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Ceiling joists with long span 1) How long of a span is too long for rough cut 2x4 inch (45x100) ceiling joists?

Pending what those more knowledgable say, my guess is this room at 5.3m is OK, but not great. It has sagged up to 50mm. Our 110 year old house isn't always straight and level yet is regarded as well constructed by our builders. Moisture and long-gone chimney likely contributed.

To finish setting the scene, I live in an earthquake prone area and want to undertake basic enhancements and also reduce the risk of cracks in the plastering (directly in the middle). Unlike foundations/subfloors which have detailed guidelines for strengthening I am seeking help understanding if a simple approach exists for retrofit strengthing ceilings.

Finally the real question that shouldn't be so specific to me.

2) How to retrofit better support for the ceiling from above?

My concept design - a "hanging beam"?

I have a limited opportunity to get timber into to attic of my hard to access roof ...

  • 6m lengths for sistering the joists are unlikely to fit
  • 4.5m lengths (as wide as the room) should fit OK, just
  • All four walls are stuctural load bearing - top plates in blue

Using hurricane braces I could do something like the following in pink?

enter image description here

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  • $\begingroup$ figure out what the ideal state would be per modern code and then figure out what you would need to get there or close enough. this should include what should be sealed and what should be vented. you may find that you have a lot more you should change or you might find a suitable modification to allow you to move components of appropriate size where you need them. look through catalogs of bracketing manufacturers; a bit of steel goes a long way. $\endgroup$
    – Abel
    Commented May 4, 2022 at 11:16

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You don't seem to have an urgent problem, as the ceiling may have stayed there for a long period of time. However, according to the modern building code for attic space without storage, the 50 mm deflection is considered excessive, as the corresponding deflection ratio $\dfrac{L}{\Delta} = \dfrac{5300}{50} = 106$ that is way too flexible based on the typically recommended ratio - $240$ for gypsum ceiling.

There are several ways to strengthen the ceiling structure, however, some methods may harm the roof members, or even impart undesirable loads (wind/snow) from the roof to the ceiling joists. So it is imperative to engage a structural engineer rather than follow the suggestions on the internet, as your safety is at stake.

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You can use 2x10s at 48" O.C. from above and use available fasteners by Simpson or other brands to hang the 2x4d from down below.

it is the common way.

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