Owool12 Posted July 4, 2023 Share Posted July 4, 2023 Hi everyone! I’m wanting to build a dropped ceiling out of a wooden frame and attach to existing ceiling joists. However the ceiling joists are engineered I joists (with OSB web) and I’m struggling to identify the best way to hang the dropped ceiling from these. I understand attaching directly to the bottom flange is a big NO when it comes to I joists, does anyone have any suggestions of how to hang from these joists? Cheers Link to comment Share on other sites More sharing options...
Temp Posted July 4, 2023 Share Posted July 4, 2023 So how do you fix plasterboard to the underside of these for a normal ceiling? Link to comment Share on other sites More sharing options...
markc Posted July 5, 2023 Share Posted July 5, 2023 Helical retaining units (screws) work well 1 Link to comment Share on other sites More sharing options...
Owool12 Posted July 5, 2023 Author Share Posted July 5, 2023 8 hours ago, Temp said: So how do you fix plasterboard to the underside of these for a normal ceiling? Sure that’s doable, but a dropped ceiling made of wooden studs, a 30kg extractor fan & plasterboard weighs a lot more than just ordinary plasterboard. I’ve heard about padding the OSB web with some plywoood & screwing through that with a joist hanger to hold a perpendicular joist… anyone any experience with doing that? Link to comment Share on other sites More sharing options...
markc Posted July 5, 2023 Share Posted July 5, 2023 30kgs as a frame is nothing weight wise for solid or engineered joists, if you were hanging a 30kg light fitting on one screw then I may be more concerned but your frame will be spanning 2 or more joists so very little localised load. If you can get access ok then screw through the web. If you are worried the bottom cord may come away from the web then don’t be, the glue used is incredibly strong 1 Link to comment Share on other sites More sharing options...
joe90 Posted July 5, 2023 Share Posted July 5, 2023 (edited) 45 minutes ago, Owool12 said: I’ve heard about padding the OSB web with some plywoood & screwing through that with a joist hanger to hold a perpendicular joist… anyone any experience with doing that? That sounds like a very good move, especially if the “perpendicular joist” is fixed to both top and bottom rail . (Over engineered but I am like that 😎) Edited July 5, 2023 by joe90 2 Link to comment Share on other sites More sharing options...
nod Posted July 5, 2023 Share Posted July 5, 2023 Our previous was I joists and I simply fixed to the underside of the joists The extractor is ridiculously heavy at 40 kg I often fit two layers of 15 mil sound boards to the underside of these joists Who ever has told you this hasn’t got a clue 2 Link to comment Share on other sites More sharing options...
Owool12 Posted July 5, 2023 Author Share Posted July 5, 2023 Thanks everyone for the insights. The extractor is 30kg so with the frame and other plasterboard it’s probably about 60kg at most. The load will only be spread across two joists, so that’s why I’m concerned at fixing directly to the bottom flange. I’m happy cutting out the ceiling plasterboard to access the web, but accessing it on both sides is a bit of a pain. Am I able to attach a wooden batten between the top and bottom flange then screw horizontally into the web perhaps? Link to comment Share on other sites More sharing options...
markc Posted July 5, 2023 Share Posted July 5, 2023 If you want to, and can fix a batten from top to bottom cord then this makes a very good fixing point, again there is no problem screwing into bottom cord but don’t ever notch it or drill a big hole through it. Bottom cord is in tension so removing material reduces its tensile strength. 2 Link to comment Share on other sites More sharing options...
Owool12 Posted July 5, 2023 Author Share Posted July 5, 2023 24 minutes ago, markc said: If you want to, and can fix a batten from top to bottom cord then this makes a very good fixing point, again there is no problem screwing into bottom cord but don’t ever notch it or drill a big hole through it. Bottom cord is in tension so removing material reduces its tensile strength. Thanks! How would you recommend fixing a batten to the top and bottom flange/cord? Would I be better screwing into the OSB web to attach the batten? Link to comment Share on other sites More sharing options...
joe90 Posted July 5, 2023 Share Posted July 5, 2023 @nod is probably right in that it needs no additional battens (and he has already done this) , the OSB is almost impossible to remove from the top or bottom cord but if your worried just run a vertical batten, 2x1 from top batten to bottom batten in the four corners of your “thing” using wood screws (or bolts if you want to be OCD) then simply hang your “thing” from the bottom cords. 🤔 1 Link to comment Share on other sites More sharing options...
markc Posted July 5, 2023 Share Posted July 5, 2023 Overkill for the loads you are talking about but a hockey stick would mean you only need a small screw fix into bottom cord while the load is taken on the top face of the bottom cord. 2 Link to comment Share on other sites More sharing options...
Owool12 Posted July 5, 2023 Author Share Posted July 5, 2023 Cheers for the diagram, I agree I probably would get away with screwing into the bottom flange but I’m reluctant to do so just on the basis that it’s strongly advised against… ultimately damaging/splitting/cracking a joist is a huge problem I don’t want to encounter. I like both those ideas, keeps it relatively simple. Another approach I was considering was screwing some plywood to the inside of the OSB and then using a face fix joist hanger to run a 2x3 joist between: 2 Link to comment Share on other sites More sharing options...
joe90 Posted July 5, 2023 Share Posted July 5, 2023 Frankly you have to do what you feel comfortable with mate. That’s a good idea above with the hangers, just bring them lower so the 3x2 bottom edge is level with the bottom of the lower chord. 2 Link to comment Share on other sites More sharing options...
Owool12 Posted July 5, 2023 Author Share Posted July 5, 2023 37 minutes ago, joe90 said: Frankly you have to do what you feel comfortable with mate. That’s a good idea above with the hangers, just bring them lower so the 3x2 bottom edge is level with the bottom of the lower chord. Makes sense, I’ve also seen some people online suggesting leaving a 10mm gap for the backer block (the ply against the OSB) either at the top (so between the ply & top flange), or at the bottom (between the ply & bottom flange). Others have said just to fill the entire gap between the flanges with ply (no gap whatsoever). Any ideas why people suggest these gaps and which would be most appropriate? I assume it’s related to where the tension is being applied on the joist? Link to comment Share on other sites More sharing options...
joe90 Posted July 5, 2023 Share Posted July 5, 2023 I can see no point in a gap at all 🤷♂️ 2 Link to comment Share on other sites More sharing options...
dnb Posted July 7, 2023 Share Posted July 7, 2023 I know that these are not the same joists as yours, but it might give a few ideas. Plenty of example diagrams of how to use I joists. https://www.steico.com/en/solutions/new-construction/roof-construction Link to comment Share on other sites More sharing options...
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