Floor framing over crawl space

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Old 03-26-16, 05:02 AM
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Floor framing over crawl space

Hello,

Yesterday I broke up flooring in a cottage bathroom, the floor had sloped severely over the last 50 years. What I found surprised me - 4" or concrete with rebar, on top of framing members (2 by of varying size) which had mostly rotted out. It seemed like a lot of effort for a floor in an old cabin - and it clearly did not hold up. The support wood was all rotted, and this caused the floor to settle significantly. Presumably because of the concrete coming in contact with the wood, and I also noticed that there is no ventilation under this portion of the floor. It is an older cabin, and has been added on to several times. There is a portion of the bathroom adequately ventilated in the crawl, and the 2x8 floor joists are fine here. The problem is, the other half of the bathroom is closed off to this ventilation, with a cinderblock foundation wall running halfway through. The good news is that I have solid rim joists all the way around, and I am thinking of using joist hangers and new 2x8s to create the structure where I removed the concrete. My question is.....without this being adequately ventilated - should I use treated lumber? I will look hard to add some air flow in there, but it looks to be all blocked in.

Is there anything I can do to prevent this from happening again? I think the main culprit was the cement contacting the wood - so hopefully I won't be in this situation again.

Any advice? My plan is basically frame the floor joists (treated or regular 2x8s) - then 3/4 plywood subfloor, then cement board or that orange tile backer, then ceramic time - and hopefully keep it dry from the top at least.

Thanks in advance - I will try to include an image to better show what I am trying to describe.

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Old 03-26-16, 05:42 AM
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It is my thinking that a leaking toilet over many years was the cause of the rot more so than the lack of ventilation. I've seen similar in second story bathrooms without non-ventilated space below. It's more that the space could not be inspected all these years and reinforces concrete will hold without support if the area is not experiencing any load.

I would put the largest joists possible if you are going to go for tile. What is the size of the unsupported span of the joists? The reinforced cement could get by with smaller joists, the newer cement board subfloors need to be stronger as the cement board itself doesn't add any structural strength. Also, plan on at least 1 1/4" worth of subfloor (1/2"+3/4") plus 1/4" cement board + tile to aprox. the correct height of the toilet flange that was set to the height of a mudbed. That combination usually works out to the perfect height.
 
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Old 03-26-16, 10:09 AM
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Thank you. I don't need to frame a very big area. Maybe 6x6 Max. I'll try to get 2x10s in there if I have enough clearance to build
Up the subfloor. Do you think I should use treated floor joists just to be safe? Or some of that pink or blue covered moisture/mold proof lumber?
 
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Old 03-26-16, 11:20 AM
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Make sure there is a 6 mil vapor barrier down on the ground and use regular lumber. If it is only a 6 foot span, you can use 2x8's.
 
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