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I'm about to take out the lath & plaster ceiling in the basement of my 1908 house (I'm hiring it out; I know how bad the mess is). Main reasons for doing this are to get access to wiring and allow me to insulate.

This is in Iowa, not a seismic zone. The basement is unfinished except for this ceiling, and I'm inclined to keep it that way for a while so we can so some remodeling upstairs.

After taking out the ceiling, should I add braces to the 2x8 joists? I'm pretty sure there are none at this time, and I'd think there is a structural element to the ceiling that keeps the joists stable.

Is there any way to know whether I need braces, apart for listening for ominous creaks? :-\

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What's the floor above this like? Jurisdiction? Quake zone? Is it a finished basement below? Moist? –  Bryce Jan 17 at 0:14

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up vote 2 down vote accepted

You are correct, the ceiling material provides a bracing effect for the joists. This is more critical on the compression (floor) side than the tension (bottom) side. There are specific ratios of joist width to height ratios based on span length where bridging or solid blocking must be installed.

I am sure 2x8 floor joists do not meet this criteria. You do not need bridging or blocking. You will actually get more creaking with bridging or blocking installed due to the extra joints rubbing under joist flexure. Of course, these are the innocuous flooring type creaks and not the ominous splintering sound of impending failure.

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On top of that, plaster and lathe ceilings are HEAVY, so you're actually reducing the load by removing it. FWIW, when I took down the laths and plaster ceiling in our last basement, all the joists were still properly cross braced underneath. –  DA01 Jan 19 at 3:27

I think the terms you want are blocking or bridging or cross bracing: connecting adjacent floor joists together to keep them from twisting and warping. This common, even code mandated, in new construction. In seismic zones these can be necessary to prevent racking of the floor in an earthquake:

Floor joist blocking or bridging diagram end view

Chances are you already have these in your 1908 structure. Removing the lathe ceiling will reduce total bracing somewhat, and increase potential for twisting. But I rather think that after 100 years the joists are hard, dry and have done whatever twisting they are going to do. In new construction, often done with wet lumber, twisting is a far greater issue.

I'd question the need to remove the old plaster and lathe. As long as the knob & tube wiring is under-amped there is no technical conflict with blow-in insulation (there may be a code conflict, relevant or not for you). And that's far easier, and far less messy. Wireless technology can be used to achieve certain electrical goals without new wires.

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Blocking and bridging provide considerable stiffness and strength to the floor—their purpose is not to keep them straight while the wood dries (although perhaps that is a secondary benefit). Adding cross bracing can actually be a useful tool for reducing "bouncy" old floors. –  Henry Jackson Jan 17 at 4:17
    
Thank you for the clarification on bridging and blocking. The illustration makes it easy. :) –  skaiser Jan 17 at 9:11
    
Thanks, had already upvoted and I appreciate the answer. I'm expecting to do some fairly big remodeling upstairs (moving plumbing, adding radiant heat, etc.) so I really want the ceiling out. –  skaiser Jan 19 at 3:05

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