Floor bounce - PosiJoist Floor Vibration checks

[Mr Punter]

The Posis make plumbing and wiring a lot quicker, especially if the joists have tight centres.  No fun drilling solid joists at 400ctrs.  You won't find the plumber and sparky charge any differently though!

[Iceverge]

With Pozis is there anything to stop you using top hung joists on a ledger plate?.

 

Could you vary the joists depth?Thickening to accommodate the required services on a room by room basis.

 

Assuming a ground floor plant room below a bathroom use 421mm joists and sacrifice ceiling height for service space? 

[joe90]

21 minutes ago, Iceverge said:

sacrifice ceiling height for service space? 

I had to pack out my ceiling joists by 50mm in the cloakroom next to the plant cupboard to get sufficient fall on a drain , just added 50mmx50mm timbers before plasterboarding.

Edited February 8, 2021 by joe90

[readiescards]

600mm posis standard spec - bloody awful for me

really regret them. 

[CC45]

I'd put the ones spec for 600mm at 400mm.  Mate did that - rock solid.

[Adsibob]

45 minutes ago, CC45 said:

I'd put the ones spec for 600mm at 400mm.  Mate did that - rock solid.

This would increase the amount of posi joists required by almost 50% (e.g. a 6m wide space would require 16 instead of 11) which seems like quite a big expense. I have about 120 square metres to cover,  very roughly it's a 7.5m by 10m rectangle plus a 7m by 6.5m, so not a small amount  of joists. Surely if the manufacturer specifies 600mm centres, then as long as they are installed in accordance with manufacturer's instructions, they will be solid. Or am I being naive? 

 

 

[joe90]

2 minutes ago, Adsibob said:

Or am I being naive? 

Well. It depends on your level of acceptable bounce, quite a few on this forum have gone for 400mm and not regretted it (I am one). One comment above said they were disappointed with joists at 600mm. Trouble is once they are in it’s too late to make it better. 

[Adsibob]

3 minutes ago, joe90 said:

Well. It depends on your level of acceptable bounce, quite a few on this forum have gone for 400mm and not regretted it (I am one). One comment above said they were disappointed with joists at 600mm. Trouble is once they are in it’s too late to make it better. 

Yes, well I want no bounce really. And I'm a big guy (100kg) with big stompy feet and my wife always moans about the floors creaking. I've specified 18mm plywood flooring deck instead of chipboard because I thought that would give more rigidity and I thought that was a good thing to pay extra for, but I guess the joists are just as important. I think 400mm centres is overkill though. Surely 450mm or 500mm would be sufficient, particularly with the 18mm ply as the structural floor.

[joe90]

10 hours ago, Adsibob said:

Yes, well I want no bounce really. And I'm a big guy (100kg) with big stompy feet and my wife always moans about the floors creaking. I've specified 18mm plywood flooring deck instead of chipboard because I thought that would give more rigidity and I thought that was a good thing to pay extra for, but I guess the joists are just as important. I think 400mm centres is overkill though. Surely 450mm or 500mm would be sufficient, particularly with the 18mm ply as the structural floor.

Not sure that 18mm ply is better than the standard 22mm p5 chipboard glued with d4 glue. Frankly the joists will give more stability than the flooring anyway. I would not instal them at 600mm (and nor would my builder, based on lots of builds) and I am only 9 stone soaking wet ?.  By the way, my span was 4M.

Edited February 9, 2021 by joe90

[Mr Punter]

The things that affect the deflection of the floor are about in order) joist span, joist depth, joist spacing and joist width.  If the span is less than 3m, do whatever you like.  More than 5m and the choices are limited.

 

Do not accept the minimum allowable deflection, as it will feel crappy.

 

Specify that you require maximum deflection of 8mm or 0.002 x span (whichever is least).  That will always feel nice and solid.

 

The decking does not make much difference to deflection.  22mm chipboard is standard and works fine.

[epsilonGreedy]

30 minutes ago, Adsibob said:

Surely if the manufacturer specifies 600mm centres, then as long as they are installed in accordance with manufacturer's instructions, they will be solid. Or am I being naive? 

 

 

Yes you are, the building trade does not want things to fall down and be sued. Standards evolve to meet that requirement and not your intangible satisfaction score for your new house.

 

Unless you state otherwise the floor will be designed to minimum standards and when you move in you will be reminded daily the floor is designed to a minimum spec, as another poster observes it will be the little things like furniture creaking as you walk across a room.

 

I am a low budget self builder but did not skimp on the floor spec. I told the joist manufacturer to upgrade to 400 centres, then I upped the wood size twice for my long 5.4m posi joist spans and once for the 4m spans. My guess is that upgrading your floor joist spec will add £1000 to your material cost.

 

500mm centres won't work with floorboard sheets sized for multiples of 400 or 600.

 

35 minutes ago, Adsibob said:

particularly with the 18mm ply as the structural floor.

 

 

Is this an improvement? Industry standard flooring sheets can cope with a lot of weather before the roof goes on. Even once the roof is on there might be weeks or months before the windows are in hence more opportunity for rain to settle on your ply.

[epsilonGreedy]

6 hours ago, Adsibob said:

Reading this thread is making me regret switching from regular joists to posi joists in my spec.

 

 

I think you are confusing instances bad workmanship and the broader debate where fussy self builders discuss floor flex.

 

The definition of an acceptable designed floor flex will run and run on this forum because it is a touchy-feely point of house holder satisfaction.  This is really a general debate but is often framed in the context of posi joists because that is what most of us choose, however it is wrong to infer posi joists are the problem.

 

Bad workman ship that leads to people suing major house builders is just that. As a self builder you can oversee the installation of the joists and floor deck, at the end of the day it is a simple and robust procedure. Providing copious mounts of D4 floor is applied and there are no floating joist ends and the strong backs are properly fixed you should be happy with the result.

 

Before the downstairs plaster board ceiling it fixed up you can stress test the first floor in a few hours which gives you a chance to sort out any creek niggles.

[Adsibob]

3 minutes ago, epsilonGreedy said:

 

I think you are confusing instances bad workmanship and the broader debate where fussy self builders discuss floor flex.

 

The definition of an acceptable designed floor flex will run and run on this forum because it is a touchy-feely point of house holder satisfaction.  This is really a general debate but is often framed in the context of posi joists because that is what most of us choose, however it is wrong to infer posi joists are the problem.

 

Bad workman ship that leads to people suing major house builders is just that. As a self builder you can oversee the installation of the joists and floor deck, at the end of the day it is a simple and robust procedure. Providing copious mounts of D4 floor is applied and there are no floating joist ends and the strong backs are properly fixed you should be happy with the result.

 

Before the downstairs plaster board ceiling it fixed up you can stress test the first floor in a few hours which gives you a chance to sort out any creek niggles.

Thanks everyone, you've convinced me of the 400mm spacing, so will insist on that.

 

@epsilonGreedy you make a good point about 18mm ply being damaged by weather. I hadn't really considered that. Would you recommend Caberdeck P5 instead. TP sells it for 17.88 per sheet including VAT, whereas the equivalent amount of 18mm ply would cost 24.40 inc VAT. So ply is actually more expensive. I just assumed even P5 chipboard would eventually soften up and ply would have more longevity, but I've not used anything other than ply previously, and I've never been dissapointed with ply. Except previously has always been in flats with no exposure to the elements.

 

How do you recommend I stress test the first floor? Just walking on it and feeling it, or is there a more technical method?

 

And does the D4 glue just go on top of the joist before the structural floor is laid onto it?

[Dave Jones]

On 06/01/2018 at 21:33, joe90 said:

I have no idea, but all I know is my builder spaces his engineered joists at 400mm not 600 because it’s firmer and I agree ( and glad he did) ?

 

this. 600 for stud walls never floors.

[Dave Jones]

8 hours ago, Adsibob said:

Thanks everyone, you've convinced me of the 400mm spacing, so will insist on that.

 

@epsilonGreedy you make a good point about 18mm ply being damaged by weather. I hadn't really considered that. Would you recommend Caberdeck P5 instead. TP sells it for 17.88 per sheet including VAT, whereas the equivalent amount of 18mm ply would cost 24.40 inc VAT. So ply is actually more expensive. I just assumed even P5 chipboard would eventually soften up and ply would have more longevity, but I've not used anything other than ply previously, and I've never been dissapointed with ply. Except previously has always been in flats with no exposure to the elements.

 

How do you recommend I stress test the first floor? Just walking on it and feeling it, or is there a more technical method?

 

And does the D4 glue just go on top of the joist before the structural floor is laid onto it?

 

use 22mm tongue and groove, glue every joint and screw not nail. You can get it that wiill withstand weather for 6 weeks if your not watertight.

 

Once you've used the expanding glue in every joint and screwed it at 150 centres it is solid. 

 

not worth skimping on as a  bouncy squeeky floor will be with you forever.

[epsilonGreedy]

10 hours ago, Adsibob said:

Would you recommend Caberdeck P5 instead.

 

 

I accepted my preferred BM's default stock decking material, I forget the band name. There are three broad categories of weather proofing:

1. Thick gloopy matt grey painted surface.
2. Thin hard plastic laminate.
3. Peel off pvc covering with taped joints.

If I were to self build again I would make it easy for the brickies to load up the internal blocks on the deck with pallets because there is a correlation between signs of minor water ingress into the chipboard with where the blocks were manhandled.

 

10 hours ago, Adsibob said:

TP sells it for 17.88 per sheet including VAT, whereas the equivalent amount of 18mm ply would cost 24.40 inc VAT. So ply is actually more expensive.

 

 

The non account public TP product prices are a nonsense, I think Travis Perkins is attempting commercial suicide. The manager of my local TP branch (now closed) ridiculed the high prices while attempting to persuade me to open a self builders account.

 

10 hours ago, Adsibob said:

I just assumed even P5 chipboard would eventually soften up and ply would have more longevity, but I've not used anything other than ply previously, and I've never been dissapointed with ply. Except previously has always been in flats with no exposure to the elements.

 

 

@PeterWtold me in another thread that the weather proof chipboard is bathroom rated so it should be much better than regular chipboard on sale at B&Q. Even so you might be correct, perhaps current chipboard floor sheets are the next cladding embarrassment for the industry and 100s of Britons will die falling through their floors in the 2030's.

[PeterW]

Ply at 18mm can’t be used on spans above 400mm as it isn’t rated for 600mm centres, neither is chipboard. For 600mm centres you would need to go to 22mm (which is hard to find in T&G Ply) so you need to compare like for like. 

[joe90]

11 hours ago, Adsibob said:

I just assumed even P5 chipboard would eventually soften up

oh no, with my build the p5 was laid when the walls were up to that level and they worked through the winter off that floor building the next story. At times in heavy rain the water was standing on it and pouring down the stairwell. Frankly I was amazed when 6 months later, after the plasterers had finished, I pulled up the plastic coating to find prestine chipboard flooring even taking the plaster stains with it.

[epsilonGreedy]

11 hours ago, Adsibob said:

And does the D4 glue just go on top of the joist before the structural floor is laid onto it?

 

 

It goes everywhere, it dribbles down the posi joists onto the floor below, it part dribbles and forms stalactites that hang from the joints and off the posi joist metal work, it splats down the facing brick internal feature wall that was meant to be a big feature of my finished house. All this is a result of applying D4 onto the top of the joists and continuously between the tongue & groove joints before the sheets are pushed and tapped together.

 

11 hours ago, Adsibob said:

How do you recommend I stress test the first floor? Just walking on it and feeling it, or is there a more technical method?

 

 

Get a heavy person to systematically stomp, jump, lung and crashdown on the heels of heavy boots.

 

You are looking for:

1. Dry creaking joints missing glue.
2. Floating joist ends where the supporting blockwork is a few mm low. What happens is that the stiffness of the sheet lifts the low joist up to the height of its neighbours.
3. Pipework or cables that rattle.
4. And hardest to provoke, thermal expansion in copper  pipes that causes highly annoying creaks during central heating cycles.

 

Edited February 9, 2021 by epsilonGreedy

[trialuser]

Also consider egger protect / D4 joins, my floor was a padling pool at times but it seems to have survived.

[Adsibob]

26 minutes ago, epsilonGreedy said:

And hardest to provoke, thermal expansion in copper  pipes that causes highly annoying creaks during central heating cycles.

And there I was thinking copper pipes are better than plastic. I've specified copper pipes and brass fittings. But I hate noisy pipes. Currently living in a rental place where the pipes are so loud they can wake us up. @epsilonGreedy are you saying that the floor needs to give the copper pipes space for expansion? Won't all pipe work be between the webbed gaps in the posi joists?

[epsilonGreedy]

22 minutes ago, Adsibob said:

@epsilonGreedy are you saying that the floor needs to give the copper pipes space for expansion? Won't all pipe work be between the webbed gaps in the posi joists?

 

 

No. Noisy expanding copper pipes are a general problem, they have to be supported somewhere/somehow for all floor types.

 

I had to take down a ceiling at my 2000 vintage developer house after years of creaking pipes. This revealed that a plumber had jammed a long run of central heating pipe up against a solid joist at a 90 degree turn, the pipe and joist were fighting each other under heat expansion.

[Dave Jones]

1 hour ago, trialuser said:

Also consider egger protect / D4 joins, my floor was a padling pool at times but it seems to have survived.

 

egger is brilliant stuff. the amount of abuse it will take is unreal.

[PeterW]

4 hours ago, Adsibob said:

And there I was thinking copper pipes are better than plastic. I've specified copper pipes and brass fittings.

 
Why..?? Hep20 is quicker and cheaper and needs less joins and is much easier to install. It also has less impact on flow and pressure reduction. 

[joe90]

4 minutes ago, PeterW said:

 
Why..?? Hep20 is quicker and cheaper and needs less joins and is much easier to install. It also has less impact on flow and pressure reduction. 

+1, I was a late convert but my new build is all hep20, no joins, all single runs with no restrictive 90’ bends, hot and cold manifold, dead easy. A no brainier IMO.