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Old 01-12-2011, 11:32 PM   #1
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Structural questions


Let me introduce my house again here. It's a house in SE Michigan built in 1917, the foundation is that it was built on beams which rest on stacks of cement blocks, but let's look past that for now - my question has to do with the structure for the second floor.

The apparent original configuration of the house was a ballon framed 22' x 22' 1.5 story house with a 9/12 pitch gable roof and a stairway going up in the middle to 2 bedrooms, the bedrooms having 7' tall ceilings and 4' knee walls. The gable ends face the east and west, the upstairs bedrooms are east and west of the stairs.

Worth mentioning, the east end of the house was extended another 9 feet. This is apparent because from inside the crawlspace, the floor joists in this section are perpindicular to the main house. The main house is supported by a triple 2x6 beam running down the center, this beam does not extend into the addition.

Anyway, the west end of the first floor is all one big open room. In the first floor ceiling, a beam is apparent and appears to be 2x6 - 5 2x6's to be exact. This beam is spanning 12' The exact amount that this beam is sagging is difficult to precisely measure because the floor is not level, there is 3" of difference of the height from the floor to the beam when the center of the beam is compared with 1 end, and when compared with the other end there is 4" of difference. The end with 3" of difference, I measured 1' from the wall because the wall is at an angle. Measured at the wall, there is 4" of sag relative to the center of the beam.

On the other side of the stairway, there is a load bearing wall dividing the dining room from the bathroom, this wall is 6 feet from the exterior wall, there is no other mid-span support for floor joists over the 7 feet between the stair wall and the original exterior wall. But there doesn't appear to be any sag? And this is where the joists would be spanning 16', and from viewing the joists in the knee wall attic they are all 2x6 joists with no sistering apparent.

I'm concerned where the sagging is visible, but I am assuming that after 93 years that this is not an imminent failure. As for the other side of the stairs, I have to wonder if anyone can explain how this hasn't been an issue?

Oh yeah.. the really puzzling part about the addition is that the floor joists in the first floor are oriented east-west. The floor josits for the second floor are oriented north-south. I believe that the addition was platform framed, kind of. The first floor walls have top plates, but the second floor walls do not have top plates.

And understanding how the house is constructed is complicated by many walls with 2 layers of drywall over lath and plaster, or 3 layers of drywall, sometimes with a layer of studs in between.

That and the frequent apperance of walls with studs oriented sideways (as in 2" thick dimension oriented to the wall thickness instead of 4" dimension) This construction was done on the kitchen wall, which the kitchen is entirely within the addition - and this wall might be load bearing if it is made so by the ceiling joist being oriented north-south, or it might not be because the floor joists are oriented east-west - then again, it is right on top of a 2x6 floor joist. Oh and this construction was also apparent in the stair wall in the second floor, and the wall studs were buckled.

Seriously, don't be afraid to answer this, I'm not asking what to do, I'm just looking to see if anyone can explain these perplexing structural conditions.

And for anyone reading this and thinking to themselves "a 2x6 can span 16'?" I believe the answer is, no it can't. Wood species in use today mostly are not as strong as what was in use in 1917.

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Old 01-12-2011, 11:47 PM   #2
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Seriously, don't be afraid to answer this, I'm not asking what to do, I'm just looking to see if anyone can explain these perplexing structural conditions.
What's the question?

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Old 01-13-2011, 12:07 AM   #3
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Why does a 16' span of 2x6 joists not sag while a 5 thick beam of 2x6's supporting 12' span 2x6 joists sags?
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Old 01-13-2011, 12:26 AM   #4
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Why does a 16' span of 2x6 joists not sag while a 5 thick beam of 2x6's supporting 12' span 2x6 joists sags?
Because the 16í span of 2x6 joists probably had no point loads, were properly specíd for the span and the above load never changed. The 5 thick beam of 2x6ís were probably under specíd for the point loads and the above load or the above load was changed. Thatís my opinion.
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Old 01-13-2011, 07:18 AM   #5
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Is old wood really that much stronger? Span tables today say Douglas Fir shouldn't be more than 12'!

Where the beam is located, without it the span would be 22'. As far as load above the floor, though, they both are the same. No load bearing walls except the attic knee walls which are 4' inboard of the exterior wall on both sides.

I might have left out some info, but then again this is a kind of unpleasant adventure getting to know this house... The first 10 floor joists under the first floor are rotted away over the bottom half of he first 18" or so, and new 2x6 sistered onto them over the first 6 feet (i.e. not all the way to the beam at the center.

I think after thinking about it, I'm concerned there is some trauma the house went through in the east half, and the ceiling beam is a sign of underlying structural problems. Then again I guess I got into this operating on the assumption that the house IS a structural problem, but with the accompanying assumption that it has lasted 93 years and that doom isn't impending, but there exists a solution to any deficiencies.
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Old 01-13-2011, 08:33 AM   #6
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The first 10 floor joists under the first floor are rotted away over the bottom half of he first 18" or so, and new 2x6 sistered onto them over the first 6 feet (i.e. not all the way to the beam at the center
Generally not the best option
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Old 01-13-2011, 09:10 AM   #7
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Yeah, I had been planning on replacing those floor joists with new Douglas Fir, I'd rather go up in size but unless I'm replacing all of the floor joists (which itself increases the scope tremendously for the repair) it complicates interfacing with the support under the floor joists.

By way of the kneewall attic, I'm able to stick a camera into the joist cavity and see a little more than I can see on my own. I did that this morning, I'm attaching an image and a zoomed in image.

One more thing worth noting, this room to the west with the sagging ceiling, the ceiling is drywall. To the east, drywall is installed over lath and plaster. So it seems the lath and plaster was removed at some point in the west.
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