Levelling old joists - battens

[OwenF]

Insulating existing suspended (ground) floor. Have exposed joists and measured approx. 15mm differential across 7m room (parallel to joist direction). Also have ~10mm fall along the joists but TBH they’re up & down where going over dwarf wall/ mid span etc.

 

Decided I will have to sister new timbers to get a level subfloor for tiles.

However existing joists are 2x4” at 400 c-c. 

If I sister with 47x95mm there’s barely any room for insulation/increased thermal bridge.

 

Another option I’m pursuing is to use 25x50mm battens either side of the joist à la sketch below.

Plan to fit battens AFTER wrapping joists with breather membrane in ‘up and over’ fashion.

AT/VCL places over top prior to 22mm chipboard.

 

I’m convinced this should be capable of taking floor loads provided suitable fixings….but is this doomed to fail?

 

[ProDave]

A lot of work?  Why not just replace them all with nice new straight joists?

[OwenF]

Well…I’ve actually put new joists in half the room, but unfortunately had to work my way across new to old.

 

Everything right of that strip of P5 is new joists. I’ve reached the left-hand extent of the white membrane. Only after moving everything across and ripping last bit of floor (on left out of shot) have I discovered the issue.

 

at this stage, new joists would be undoing a fair bit of work but seems like I’m in for it either way

 

 

[saveasteading]

Easier is to counterbatten, with packing to level.

[Iceverge]

If counter-battening would make the floor too high I have another suggestion. 

 

Buy some 3.6mm plywood and rip it into say 300x50mm sections. Use a laser level or string line and tack or staple the ply in place to get it close enough to level. 

 

When the floor is finished use some self levelling compound to take any last imperfections out if needed.

[OwenF]

Yea counterbattens isn’t an option due to needing to maintain floor height 

 

I’d like to replace all old joists but I think 35week pregnant significant other might kill me

 

I’m liking that two people have suggested shim/packing (albeit different approaches). General wisdom seems to be in favour of sistering over packing, but I’m wondering if plywood shims might be enough to take out the worst (as @Iceverge says) without serious compromise to integrity.

Edited March 20 by OwenF

[Russell griffiths]

Why not sister but with ply, 18mm ply ripped to 100mm strip, fitted to the side would significantly strengthen the original 

level these all up and it would give a nice flat top to then cut a full length shim/ packer to glue onto the top. 
some might only be 5-6mm so a table saw would be needed. 
 

mad idea 🤷🏻‍♂️🤷🏻‍♂️

[Dave Jones]

sister up with 3x2 so you have a good fixing both into the old joist and for the new flooring. start from highest point and level to that everywhere with a laser.

[Alan Ambrose]

Careful use of plastic spacers which come in 1mm increments? Take up majority of fall with thin-ish ply and then spacers. Do remainder of <1mm levelling with tile fix and/or 'self-level' screed?

[Shelterstone]

I've just done this to my whole house, in the past I've sistered with rips of ply but this time to maintain space for insulation I cut firrings from lengths of 6x2 using a tracksaw. Glued and screwed these to the top of the existing joists. Was actually a lot quicker than expected and easy to get bang on if you use a laser to measure up.