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Strengthening narrow joists before new ceiling

  • 09-05-2023 3:31pm
    Registered Users Posts: 1,206 ✭✭✭

    I've been reading a few boards threads on attic conversions and flooring over joists and strengthening joists but I guess each house can be different so I thought I'd ask for advice on this house. It's my brother's terraced house in Marino and he's been getting a few different opinions on how to put up a new ceiling after getting someone in to take down the old asbestos ceilings.

    The problem is that the joists are pretty narrow - about 2x3s - which are nailed to the rafters at each end (the span is about 7 metres) and supported by hangers and a single runner. Not sure what you'd call this style of roof construction but I've (attempted) made up a picture to illustrate. Concrete internal walls, none of which are supporting the roof. The joists are fairly close together - about 35cm or so. Oh, and there are purlins but I forgot to draw them.

    The structural engineer says the ceiling will crack if he puts one up without reinforcement. Well. Yes. I could tell that after my brother stepped on one of the joists and the nail pulled out of the runner. Dropped about an inch before it hit the top plate of the bedroom wall. Lots of swearing.

    My feeling is that sistering might help but not enough - it's still a 3+ metre span. Also it's a pain to do. My personal inclination is to:

    • Use metal brackets between the runner and the joists to stop the nailed runner pulling out. Either that or pull the nails and use screws. Or both.
    • use blocking (bridging/noggins whatever you call it) between the joists to stop them moving laterally. It would also spread the load enough to stop the joist bouncing up and down.
    • Possibly add another two runners and support them with hangers to the rafters?

    An alternative would be just to run new 2x4 battens perpendicularly across the top of the existing joists. I suppose this would also increase the height to support more insulation (assuming we add a bit of ply flooring for storage) and should add strength.


  • Moderators, Home & Garden Moderators, Science, Health & Environment Moderators Posts: 17,677 Mod ✭✭✭✭DOCARCH

    Mod Note: Per the forum charter, structural advice, and issues relating to the specification of structural components, is not permitted.

    If you are not happy with the advice your structural engineer has given you, maybe engage/consult another structural engineer?

  • Registered Users Posts: 38,970 ✭✭✭✭Mellor

    How big are the rafters?

    From the scale you've drawn it, it looks like the rafters rafters are supporting the roof on their own. And the joists are hung from the rafters to support the ceiling only. Joists are not trafficable. If he wants to convey the attic, its a replace the joists job. If it's not accessible, then the current joists might carry a lightweight sheeted ceiling. I'd probably try to put a few more supports across the roof connecting adjacent joists