What thickness plywood to revive sagging sofa?

The sofa in IHN towers has seen a bit of action, if you know what I mean, eh, eh, nudge nudge, wink wink.*

*Clearly what I mean is “people sitting on it every night to watch telly for the last four years”

The springs in the base have gone a bit baggy, and I’ve looked at the ‘sofa-saver’ things you can buy but the base sofa is quite deep (600mm) and they’re not deep enough (they’re generally about 480mm). So, I’m thinking of getting a piece of ply cut to size to place on the base, on top of the sprung bit, to bring it back to life a bit. It needs to have a bit of give, so I’m thinking that 6mm would be the right thickness. Sound about right?

FWIW, it’d be about 1600mm x 600mm, supported along the long edges on the sofa frame.

so I’m thinking that 6mm would be the right thickness. Sound about right?

For splinters on your arse?

12mm minimum. I’d go for 18mm if it was me.

Really, that thick? The sofa-saver things you can buy are 5mm MDF. It’s not going to take the whole weight of me, the Mrs and the dog, it’s just got to spread it out across the existing springs (most of which aren’t too bad)

Is it big rubber bands?

I have a roll of that somewhere plus the end pieces.

I tried a piece of 18mm MDF on our sofa, but it was far too rigid. Thinner plywood would probably be a better idea, as it will flex, whereas MDF won’t.

We’ve 6mm ply under our cushions.
I’ve also planks of wood screwed to the base and back of one sofa.
Clearly all about the use our sofa has had (fnnnar) and the crap quality of Debenhams sofa’s…

Don’t use mdf…. It’s essentially just paper compressed. It’ll absorb moisture and quickly lose its structure /rigidity.

9mm birch ply will have a bit of give yet be strong enough to take the weight of adults sitting om it. It will however give over time.

I’d go for 12mm birch or,15mm shit ply.

And four years for a sofa? That’s shit….!

Like Alpin said – 9mm. That’s what we’ve used so it still gives a bit. Our cushions are the loose fill type so anything thicker would be like sitting on a plank.

Cheers all

It’s not ‘rubber bands’ under the cushions, it’s those long flat s-shaped springs. They’re within the upholstery of the base of the sofa, so I can’t get to them. To be clear, the base of the sofa has not gone completely, just a couple of the springs are ‘giving’ more than ideal (the ones under the bits where we always sit).

The ply I’ll be putting in won’t be taking all the weight, it itself will be supported by the springs, most of which are fine. There needs to be a reasonable amount of give as the cushions aren’t that deep.

Aren’t the springs accessible if you take off the plastic hessian stuff under the sofa? I replaced the ones (on my wife’s side, I may add) – you can buy all the bits on Ebay (you probably know that).

I reckon 6mm will be fine to spread the load.

Forgot to mention I bonded on some inch thick upholstery foam too. Lush!

I reckon 6mm would be fine too. Its not actually taking all the weight is it, more spreading load through cushions.

However to be safe, I’d say 9mm artisan Baltic Birch Plywood, veneered with American Black Walnut.

I think 9mm would still be pretty stiff even across that fairly large span – through the cushion I doubt you’re arse could tell the difference between 9mm and 18

6mm would seem pretty flimsy of it was in unsupported span but its not

No point spending money on birch ply for something you can’t see – its main attribute its it looks lovely. But it goes mouldy and black  if its under things like cushions and mattresses where it can be a bit humid.

A sheet of bog standard 6mm ply would be fine and should only cost about a tenner or so. If theres a choice of bog standard 6mm plywoods at the merchants theres sometimes either Eucalyptus faced ply which is made of a few thin plys and and usually a reddish tinge and another one – (I forget which) which is one thick ply and two thin face veneers and usually a more yellow tone. The former is a bit more resilient and the cut edges are a bit more well behaved and lest prone to splintering and splitting – but either would work fine.