Leveling and strengthening bouncy floor

[muh3]

Hi all,

I have a bit of trouble with my upstairs bedroom floor. It is quite bouncy.

Span is about 4m. Joists are 2x7 and 42cm center.

Second problem I have is that the joists are not very level. There is a 9mm drop over 9 joists but some individual joists drop up to 3mm to the next one. I want to lay laminate in this room, so would prefer it level all over.

Could I sandwich the joists in ply wood and leave the ply proud over the joist by nothing to the 9mm and then backfill the joist with a dry send and cement mix and put 18mm ply sheets over.

Would that work or am I going mad?

Thanks.

 

[pinenot]

Are the joists dwanged (noggins) and if so what depth are the dwangs in relationship to the joists themselves, full depth?...pinenot

 

[muh3]

X struds in the mid span. One side sits on a brick wall with a plate on top which support rafters. The other side goes through a brick wall.

 

[tony1851]

7x2 on a 4m span will give you a bouncy floor because they are over-spanned by modern standards.

Instead of sandwiching the joists in plywood, why not just fix some 6x1 softwood pieces to one side of each joist, levelled as you suggested. If they are well screwed, they should help to stiffen the joists a little.

If you can glue and screw the plywood subfloor to the tops of the (raised) joists, that will also help to stiffen the assembly.

 

[pinenot]

Am I right in thinking, you are saying X struts (noggins) in the central span? Then can you advise of the span(s)??...pinenot

 

[muh3]

@pinenot: It is a herringbone structure in the central span. Span is about 4m. Some are only 3,5m there they go around a chimney.

@tony1851. I think trying to fit the ply floor to a 1 inch surface might be a struggle. especially on joints.

 

[pinenot]

Yes herringbone, I thought so. These keep the joists perpendicular, and allow for maintenance in that direction, but allow horizontal displacement to take place unfortunately. I think with these spans you could place a row of full depth dwangs equidistant each side of the herringbone dwangs and their respective walls, leaving the herringbone undisturbed. As said earlier, assuming the ceiling could be propped from below and the props screwed up until a level floor was achieved, the full dwangs would give a torsion box effect i.e. when the props are backed off the floor & ceiling will remain all but flat as it were and the resulting floor, as you want it...pinenot

 

[muh3]

Hello unfortunately I can't disturb the plasterboard ceiling underneath. My better half would kill me

@pinenot, ply wood sandwich and 3 full size noggins one either end and one in the middle not an option?

I don't think any of the joists have dropped, I believe they have been fitted like that, some are also a little bit smaller then others.

House is 1940-1950.

 

[pinenot]

Sorry, I never suggested any damage or infringement to the ceiling, using load spreading battens under screw up props will cause no, or at least very slight hairline cracks at worst.

In your original post you say 9 mm drop over 9 joists and 3 mm drop between some neighbouring joists, now you say you don't think the joists have dropped!... My intentions are genuinely to offer a working method that will give you the minimum of work achieving your goal, if you have some hidden criteria that you would like to share, please let us know...pinenot

P.S. I never mentioned ply wood sandwich, you must be confusing me with someone else.

 

[tony1851]

@tony1851. I think trying to fit the ply floor to a 1 inch surface might be a struggle. especially on joints.

But how would it be any easier with plywood both sides of the joist?
Presumably your plywood would be thinner than 1" (say 0.5" to 0.75").

@pinenot; putting those pieces between the joists and jacking up from below would not work (even if his better-half had allowed it).

 

[muh3]

Hello,

appreciate all comments thx.

@tony, with the ply screwed both sides i can still screw the ply floor in the joist. Hope you know what I mean. Bud joining the ply sheets on the 1 inch peace might be difficult. but I guess using a 6x2 would work.

@pinenot
I suggested the ply.
I did make some photos.

The room

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• muh3
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The roof side. the joists just sit on the brick wall, I did put some noggins in at the bottom to stop them twisting on the end before I started looking at getting it all level.

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• muh3
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The X

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• muh3
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The other side

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• muh3
• 7 May 2013
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And the left side

I believe the hight difference has been build in then just sitting the joists on the brick wall and using slight different sizes of joists.

I do not think the joists them self have moved much over the years. The plasterboard ceiling beneath has a crack right in the middle could be there the herringbones are.

@pinenot will your suggestion help in my case? I also have a bounce I would like to get better.

Thanks for looking.[/img]

 

[pinenot]

Yes, the whole idea is to create a stable framework for the floorboards, ply sandwiches either side of joists are not the kind of thing I would recommend, sorry. You have suggested that the joists used to build in the first place, may have been of different size, that's hardly likely. In the trade, uniformity of timber sections is the norm, especially when building things like floors, at least that's my 40 years + experience, but true to say not all of the timber arriving on site were perfectly strait, and in the case of floors, that's where dwangs come in.

I have in the past used short joists as levers to set a pair of joists flush before inserting the dwangs. The short joist being slipped under the lower joist and levered down to lift it, lever action released. And whilst the joist may not be level, they are at least in the same plane and in line with each of its neighbouring joists.

But that's enough of that, this is what I'm suggesting for you
View media item 60992 existing joist miss-alignment



View media item 60998 screw props in place, joists aligned and dwangs fitted (full depth & flush with joists) View media item 61000 The finished job

This is typical tradesman's job in my view and the floor/ceiling will remain relatively flat and together when finished. It's a job I've done many times over the years, a bit of a set up, I know, but worth it in the end...pinenot

 

[tony1851]

@ pinenot;
If you've had 40+ years in the business, you would know that something like that would not work.
If the joists are uneven, either they have been built-in that way, or the timber has undergone long-term creep. Whichever it is, jacking up the floor and nailing blocking pieces between the joists will not achieve anything.
You are advising people to do things which will be, at best, a waste of time and at worst, unsound.