My loft joists are horribly uneven.. what to do when converting?

[untrepid]

I installed a loft hatch today, and in the process found that the joists on either side are at different heights, an entire inch apart! I'm looking to board out the loft in the new year, with 2x4's across the existing joists and insulate and all that good stuff, but now I've to work out how to deal with these irregular joist heights.

Given this is for "storage" only, not fitted rooms per se, should I pack out under every lower joist across the whole area, creating a truly flat space? Maybe look for high spots and cut the joists at that point, letting them slope slightly down on either side?

I certainly presume I should want to screw the cross joists to the existing joists at every single crossing point, especially now to avoid just a couple of high joists to take the load... right?

 

[Doggit]

You haven't mentioned loft insulation, but you'd normally do 100mm in between the joists, and then 170 over them, if you use 3x2s across the joists, then you could fit 75mm of cellotex etc to get the 170mm instead. You first need to find the highest point, and then work from there; secure the first joist, and then level it by packing under it as you go. You can drill into the 3x2 an inch, and then use 4" screws to secure them, but you might need longer ones (or drill deeper) as you get across the loft. Some times you'll be able to use plastic packers, but you'll also need wood fillets as well; check if your local woodyard can supply wood in a few thickness, say 10mm, 15mm and 20mm etc. You can also use 3mm hardboard for packers, and all sorts of offcuts.

 

[untrepid]

The existing insulation is pretty old, but looks like it was technically 100mm at one point in the past. current joists are a shade over 3" so I was planning on another 100mm across them. I also want to insulate the roof itself with 100mm, but haven't explored it much yet in terms of possibly needing a membrane etc., but ultimately hope to have 200mm on the ceiling, 100mm on the roof, so would like to think that the two combined count as more in total. As such I don't *think* celotex need come into the equation at all. All finding it surprisingly hard to find information about online though.

 

[Doggit]

If you're joists are just 3", then I'd first glue and screw a 2x2 batten to the existing joists, and then run the 3x2s across. A 3" joist isn't going to take much weight before it starts to bow, although you haven't mentioned the span.

Of course, your other option is to sister 4x2s alongside the existing joists, and this would let you level them as you go. This time you'd start at the lowest point though, and you'd screw the 2 joists together, and then add some noggins to keep them stable.

 

[untrepid]

14" between existing joists. Spanning up to 3m max. I don't plan on having anything particularly heavy up there, so whilst I'm sure you know more about this than me, I don't see how they'd bow esp when all cross joists are fixed to the existing ones. If the worst comes to the worst, the mother in law only weighs about 7st.

I presume the reference to sistering them is to then put cross joists above them still, in order to get the strength and insulation depth?

 

[Doggit]

You're adding the cross joists, and then the boarding, and then putting stuff up there as well, and it can all add up. And then the next person comes along, and puts their 24st MIL up there, and then finds her sitting downstairs with firewood and dust around her. Because you're putting packers under the joists as you level the floor, you're not adding as much strength as you normally would, but jokes and everything aside, I think you're method will be fine for that size.

Sistering the joists would allow a deeper method of putting in the insulation, and more importantly a different method of leveling the floor, but it wouldn't add much to the strength of the joists.