Simple Bench Seat Advice

RonnieL

I'm not the handiest, but a couple of years ago I undertook a project to add a bench seat to our garden. At the time, I asked around about materials to use, and people said "composite". I found some the right colour, and ordered it. The moment it was delivered I thought "S%*t, that's heavy". I ploughed on regardless and did a reasonable job creating the seat, but it weighed in at something like 60KG. Wife insisted I do something to stop it breaking someone's hand (fair), so I ordered a pair of the most powerful gas struts I could find. Worked well, I thought, but over time, and presumably because of the constant force of the struts, the seat has warped, and was basically tearing itself apart.

So, I'm going to give it another go and I'm looking for advice from people who know what they're doing. The soft close requirement remains. ChatGpt suggested a sheet of marine ply, which seems reasonable? Once bitten twice shy, so even with marine ply, I'm worried about the weight of it, and finding the right hinges so it'll stay open when the kids want to get a ball, and close softly also.

I've already dismantled the seat, except the frame attached to the walls, which you can see in the second picture below.

It's about 178cm wide, and 60cm deep.

So, what I'm currently thinking is a sheet of 18mm marine ply, and a pair of these hinges → https://silvershardware.ie/pair-of-bench-seat-hinges-with-spring-12kg-capacity-p-963.html

Thoughts? (thanks in advance)

Lumen

18mm marine ply in that size is going to weigh maybe 10kg for a single layer.

Plywood, in general, performs badly outdoors. Marine ply is better than regular ply but it still needs to be properly finished (several layers of outdoor rated varnish or paint) in order to be waterproof, and a horizontal sheet is the worst case because it will collect water, and if it sags it will collect even more.

Boat building grade marine ply is more expensive still, and still needs proper finishing so might not be worth the extra. It is prettier but that only matters if it's going to be varnished rather than painted.

RonnieL

Thanks for the reply. When I did the bench the first time around, I made sure there was a bit of a slope to avoid water pooling, so I'll aim to do the same again.

After posting last night I found some more information about marine ply density, and now I'm thinking I'll need to use a sheet of 12mm ply for the top, and I'm thinking I'll build a base for that to sit on from treated timber. I just need to figure out how much that will weigh.

Regarding the water proofing - chat gpt suggested sanding, and then using an epoxy sealant, especially around edges. When I googled that, some products came up, but I'd be open to suggestions if anyone has any?

10-10-20

Epoxy no, but I have used Owatrol Deck oil to protect a bench here to very good effect. I applied about 4 coats onto treated timer from a builders' providers. You can then top-up easily as needs be.

https://www.owatrol.ie/product/deks-olje-d1/

https://brooksonline.ie/timber/treated-timber

Lumen

https://www.boards.ie/discussion/comment/123536343#Comment_123536343

I've used epoxy, oils and varnishes (water and oil based) on outdoor projects including with boat building rated ply.

Probably wouldn't use epoxy as it might crack under flexion.

Decking oil not ideal for horizontal surfaces (decking is fine because it's fundamentally more durable than ply).

Water based outdoor poly varnish works and is nice to use. Yacht varnish has best waterproofing but nastier. Either way the more coats the better.

GreeBo

Do you need the entire seat to be fully open underneath?
Easiest way would be to divide up that space to match the bracing on the seat section, and have the dividers run to the ground.

Long term I would probbaly use some angle iron as an exterior frame. Composite will always sag, especially in warm weather.

RonnieL

Job done. Time will tell how it lasts, but it's better than it was.

Thanks for all the input, much appreciated.