sistering floor joists

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Old 01-19-14, 09:36 PM
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sistering floor joists

I own a 130 year old row home in Philly. I have neighbors on both sides of me (party walls.) The party walls are brick, there are two rows of brick between each row home.

There has been some settling over the years and I wish to level the floor and lay a new subfloor, floor, etc.

The current floor joists span 16' and they look to be 3x7 (true) lumber. I'm pretty confident I can sister the lumber to the existing 3x7's on my own. I'm also familiar with the need to use a laser level, and the need to find the highest point, and raising everything to that point. My questions are:

The existing floor joists rest on top of bricks. For the sistered lumber, do I need create pockets in the brick, next to the existing floor joists? Or can I simply sister to the joist and not have the new lumber rest on bricks?

On sistered joists, does the load transfer from the new sistered joist to the existing 3x7s, thus not requiring me to rest the sistered joists on top of bricks? What lumber should I purchase for sistering?

In some cases, the 3x7's will need to be replaced. Where does one purchase 3x7s? Are there alternatives?

If I need to install a 3x7 joist into the existing pockets in each party wall. How am I going to slide it in? I'm assuming the length from pocket end to pocket end is 16'8'. (16' span + two 4" bricks) There is no way I can install that length of lumber without poking a whole into my neighbors home. What does one typically do to replace floor joists in a row home or brownstone where you have neighbors on both sides? Is there a steel bracket I can purchase?

Many thanks in advance!

Doug
 
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Old 01-20-14, 06:45 PM
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I would use 2x8s & notch the ends to get them on the brick. Lay them on the side & flip them up. You don't have to touch the neighbor's wall.
 
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Old 01-21-14, 09:46 PM
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I would not run full-length sister joists. For the simple reason that the majority of the bending moment applied to the existing joists (which is what is causing them to sag) occurs in the middle 2/3 of their length. You could easily install 12' 2 x 8s, centered on each joist.

Good luck on trying to find true 3 x 7s for replacing the bad ones. A building salvage outfit might be a good place to start. If that doesn't work, a single 2 x 8 has a Section Modulus of 13.14 C.I., while your existing 3 x 7s each have a Section Modulus of approximately 24.5 C.I. Doubling up 2 x 8s (glued and screwed) to replace each 3 x 7 will work quite comfortably (26.28 > 24.5). Getting a full-length member into position may be another challenge, however, despite Pulpo making it sound easy. I think the limitations of your basement walls and the proximity of adjacent joists could make difficult getting the long members into place. Without seeing what you have, one possibility I can think of would be to remove (and later replace) the flooring, enabling the full-length members to be dropped in from above. Another (long-shot) option would be to use lumber flexible enough that a full 16' piece could be jacked and tensioned with cables, to form a curve, enabling it to squeeze past the walls while oriented in a concave direction, looking down at it.
 
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Old 01-22-14, 08:12 AM
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When planning for your sistering operation take into consideration any wiring, plumbing or ducts. Sometimes things must be removed or relocated but usually removing their mounting fasteners will allow you to move them around enough to get your lumber in place. Just be very mindful that wiring and plumbing is not load bearing. Letting a long beam fall on a pipe or wire can ruin your plans for the rest of the day.

You did not mention what is under the joists you want to sister. Is it finished space or a basement or crawl space? Is installing a beam down the center of the room to support the existing joists an option? Quite often a beam is much easier to install and can raise sagging beams and really improve the stiffness of the floor by cutting the joist's span in half.
 
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Old 01-22-14, 07:49 PM
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Hi everyone.

It's not the joists that are sagging, it's the foundation that has settled over the course of a century. The joists were just along for the ride (so to speak). I was mistaken regarding the dimensions. The joists are 3x9 true. The joists support the first floor and are visible from the basement, which is not finished but has standing room.

So here is what I'm struggling with. The span is 16', the existing joists are 16' 8". Each side of the existing joist rests on 4" of brick. How do I fit in a 16' 8" sistered joist next to the existing joist? I don't think I can do that without breaking several feet of additional brick above one end of the pockets.

I spoke to a structural engineer. All weight is transferred from the sistered joist, through the bolts, to the existing joists. So that answers that.

When I do lay a sistered joist on brick, it'll be with a mason's help, and only for those those joists that look questionable. i.e. there is a large crack in one of them.
 
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Old 01-22-14, 08:03 PM
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As Bridgeman45 said, I made it sound too easy. You may have to temporarily move adjacent joists, to one side, to install the new joists, on an angle. Then pivot them into position. If that won't work, Bridgeman's other suggestion of removing some of the flooring may work. Can you post some pics?
 
 

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