bouncing joists cracking plaster

[ajrobb]

I've taken down the lounge ceiling, several of the joists are floating above the supporting wall and I can get a steel rule (or more) between the bottom of these joists and the wall. This allows the joists to bounce, which causes movement and cracks between the ceiling and walls.

If I push the joists up with my hand, I can get a 5mm wedge under some joists. Other joists don't budge.

How can I stop this bounce?

 

[hotrod]

Are you saying the supporting wall that provides mid-span support for your joists doesn't extend high enough in places - effectively leaving your joists "floating" and deflecting excessively. Could you pack it up where required to meet the underside of the joists?

 

[ajrobb]

There is no mid-span support, just wooden herringbone trusses mid-span. The joists run side-to-side between party wall and end wall over a 3.6m span.

Actually, the bottom of the joist sockets seem pretty level. I think it is the joists and floorboards that are warped slightly. Also, the central herringbone trusses might not be letting the joists sit down in the sockets. There are also quite large gaps at the tops of some sockets. I've got the skirting boards off, so I could pack these gaps with something. I'm worried that joists might need to expand into the sockets and packing them solid might push the walls out.

I don't have a ceiling up right now, so there is not much weight on the joists - just the 18mm T&G boards. I've just taken down a 9.5mm plasterboard ceiling and I intend to replace it with heavier 12.5mm Soundbloc. Perhaps that weight might be enough to pull the joists down.

 

[HERTS P&D]

This is what we are doing to our through lounge/dining room

1. Remove all coving and ceiling.
2. Pack all joists with timber glued wedges (level)
3. Foam fill every small gap around joists.
4. Supply and fit new loft insulation to joists, held with 4'' nails or chicken wire. (will make a big difference in colder months)
4. Supply and fit new 9.5mm plaster board.
5 Supply and fit second sheet of plaster board grip-filled around all edges and staggered joints
6. Re-plaster ceiling
7. Coving
8.Paint.
9.Refit lights.

Job Done.

Andy

 

[Static]

Were they creaky/bouncy before you removed the ceiling.. ?

 

[noseall]

Pack the joists with slate or cement board. Don't pack with timber as suggested as this is the reason you have bounce in the first place.

The joists have shrunk across the grain which is what wood does
It usually settles though.

Use 12.5mm plasterboard as 9.5mm stuff is naff.

We try not to fit 9.5mm plasterboard anywhere.

 

[HERTS P&D]

CEILINGS.

2 sheets of 9.5mm is far better (with staggered joints) on a ceiling than 1 sheet of 12.5mm, which can crack the joints.

2 sheets of 12.5mm will be to heavy.

Packing timber wedges will be ok for packing the joints as they can be wedges.

Andy

 

[noseall]

CEILINGS.

2 sheets of 9.5mm is far better (with staggered joints) on a ceiling than 1 sheet of 12.5mm, which can crack the joints.

2 sheets of 12.5mm will be to heavy.

Agreed.

I didn't read your post properly.

I seem to be doing it a lot lately.

Alzheimer's?

 

[ajrobb]

Were they creaky/bouncy before you removed the ceiling.. ?

Yes, especially by the front window, away from the work I'm doing to the bathroom.

I'm going to take out the bottom nails from the wooden herringbone trusses around the worst bouncing joist, just to see if that allows the joist to settle on the supporting walls. IF that works, I'll do the same for all the herringbone trusses to release any residual loads.

After I've repaired the joists (another post) and replaced the partitions upstairs (another post), fitted a new kitchen and bathroom and just before I put the ceiling back, I can screw the bottom of the trusses back. That'll take me a few weeks, giving the floorboards a chance to move.

The wooden herringbone trusses are about 36x36. Would steel herringbone trusses give a significantly stiffer floor than the wooden ones? Now would be a good time to change them, before I put partitions back.

 

[Nige F]

Pack the joists with slate or cement board. Don't pack with timber as suggested as this is the reason you have bounce in the first place.

The joists have shrunk across the grain which is what wood does
It usually settles though.

Use 12.5mm plasterboard as 9.5mm stuff is naff.

We try not to fit 9.5mm plasterboard anywhere.

Pack with a resin ( like used for resin anchors) using a mastic gun I did @ my last house -on the bouncing stairs AJ are you in E. Sussex I know a good plasterer

 

[ajrobb]

I know a good plasterer

I have sent you a friend invitation so you can give me their name in a PM.

2 sheets of 12.5mm will be to heavy.

This is the first time I have seen this. I thought it was a common acoustic treatment to put up 2 layers of 12.5mm Soundbloc.

 

[Static]

The wooden herringbone trusses are about 36x36. Would steel herringbone trusses give a significantly stiffer floor than the wooden ones? Now would be a good time to change them, before I put partitions back.

The herringbones are just to stop the main joists twisting so shouldnt be taking any load.. but with older buildings everything tends to end up taking some load or movement when things sag and shift over time..
Just remember to nail back the bottoms when your done packing out the joists as other posters have suggested..

 

[ajrobb]

The herringbones are just to stop the main joists twisting so shouldn't be taking any load.

I thought they were there, like noggins, to make the floor less bouncy when you walk across it. The floorboards and plasterboard will help stop the joists twisting. I found this Trada PDF document on Strutting in timber floors:

"Strutting between joists is installed in both site-built and prefabricated floors to achieve several important structural effects. These include providing load transfer to adjacent members (load-sharing) and lateral support to joists to avoid torsional instability. Research at TRADA and other centres has also demonstrated that strutting, in the form of solid timber blocking or herringbone strutting, reduces the effect of vibrations which have frequencies above the fundamental frequency of the floor."

One thing this document points out is that I should have end-blocking between the end joist and the wall. I might fix the end joist through the end block to the wall with a couple of hammer fixings.

I've used a Point Master to inject sloppy mortar along the sides of the joists where they go into the end wall. Some of these gaps were up to 40mm. There are just single chopped-down blocks between joists. Oh, for a Tony tray.

Immediately, the floor upstairs started to sound 'normal'. I'll leave it to dry over the weekend before giving the final verdict.

In the party wall, the spaces between joists are filled with bricks and the gaps are much smaller so I'll leave those. There aren't any problems with smells or excessive noise from neighbours, so I assume they are reasonably sealed.

I also found this PDF document for timber floors:

"The masonry is accurately cut to allow an even mortar joint (of between 10mm and 15mm) all the way around the sides and tops of the joist. The mortar joint is to be struck or recessed, to allow the later installation of a good bead of silicon mastic between the blockwork and the joist. The silicon mastic bead must be sufficiently generous to cope with any anticipated shrinkage in both the masonry and the timber."

Unfortunately, I have not left a recess around the joists to allow a mastic bead to be installed. However, the timber is now over 30 years old, so most shrinkage should already have happened.

 

[Static]

Yer they will remove some of the floor bounce, but only if they are well screwed/nailed down.. in the end the floor boards will spread deflection better than relying on the herringbone

 

[ajrobb]

in the end the floor boards will spread deflection better than relying on the herringbone

Funny, that's what TRADA says - thicker floorboards rather than more struts to stiffen a floor. I also expect SOME improvement when I put up 2 layers of plasterboard.

Small point - struts CAN reduce the modal density, which translates to less high-frequency sound transmission. (I worked in Modal Analysis for over 10 years.)

I anticipate good results near the wall when I install an end block between the end joist and wall. It should make the floor stiffer by the window AND protect the plaster join below.