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Re: Converting ceiling joist to floor joist.

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I believe the smart way is to build a new floor above the existing ceiling, which is left in place.  Space the new floor joists 1/2" or 3/4" away from the ceiling joists both vertically and horizontally with plywood spacers, so that when you nail the new floor sheathing to the new joists you don't damage the existing ceiling.  This is especially important with old plaster ceilings that can be knocked loose if you nail onto the joists.

This method provides a floor that is separate from the ceiling, which is good for sound separation.  Also it provides space for wiring to be threaded through.  Obviously it requires that the new joists run the same direction as the old joists if they are set down between each other, which may not always work.

Unfortunately, sometimes the "smart" thing to do is to just demolish the existing ceiling and build a new floor in its place.  (I say "unfortunately" because this uses more material and is thus wasteful, but it may be the fastest and maybe even the cheapest way to accomplish the job, especially if labor is expensive.)  On many of my jobs like this the people want to live in their home during the work and leaving the ceiling in place is a real plus.

Now, I believe what you're really asking is how to somehow strengthen existing 2x4 or 2x6 ceiling joists with something smaller than a new 2x12--possibly by adding a new 2x6 on top of an existing 2x6--to provide a floor joist.  I don't think that's either feasible or "smart" unless the labor is free and you don't care at all about squeaking floors.

Ralph Hueston Kratz, S.E.
Richmond CA USA

In a message dated 9/17/05 8:01:32 AM, hadiprawira(--nospam--at)yahoo.com writes:

Fellow Engineers,
 
A fellow architect was asking me, what is the smart way to add a new story on top of an existing structure without replacing/sistering the whole ceiling joist (2x8 @16"oc) to accomodate new loadings (need 2x12@ 16"oc) assuming the wall and the foundation below are adequate.
 
The question sounds very simple, but is it practical to build up a composite member from ex.  ceiling joist? what is the practical way to go? Anybody?
 
As always, thank you in advance!
 
Djohan.