I am always referring to C 24 wood (except for the decking boards, which are not strength graded).

 

[Martin Östlund]

You can disregard the dimensions of the decking as long as the c/c distance between the beams it rests on is no more than 600 mm (for 28x120). The load-bearing capacity and strength of the joists depend entirely on how densely they are spaced and their span. The critical point in your case is primarily the bearers. If the distance between foundation pads and the c/c distance is too large, the load (shear force) on each pad becomes too great. I can say directly that single 45x145 as bearers lead to very small pad and c/c distances. It is better to use double screwed together (no need to glue in this case) 45x145.

There is no single correct solution, but you can see it a bit as a zero-sum game. Example: Double screwed together 45x145 as bearers with a 1 m pad distance (i.e., slab distance) and 1.4 m c/c distance. Then you can use 45x120 on c/c 600 as joists.

I am always referring to C 24 timber (except for the decking which is not strength-classified).

Hello,

Thank you so much for taking the time to read & respond.
I'm not sure if I understand you correctly, but do you think that bearers 45x145 and joists 45x145 with cc30 are too weak?

A detail I forgot to include in my original post (if it has any significance at all) is metal joist hangers 45x97mm and for these anchor screws 4.8x40.