Levelling Joists

[cruddler]

Hi,

I've been through various threads on the subject on here but I just wanted to gather opinions before I commit to a solution.
I am renovating an old house in France and am in the process of converting the attic space into bedrooms. The joists are in good condition and seemingly adequately sized at 22x8cm spanning 6m in an 8mx6m space. The problem is that the joists were clearly installed with no intention of using the space above resulting in a rather impressive roller coaster effect. See pic if Ive managed to attach it.

So, prior to laying a floor, what would be the best method for evening out the floor as much as possible?

1. Buy batons to go along each joist and somehow cut them to shape?
2. Sister one side of each joist with wood and create a new level upon which to secure the floor?
3. Something else?

Although potentially a lot more expensive than 1), it seems to me that Sistering is the more sensible solution and hopefully more straightforward but if that is the case, could someone advise on what size pieces of wood should I use and how should I secure them? Are decent screws sufficient or should they be bolted?

Any help much appreciated, thanks

 

[TicTac]

Option 2 might be more expensive, but it's also the best option. What is the height difference across the room you've got to sort out. Makes sure you've got at least 2" to sister alongside the joists though, and possibly even 3", and work from there. Screws and nails should be fine, but screw the boards down afterwards so the new joist don't get knocked downwards.

 

[cruddler]

Hi, yes just realised I forgot that crucial bit of info. From the highest corner to the lowest (which is diagonally across) the fall is about 6cm although at a couple of points in between, it is nearer 8cm

 

[Notch7]

Sistering has got to be the easiest solution.

3x2 or 4x2 I reckon. 80mm x 5mm csk screws

*** edit *** make that 6 x 2

 

[TicTac]

Then you're going to need to start at the highest point, and use 3x2s, and as you get to the lowest point, you'll need 6x2s, and possibly 7x2s with bolts. The 3's and 4s should be okay with nails and screws, and bolts every 500mm for the thicker joists.

 

[cruddler]

Thanks for all the replies and confirming the correct approach.
As sizes are slightly different here in France, could I get away with 38mm widths as opposed to 5cm (2in)?
Also, in terms of the bolts, are these what I need? https://www.screwfix.com/p/general-...-bright-zinc-plated-m10-x-150mm-50-pack/53448 along with square washers each end?

 

[phatboy]

You only need a washer at the end with the nut on

I think even in the UK, loft joists are often 38mm?

 

[TicTac]

38mm should be okay, but if you've got 2x38mm making 76mm then those bolts will be too long, as the thread will give out before the nut starts to tighten. 80s will be too short, so 100mm bolts will be fine. If you want to use square washers, then you need a bolt with a hex head on it (easyfix set screw), and if you've only got 38mm joists, this might be more sensible, or you could use some turbo coach screws as well to make the job a bit quicker.

 

[cruddler]

The existing joists are 8cm wide so adding 150x38mm beams is going to make them 118m wide so surely I will need 130mm+ bolts?

 

[cdbe]

Hi, when I did this I used big sheets of 18mm plywood, one on each side of the joist - I just ripped them down to the correct height I needed along the joist and ensured I had at least 100mm downstand - worked fine for me, I think I fixed them with lots of hefty woodscrews.

 

[cruddler]

Hi, how did you then attach whatever floor you laid on top? Surely 18mm is too narrow to then screw in to or did you then pack out the joists and screw through that into the joists?

 

[catlad]

Another option if height is not an issue is to build a frame perpendicular above your existing joists out of 3x2" or similar and then level that. quick option!

 

[cdbe]

Just into the joist - with long screws - in my case the max "sag" was only about 50mm but the whole thing was very rigid when done.

 

[TicTac]

The existing joists are 8cm wide so adding 150x38mm beams is going to make them 118m wide so surely I will need 130mm+ bolts

Yup, you're right; forgot the original post when you mentioned that french joists are 38mm. Use the 150s every metre, and the turbo coach screws in between.

 

[cruddler]

Thanks for all the advice. Have gone with 38x150mm pieces and ordered a load of 150mm bolts.
Catlad, that was also suggested by a local but height would have been an issue
Cheers