How to build an outdoor box seat?

[Bolton]

 

 

Hi team, I am building an outdoor box seat with storage. It will be down in a sunken firepit (which has drainage) however my question is about material. 

I was thinking of building a base with marine ply and facing it with aluminum and marine ply built seat faced with external cladding. Is this the best way to stop any water getting to the ply ?

Pictures is of area it’s going so it will have rain contact. 

Also is thick marine ply enough or should I use treated pine for a frame first ?A Bunnings video I watched just uses mdf however that was inside but no pine frame as of such. 

My second option was to build a seat on metal feet. 

[MitchellMc]

Hi @Bolton,

Marine ply, despite its name, isn’t fully waterproof and only performs slightly better than standard ply in wet conditions. To prevent water damage, it needs to be completely sealed on all sides and edges with multiple coats of exterior paint or sealant.

If durability is a concern, especially given the exposure to rain, you might consider constructing a frame from H3-treated pine and cladding it with Merbau decking. This would provide better longevity in outdoor conditions compared to relying solely on ply.

If you do use marine ply without a frame, make sure the lid is well-supported to prevent sagging over time. Also, just a note—if you’re planning to use the firepit while the bench is inside the sunken area, it’s worth reconsidering. That space was likely designed with safety clearances in mind, and adding flammable materials could pose a risk. 

Please let me know if you have any questions.

Mitchell

[Bolton]

Thank you Mitch, I think I’ll use a frame with h3 timber. I’ll then use external cladding or decking you mentioned. I plan to put a support for the lid midway as it’s only 2 metres long. 

The fire pit is being swapped out for a gas one that is much smaller too. 

Thanks as always. 

[Bolton]

Hi @MitchellMc , it’s coming along nicely. I just need to know what you would use as a ‘lid’. I’m using vj hardie board for the facade however it’s 2.2m wide and a storage inside so I think that may bow. Cushions will be on top of the lid so you won’t see it. 

Which material should I use ?

also which screws, filler and cutting tool is best for exterior vj hardie ?

Thanks

 

  

hanks

[JacobZ]

Hi @Bolton,

You could use decking such as this 90 x 22mm Decking H3 Treated Pine or SpecRite 90 x 19mm Merbau Pre-Oiled Decking, but it would need to run vertically so the span is not over 450mm. This will give you a good strong lid that can support the weight of those sitting on it without any issues.

Alternatively, you could use plywood, but it would need to be fully sealed with exterior paint to ensure it does not rot.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[Bolton]

I have some marine ply on top just now as a temporary measure and it’s split within 2 weeks fully treated !! 

Would Hardie board work or even the weatherboard I have on there now ? Obviously I’d need to build a top frame which I’m hoping to build with 70x35. 

[MitchellMc]

Provided the cement sheeting/board is adequately supported, it should work well for your lid @Bolton.

Mitchell