Replacing a bouncy floor

[MadAlicesDad]

After a break of a few months the next project has started.

It's a late 19th century house with 450mm thick stone faced walls with rubble infill. The ground floor living room has a wooden suspended floor that bounces like a trampoline. I've taken up some of the boards and from what I can see of the joists especially near the walls there is very little solid wood to screw into, many have the texture of Crunchie bars!


So, I need a new floor (and some more air bricks, but that's a different story...)

The total span is 4m and the current joists are nominally 50x100mm at 400mm spacing. They are supported by a wall at approx mid span so the original design does meet building regs (50x97mm timber at 400 spacing = max span 1.98m)

However, when I replaced the joists in the similar sized back room with like for like, there was still a bit too much 'give', therefore I'd like to beef this up a bit and go for 50x147. Unfortunately, the joists sit on a plank of wood (wall plate?) on a stone ledge 100mm below the floor surface so if I were to rest the joists onto the wall plate, the floor would be too high.

It's part of a total back-to-bricks refurb of this room so I have a fairly clean sheet to work on. My thoughts are to fix 50x100 timber along the ledge (effectively a tall wall plate) and hang the joists off this (see diagram).


Questions:
Is this a sensible way to do it?
What sort of joist hanger should I use?
Is there another way?

TIA,
Mike

PS Apologies if my terminology is wrong and please feel free to correct me

 

[endecotp]

You could keep the same joist size but reduce the spacing.

 

[MadAlicesDad]

Thanks for the suggestion. I considered this, but the strength of a beam is proportional to the cube of its depth, so a 25% increase in depth gives nearly double the strength (half the deflection) for a given load. To achieve this improvement by adding more joists would require 200mm spacing which is non-standard and too close together to do anything sub-floor (eg radiator plumbing) once in place. Plus I'd have to buy 60% more wood (volume-wise).
Mike

 

[ree]

1. Essentially, your floor is bouncy because the joist tails have no bearing - they are rotted, and perhaps some of them sit loosely on a possibly rotting wall plate. Plus, 2" x 4" is barely adequate for a 2m span.

2. All wall plates on external walls should be eliminated. All of them.

3. New joists should be capped and wedged in masonry pockets.

4. Air bricks should be inserted for thro ventilation - do it with the floor open. You will be able to locate them facing the joist bays.

5. All wood debris must be removed from the oversite.

6. Do you intend to lift the floor back to the intermediate knee wall (or further?) & install 2" x 5" joists? If so the knee wall will have to be dropped to accommodate the new depth.

7. Is the knee wall honeycombed and does it have a DPC and a wall plate?

8. Do you know how to lift T&G boards without ruining them?

9. Pics of a larger view of the room and outside showing ground level would help.

 

[MadAlicesDad]

Thanks for the reply Ree, I missed it until just now, but I have been busy in the mean time.
Taking your points in order:

1 A perfect description! Undersized rotten joists resting on spongy wall plates.

2/3 - The outside wall is all different sized stones with no courses to follow and held together with soft lime mortar so accurately building joists into this would be a nightmare. I can't see a way to do it without a wall plate (on a DPC) on the existing ledge on the outside wall. The internal wall (the other end of the joist) also has a ledge and a wall plate.
Is building joists into pockets preferable to using joist hangers?

4 - Work in progress. Two holes to the outside made so far and a hell of a draught coming through. The outside ground is at internal floor level so I have to fit a stepped duct into the wall. Time consuming but I've worked out how.

5 - All will be cleared, along with the old fireplace and hearth debris, plumbing bits, fag packets and other detritus.

6 - I'll lower the knee wall* to accommodate deeper joists (*if you mean the wall across the middle of the room supporting the joists mid span).

7 - Honeycombed - No, but it will have gaps in it once I remove the top course (see 6 above) and some of the lower bricks.
DPC - No but it will have.
Wall plate - Sort of, but made of same Crunchie bar material as some of the joists!

8 - Not applicable; the boards aren't T&G. The big problem was where I'd screwed them down and filled the holes before sanding. They are all up now and will be replaced with chipboard.

9 - Will do tomorrow.

Cheers,
Mike

 

[MadAlicesDad]

I've edited the previous post; the end of the joists at the internal wall are also on a wall plate but seem fairly good condition. Photos to follow.
Mike

 

[MadAlicesDad]

Some photos to educate, inform or shock...
General view:

Joists to left of fireplace

Joists to right of fireplace. Note new(er) wall plate and two replacement joists. They've been in for approx 25 years and seem good condition.

Joist crossing sleeper wall in middle of room.



A sorry state...
Mike

 

[MadAlicesDad]

Air brick hole viewed from outside

and inside

Note driveway and top of joists are at similar level.
Periscope air brick recessed into wall and ledge (to go under wall plate).

 

[Deleted member 174758]

3. New joists should be capped and wedged in masonry pockets.

You might do that in California, Ree, but I've found that many structural engineers frown on it over here, especially in older buildings where there may be no or inadequate DPM in the walls and pocketing might be another place for rot to start (in the north of England we get a little more rain than California). We are generally instructed to install treated wall plates using chemical anchors or Rawl anchors and to support the new joists on jiffy hangers, although occasionally we are old that zip (ZP) hangers fixed directly to the masonry are acceptable (although I don't really like them unless the masonry is almost perfect)

Another thing I often do on longer floor joists is to either solid strut or add mertal herringbone strutting between joints to remove the tendency of long joists to twist or bounce. It works exceptionally well

 

[ree]

MadAlicesDad,

Thanks for the annotated pics. With the new information, here's what i would do:

10. Lift all the floor boards in the room.

This will enable you to see the full extent of any rot and also enable you to replace damaged joists.

11. Remove the remaining sections of wall plate and rubble on the ledges. Wash down with anti-fungal chemicals.

12. It might be in your interest to replace complete lengths of joist rather than do tie-ins & bolt-ons.

13. Use 300mm x 30m rolls of DPC to cap the joist ends and wrap the joists that are tight to walls with 300mm DPC - protecting them from masonry contact.

14. Protect from any masonry contact the new sections of wall plate (wrap with 300mm DPC), and bed the wall plates level on to the masonry ledges - similar to what you have now.

15. The joists can be pinned to the plates by Ell brackets.

16. Use Simpsons HSA Joist Struts at appropriate intervals.

17. An intermediate wall/plate/joist pic shows signs of decay - its probable that the plate must be replaced (bed the new plate on a DPC). The rotted joist, & perhaps other joists, must also be replaced.

18. Post pics of the exposed hearth area - its probable that remedial work (the trimming joists) is needed there. Has damp been an issue on the chimney breast?

19. In pic 6 - to the left of the cable - there is a white shiny object: what is it?

 

[ree]

JobAndKnock,

What nonsense.

Technical information is good or bad - its origin is irrelevant.

DPC's are commonly used in walls, not DPM's.

Wall pockets & capped timber has been the typical method of supporting joist tails in UK remedial work.

This is remedial work: the wall is damp, moisture is penetrating, and yet you would fix metal ZP hangers to that background - neither would you wrap any wall plate, instead, you would rely on "treated" timber?
"Treated" timber in contact with the above conditions would probably last 2 - 3 yrs tops before it began to rot.
Your suspended plate would interfere with the air brick ventilation, and cause further work.

We have never once, in years of remedial work, used a SE to advise on a few rotted joists. It borders on getting an architect to design the joists. We are talking about a residential house.

 

[MadAlicesDad]

Ree, many thanks for the comprehensive post. I'll provide more detailed answers later, but all timber will be replaced and the hearth will be removed entirely.
However, you seem to be suggesting a wall plate with joists resting on it (ie as original) but this doesn't allow me to increase joist depth. Or have I missed something?

Mike

 

[ree]

I understood that you didn't want to increase joist depth. I was wrong.

20. However, if you do then dont use any wall plates: hack into the masonry for pockets - you've managed to do it for the venting.

21. Or, simply rest the capped joist tails on the ledge & use Joist Struts within 400mm & 1000mm of the bearing.

22. Drop by one course all intermediate knee walls.

23. Post pics of where the next room knee wall meets the old floor - presumably the joist tails cross?. Are the rooms knock-thro's?

24. When all joists are fixed and level, and most chipboard is down (but leave off the last sheets near the external wall) then test the floor for bounce.

 

[MadAlicesDad]

Ree,
Q19 - it's a bit of discarded expanding foam. Not mould or fungus (although that wouldn't surprise me if it were)

Q18 - no damp on the chimney breast; the chimney has been removed at top floor ceiling level so there is a hole exiting into the loft. With the gaps around the previous (decorative only) fireplace, there would have been some airflow.

Q23 - there is no knee wall in the adjoining rooms (a hallway and a study) as they are small enough to span with single joists, the ends of which rest on ledges, not pockets in the wall. This is the original room design, not knocked through.

Regarding 11-17, this was roughly what I planned to do, ie remove everything and use DPC between any wood and masonry as you suggest, lower the knee wall by a course, use herringbone struts etc.

However regarding using deeper joists, removing the wall plates will only give another 15-20mm joist depth and I'm loath to hack into the ledges to support anything structural as it is difficult to get exactly the bit out that needs to come out without disturbing other stones (hence over sized holes around the airbricks). I also foresee problems around the hearth as I've yet to dig it out to see what is underneath that could support joists - currently lots of crumbly wood on damp clinker/ash.

I was thinking it would be easier to get a wall plate level and work from that. Is there any reason why my original suggestion ie a 'thick' wallplate and joist hangers, using DPC and new timber is a no-no? I was thinking of something like this http://s7g3.scene7.com/is/image/ae2...1,281&layer=1&size=281,281&src=ae235/37986_A3.

I've got a bit of time to work it out; I need to pull down the ceiling and knock off plaster before I lift and remove the floor completely, but suggestions and advice very welcome in the mean time to help me plan it.

Mike