Supporting joists under bouncy floor

[Flipjango]

We recently had a kitchen extension done to our Edwardian house and went with suspended timber floors so we could run exposed floorboards through the house to match the other rooms. We had no issues until the kitchen was fitted and heavy quartz worktops installed last week, and now suddenly the kitchen is like a trampoline- everything on surfaces rattles when someone walks through the room, and you can feel the vibrations across the room. I saw everything being built and whilst it’s passed BC and generally looked ok, my suspicion is the span of the joists (3.7m) might have been a bit long and there should have been a supporting wall mid way.

We’ve had some issues with the builders and don’t think we’ll be getting them back so I’m hoping to to try and deal with this myself. The floor is all sanded and finished and the kitchen fitted and worktops in place so lifting the floor all up isn’t really an option now and there is no crawl space underneath,bexceps about a foot or air under the joists, so I was thinking that perhaps we could lift the central three or so floorboards to gain access to the underneath (these thankfully don’t run under the kitchen except a bit that’s under the island). Then I was wondering if we could perhaps hammer something like CLS studwork timber supports between the earth sub-floor and the joists, under tension (perhaps jacking up with a car Jack first?). I have a feeling digging a concrete foundation for these would be good but this might be hard to do within a restricted space, so was thinking maybe laying a paving slab under each support to spread the load on the floor. Does this sound like a feasible plan to reduce the bounce, given the restricted space we’ll be working in?

 

[oldbutnotdead]

If you can access the void then yes, timber props or piles of bricks mid-span will be fine- as you said, jack joist up til its straight, insert prop, move to next joist.If you do use timber, make sure there is dpm between timber and subfloor, if you use bricks again dpm between bricks and joists.
Timber again plate the legs to the joists so they won't fall over if load is removed.
If the subfloor is 50 mm concrete, you can get away without a spreader, 300 x 300 would be overkill.

EDIT Is there insulation between the joists? There should be

 

[Flipjango]

Thanks @oldbutnotdead. The subfloor is earth, though I believe they hit sandstone when digging so mostly rock with a bit of earth on top I suspect. There is insulation so we’ll have to cut that out and put back and retape after, but it’s just celotex between the joists so shouldn’t be too difficult.

I did wonder if we could use underpinning jacks is we have sufficient space- this might be easier than jacking up? Don’t mind the cost if this’ll make the job easier. Would that still need DPM?

 

[oldbutnotdead]

Tut tut, naughty builders, earth subfloor will be allowing loads of water vapour into the subfloor.
Usual is DPM and 50mm concrete, acceptable is DPM and a load of gravel on top (to stop the dpm lifting in case of flooding).
If you can get enough access to the void, consider this as a retrofit before putting your jacks in.

The dpc on legs or whatever is to prevent moisture transfer from the ground/wall to the timber. If your jacks are metal you might want a bit of dpm under them but probably redundant if they're designed to live in the ground for 50 years

 

[Flipjango]

Tut tut, naughty builders, earth subfloor will be allowing loads of water vapour into the subfloor.
Usual is DPM and 50mm concrete, acceptable is DPM and a load of gravel on top (to stop the dpm lifting in case of flooding).
If you can get enough access to the void, consider this as a retrofit before putting your jacks in.

The dpc on legs or whatever is to prevent moisture transfer from the ground/wall to the timber. If your jacks are metal you might want a bit of dpm under them but probably redundant if they're designed to live in the ground for 50 years

Just lifting boards now so will confirm subfloor when we get to it. Truthfully I can’t remember what’s under there.

 

[Flipjango]

So @oldbutnotdead it gets a little more complicated. Under the joists is a dpm but no gravel. The void is tiny and the ground underneath completely uneven (I have a feeling there is a big lump of concrete under there), so gaps are as little as 100mm in places, as much as 160mm in others. But as we're only lifting a small section in the middle, three floorboards wide, with such a small void it'd be near impossible to put gravel on top, and there are pipes and wires running across the void as well. So the problem we have now is working out how to support the joists on an uneven floor. In places the supports would be resting on slopes on the floor. We obviously can't dig down or risk damaging the DPM so no idea how to deal with this. Any suggestions?

As an aside, I am still not completely sure they are correctly sized - they measure 44 wide x 165 high. Looking this up suggests that standard 47x170 joists (which I assume mine are after being planed) can support a span of 3.77 if C24 and 3.38 if c16 but I can't find anything on them to tell me which they are. Span is 3.70. Anyway, neither here nor there now, as we just have to deal with what we've got, I can't face ripping out the whole kitchen to lift the floor and replace now.

 

[robinbanks]

Any suggestions?

Pour a concrete slab?

 

[foxhole]

Engineering brick topped with short (4x2) legs screwed to side of existing joists .Each fix will require slight jacking before fixing to keep all legs “loaded”.

 

[Flipjango]

Pour a concrete slab?

I don't know how we'd be able to spread it across the floor as only a small section is being opened up due to the kitchen being fitted already

 

[Flipjango]

Engineering brick topped with short (4x2) legs screwed to side of existing joists .Each fix will require slight jacking before fixing to keep all legs “loaded”.

Thanks, what do you mean by 'legs' please? Also, would it be ok to put the engineering brick directly on the dpm? Worried it might cut into the plastic?

 

[oldbutnotdead]

Yeah your builder has been very naughty- take it Building Control weren't involved (minimum clearance from joist to subfloor is usually 150mm).
The engineering brick under a timber leg is a decent call if there is room. You'll have to plate the leg to the joist (otherwise the insulation won't go back in) and ideally cut the bottom of the leg to match the angle the brick is sat at. Or put a mortar bed under the brick so it is level.

 

[mrrusty]

So, the joists are marginally sized, and the floor is a bit bouncy. I think, with a small access some lateral thinking is needed. An idea that springs to mind is to make up some custom jacks for each area with a top and bottom base plate, folding wedges between and a long piece of threaded bar to both push the jacks in to position and then spin them to pull the folding wedges together and raise the jack. Bit of knife and fork engineering needed. You could custom build them for each position at different heights.

bit like this

 

[Flipjango]

So, the joists are marginally sized, and the floor is a bit bouncy. I think, with a small access some lateral thinking is needed. An idea that springs to mind is to make up some custom jacks for each area with a top and bottom base plate, folding wedges between and a long piece of threaded bar to both push the jacks in to position and then spin them to pull the folding wedges together and raise the jack. Bit of knife and fork engineering needed. You could custom build them for each position at different heights.

bit like this

I like this idea but can’t quite work out what I’ll need to do it. Would you be kind enough to treat me like an idiot (!) and give more details please?

 

[Flipjango]

Yeah your builder has been very naughty- take it Building Control weren't involved (minimum clearance from joist to subfloor is usually 150mm).
The engineering brick under a timber leg is a decent call if there is room. You'll have to plate the leg to the joist (otherwise the insulation won't go back in) and ideally cut the bottom of the leg to match the angle the brick is sat at. Or put a mortar bed under the brick so it is level.

All done with full b/c and inspected. I think when dug put, the void was 160mm but someone has emptied the bottom of the cement mixer into it, hence the shallower sections. Can’t see due to the bpm but it feels like that’s what it is.

So, you’d plate the legs to the side of the joist rather than hammer them underneath?

 

[mrrusty]

I like this idea but can’t quite work out what I’ll need to do it. Would you be kind enough to treat me like an idiot (!) and give more details please?

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-To...1687162421&sprefix=wedge+jacks,aps,76&sr=8-97