Hey all, I will be installing a new ceiling in my kitchen next week. The joists have been replaced and are level upstairs, however downstairs there is some variability between the joists. I'd like to level these off before installing the plasterboard. My initial thought was to run batons across the joists at 90 degrees to the joists and pack the batons out with shim or trim them on the router where required. Now I'm thinking of perhaps dropping some form of hanger down from the joist to a set level and then fix the batons to that? Any advice or tips are greatly appreciated. Thanks Mark
Find the lowest point of the ceiling where it protrudes or sags the most and work outwards from that point by planting timber on alongside the existing joists rather than faffing about trying to pack and shim. It's more important to get the joists flat rather than fighting to try to get them level, as its only a ceiling you'll never notice if it's slightly out of level, but you'll notice if there's dips and bumps everywhere.
I work off the floor with timber baton to find the height either end or room. Fix timber then long straight edge across ceiling to find a level. Normally fix to what's there along the side of existing timbers
I recently had a similar problem with about 1 1/2" variability in flatness across a large (4.5x5m) room in a 1902 house (not because it had sagged particularly, I think it was just made that way, and the old laths could accommodate what plasterboard could not!). As the room was high, I could afford to lose a little height. The "install alongside" method was my first thought, but not so easy because of services and cross braces, so what I did was screw metal angle brackets on every joist underneath with lots of 5x50 screws - 3 in each bracket (technically not ideal because the screws are in tension, and if the screws pulled out, the ceiling would fall down!, but I figured with enough of them would be OK). This left me with brackets hanging down on every joist spaced for cross joists. With a laser level I marked the brackets to get a level ceiling and then screwed 2x3 at 90 degrees to the existing joists picking up a bracket on every crossing. Result was a super-firm and absolutely level ceiling, with about a 4" loss of height. Couple of years on and it's absolutely fine.
I had this problem with a new 7m x 4m kitchen extension ceiling I had to board out and skim. I did what Jord and Wayners did and battened along the unlevel joists . The customer asked why is this necessary, carpenters came recommended, they used lasers, now I have to pay you for this extra work.