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Old 11-12-2011, 09:15 PM   #1
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Tricky subfloor


The builder didn't show up, and I have a deadline, so I need to do this work myself (choke). I built a small deck last week (with internet instructions, very good) so I figure, how much harder can a floor be?

I need to construct a subfloor in a partially renovated section of a 2.5 storey century old home. The floor is partially dirt and partially concrete, covered in spray foam. The whole room is about 13 feet wide, and 27 feet long. In the front section, one side of foundation is rubble, topped with properly constructed walls, and the other (interior) side foundation is a big long box (approximately 2' X 2' X 14') full of stones and sand or something, with a nice firm top. I can't replace that foundation box without the house falling down, it's there to stay. Because of the dirt in the middle of the floor, only the foundation at the edges can bear the weight of the joists. The joist span in the front portion of the room will be a little under 12'. I can't change the height of the foundation box (which is partly topped with huge 6" X 7" rough hewn beams supporting the floor in the adjoining room), and I can't change the placement of the doors (set in brick), especially since this very unique house has three front doors, all the same height and more or less symmetrical. So my final floor height is not negotiable. The result is that at the edges of the joist span, the lumber can only be (true) 6" in height (i.e. not higher than the 6 X 7 beams). So I figure I need to use 2" X 10" joists, and notch out the bottom at the edges so that they are 6" at the edges, and hang the notched portions on faceboards using 6" joist hangers. At the back portion of the room, instead of the big box of stones, there are ordinary building blocks, and the span is between 13' and 14' (I don't know to the inch) because there is no big box of stones supporting the interior wall. I hope I am explaining this clearly.

My question is, how do I safely construct the subfloor, given that the ends of the joists need to be notched down to 6"? On the one side of the room where the wall is constructed on top of rubble, I could put a second face plate (to hold the joist hangers) inside of the regular face plate, have it hang down lower, and thus use unnotched 10" joists. However, I am not sure that it would be structurally superior to have two parallel face plates, given that the joists would then be nailed through one face plate which is only secured at the top, and also through a second face plate against the wall: A 3 inch depth. I think that maybe this would be structurally inferior to using a notched 2X10 for the full span of the room. Should I compensate for notching the 2 X 10's by using 12" centres instead of 16" centres? I can put pieces of wood half way, or 1/3 and 2/3 of the span of the joists, to attach them together and increase stability, but if I do this, should I use 16" centres or 12" centres to hang the joists? Has anybody else encountered this situation? I could use some advice here.
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Old 11-13-2011, 09:23 AM   #2
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Tricky subfloor


Quote:
Originally Posted by Picha View Post
The builder didn't show up, and I have a deadline, so I need to do this work myself (choke). I built a small deck last week (with internet instructions, very good) so I figure, how much harder can a floor be?

.................................................. .................................................. ..........................................

I could use some advice here.
I'm a big fan of DIY...but this one sets off lots of alarms.

Big difference between a small deck and a sub floor....it sounds like you have some idea of what to do....but this is one area you don't want to make mistakes.

My advice....since this is the first time....get a different builder....
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Old 11-13-2011, 11:44 PM   #3
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Tricky subfloor


Because you would be past the maximum notch found here: http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf


You could use some top mount hangers for solid wood joists under the remaining lumber after the bearing notch is removed, so it doesn’t split there: http://www.strongtie.com/products/ca...flange_ww.html


Make sure your ledger is solidly fastened to ? with lag bolts at least following the Deck Code minimum attachment here: http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf




And, welcome to the forums, Picha!


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