Started by 22301butch, November 19, 2004, 09:10:39 AM

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ref:20x30 1-1/2 story plan
are the floor joists 20' long with no center support?
what size are the second floor beams(4'centers)?



You have some choices for the main floor joists. See floor framing and cross section on the foundation plan sheet 1C. You do not need an interior beam support if you can find the quality of joists I call out. The I-joists option is what I would use as these can be found everywhere.

For the loft floor see the Loft Framing sheet 3. You use 2x12 and can build either a standard joist system with drywall ceiling below (where you need to hide plumbing) or an exposed beam and decking ceiling where you want a more open higher ceiling below. You can do both as shown in the framing plan or do one or the other for the entire floor. Both will span the full width of the building.

Note: This assumes a 30 psf floor load in the loft - this is standard. If you will be hauling a grand piano up that little loft stair then you can build the loft floor the same way you do the main floor (rated @ 40 psf).


Do you see a problem with adding a third 30 ft beam running down center with same support post spacing?  Joists would then be 10'.  This would be a single story application.  I'm considering this as it would be easier from a material handling and solo builder perspective.  


Sure, you can use three beams and then reduce the floor joists to 2x8s.


For what it is worth...
I built the post foundation with a third beam down the middle and have not used it.  I went with the I joists and the floor turned out great!  Very stiff and the I joists were very easy to work with in addition to costing me less than the 2x12s would have.  I was able to set all of the joists and deck the floor in a single weekend.

I went with 12 foot walls and am putting two bedrooms upstairs.  I wanted to use the I joists again but wanted to keep the open ceiling over one half of the space.  I had the local lumberyard work this up and they recommended 3 1/2 x 14 LVLs (doubled 1 3/4 x 14s) on 4 foot centers.  This seems like it may have been a bit of overkill ( I think this ends up being a l/480 40 psf floor ) but the additional cost was not very much.  I plan to sand and varnish the LVLs and I think they are going to look great.

If I was doing it over I wouldn't bother with the center beam and I would definitely go with the I joists.

Jack Landers

Wood I-joists with OSB webbing are very strong and well engineered but are a big danger with regard to fire.  

The webbing is often as little as 3 or 4 8ths of an inch thick. Sure it's strong, but an open flame will burn right through it in a matter of minutes. A 2'' thick joist resists the open flame much, much longer.

If you have a fire in your home it is far more likely to be a total loss with I-joists. By the time the fire department arrives it will usually have collapsed. There have also been problems with homes built with these joists involving firemen rushing in to save the house or occupants because they know that the fire has only been burning for 10 minutes and generally that means that they still have time to safely do something. But the clock has actually run out due to the thin joists and the burning house collapses on them. This has actually happened.

The convenience of skipping a beam is not worth the risk of dead firemen, in my opinion.


You do want to cover the bottom of the I-joists with drywall. Two layers are required if it is in the ceiling of a garage.


Fire has also been a concern of mine.  We live in whats called a hot zone (The very dry west) Have been questioning myself whether or not to use board and batton siding.  Cedar shakes are definetly out.  I've even worried about wood ceilings and floors.  Beams and larger dia. lumber burn at a much slower rate, giving you more time to do whatever it is you do when your house is burning.  (my wife could spend hours in the bathroom getting ready for the escape.)  Anyhow, I've read about a solution called sodium silicate.  It sounds like just the ticket.  Is anyone familiar with it?  Is it ligit?  Does it look good on wood?  
We live in a hot zone

Bob Lawrence

I used Hardiboard siding on my recently completed project. It does not burn. Also used metal roof and concrete floor. This doesn't prevent destruction but, with dry wall interior makes a fire even starting
much less likely. Could have used metal studs and been even safer but, feel pretty secure. Hardiboard is not much more expensive, is very easy to install, looks great and requires much less maintenance. The entire outside is fireproof as I used all Hardi trim and soffets. No wood on the exterior at all.


Here's an article concerning fire-proof plants:

Maybe these could be used to help protect a house from brushfire.


Thanks for the information on fire.  I've been burnt out.  It wasn't fun--coldest night of the year, I was standing out there without my car keys, in my holstein cow pattern pj's.  I was very glad that the truck didn't catch on fire.

I assume this holds if you are using the same kind of I-joist as a rafter.