Supporting joists under bouncy floor

[Flipjango]

Aa

I think you'd need to experiment to get the best design, but essentially a wooden post cut so there is a folding wedge, a plate top and bottom to spread the load and make it so it doesn't topple over, and a threaded rod (which you can buy from the builder's/engineers merchants in various lengths) which with various lock nuts/welded nuts, washers and a bit of bracketry will close the wedges and raise the jack when turned with a spanner on the end. The wedge post will be very solid when raised.

Think I'd think about getting a 75x75 or 100x100 treated fence post to cut up. If you cut them to height so the wedge is 80-90% home when raised in the correct position, you'll get a very solid support. You'll need something to keep the top and bottom bit's of the post in line - bit of ingenuity needed - depends what access to tools you have.

Or even buy a few really cheap scissor jacks, and modify them so you can easily position them without toppling over. https://www.amazon.co.uk/Quality-Tonne-Scissor-Handle-Octavia/dp/B0721MSB52/ref=sr_1_97?crid=2XIGJI4LTSBDC&keywords=wedge+jacks&qid=1687162421&sprefix=wedge+jacks,aps,76&sr=8-97

the bit I’m struggling with is the timber post cut so there is a folding wedge. What hinges it?

 

[mrrusty]

Nothing "hinges" it - it's just a wedge which when pulled in by the threaded rod pushes the other parts apart. The wedge needs to be nearly fully home at the point the post starts to take the weight.

strictly speaking it's not a "folding wedge" which is a pair of wedges, but the principal is the same.

Wedge - DIYWiki

wiki.diyfaq.org.uk

 

[Flipjango]

So my question now @oldbutnotdead @mrrusty i need to put something like a brick or a paving slap or even a timber post onto the dpm without tearing or puncturing which might be difficult. Thinking of a bed of mortar, but worried it’ll just dry and slip away as the dpm is just a sheet of plastic at the end of the day. What’s going to happen? What about using something like cempolatex fibreflex self levelling to make a level bed beneath each raft?

 

[mrrusty]

I think you are over-thinking it. You're only trying to take out some bounce, not hold up the whole house. Presuming it's dry under there, If you mount something(post/jack whatever) onto a square of 18mm ply perhaps 300 x 300mm it'll spread the load enough to not puncture the dpm. If the surface is bumpy/sharp underneath the dpm perhaps a piece of 10mm polystyrene between ply pad and dpm - it'll take the dints of the sharp points underneath and still be tough enough to spread the load and give you support to take out the bounce.

The compressive support strength of a 300x300 bit of polystyrene is in the order of about 2 tonnes

 

[oldbutnotdead]

Given the limited space under there you might struggle to fit those wooden jackwedges in (like the idea though).
The ply base on a polystyrene tile is a sound plan.
The legs really need to fit under the joist (pain i know) rather than screwed to the side of the joist cos otherwise the insulation will need cutouts (not ideal for thermal performance).
I'd jack a joist up, measure clearance, cut leg roughly to length, offer it in with base etc and trim it til it sits vertically (if you're on a slopey bit) and fits snugly under the joist.
Release jack, move to next joist.
If you find the floor is a bit up and down after your first run, use a straight edge to find the hump (or the dip), jack up, adjust, release.
Once you are happy, skew-screw through each joist down into each leg.
Yes 6 x 2 is a bit marginal for that span, specially with all that load from the kitchen units.

 

[Flipjango]

I think you are over-thinking it. You're only trying to take out some bounce, not hold up the whole house. Presuming it's dry under there, If you mount something(post/jack whatever) onto a square of 18mm ply perhaps 300 x 300mm it'll spread the load enough to not puncture the dpm. If the surface is bumpy/sharp underneath the dpm perhaps a piece of 10mm polystyrene between ply pad and dpm - it'll take the dints of the sharp points underneath and still be tough enough to spread the load and give you support to take out the bounce.

The compressive support strength of a 300x300 bit of polystyrene is in the order of about 2 tonnes

Thanks, I am probably overreacting, you’re right.

Is it ok to put timber directly in the dpm?

 

[mrrusty]

Is it ok to put timber directly in the dpm?

No - you need to spread the load with a plate - unless it's concrete underneath, just a post will sink down. Or if you mean is it OK to put a timber plate directly on the DPM, yes, unless it's running wet.

Once you are happy, skew-screw through each joist down into each leg.

I suspect with the restricted access, this might be an issue. TBH, I think I'd go and buy a few of those cheap scissor jacks, mount them on a plate, push them into place and jack them up. They won't go anywhere, and at least you can jack them from a distance away. Even a small spike on top (screw poking up to dig into underside of joist?) would be enough to stop them toppling sideways, if it was thought necessary

 

[Flipjango]

Something like this be ok? Polystyrene Foam Sheets Boards Slabs https://amzn.eu/d/0sT0dEw

 

[Flipjango]

No - you need to spread the load with a plate - unless it's concrete underneath, just a post will sink down. Or if you mean is it OK to put a timber plate directly on the DPM, yes, unless it's running wet.



I suspect with the restricted access, this might be an issue. TBH, I think I'd go and buy a few of those cheap scissor jacks, mount them on a plate, push them into place and jack them up. They won't go anywhere, and at least you can jack them from a distance away. Even a small spike on top (screw poking up to dig into underside of joist?) would be enough to stop them toppling sideways, if it was thought necessary

Yes, I meant the ply plate on the dpc. Scissor jacks seem a good idea if they fit under. It’d be a lot easier! Can’t quite work out how I’d modify the tops though as all seems to be welded together.

 

[mrrusty]

The top bracket on the jack is probably either rivetted or welded on. Nothing an angle grinder wouldn't sort - or even a hacksaw. However, if you can slip a piece of wood through the top bracket and still have enough room to slide the jack in, I wouldn't bother.

I suspect you have enough ideas now - you're just going to have to have a go and see how you get on....

 

[Harry Bloomfield]

TBH, I think I'd go and buy a few of those cheap scissor jacks, mount them on a plate, push them into place and jack them up. They won't go anywhere, and at least you can jack them from a distance away.

A local car scrap yard will have hundreds of jacks, in the boot of many of the cars, ask for a deal on however many you need.

 

[robinbanks]

I don't know how we'd be able to spread it across the floor

I didn't suppose it would cover the entire area.. It's mid span support that is required; perhaps I should have said "beam" instead of "slab"..

 

[phatboy]

I'd put a 450mm square paving slab to spread the load, and then a timber leg bolted into the joists, or a brick flat or on side depending on the gap. and bits of slate where required. Very cheap way to do it.....