Sistering a joist with no space on any side

Question

I am doing some work to the area underneath my master bath. While opening up the walls we found some large holes drilled into the joists for the plumbing running through it. In one case I have two partially overlapping 3" holes 1/2" from the top of our 2x8 joist.

I am going to be replacing this plumbing because it shows signs of leaking around many of the joints and the drain line has an portion with an upward slope. I need to be able to drill new holes to be able to get the drain line to have a downward slope. To feel comfortable drilling new holes into this joist, I would like to sister a new joist.

I run into some trouble with sistering this joist because it has other structural components on all sides of it. The joist I am sistering runs from the center of the house to the back of the house. On one side of the joist is the joist that runs from the center of the house to the front of the house. The joist I am sistering is also the last joist before the side of the house. There are short 2x8s connecting the joist to the side of the house. The second story of my house slightly extends over the side wall framing. These short 2x8s are 2'. The distance between the framing of the side of the house to the joist is 15". These short 2x8s run all the way along the joists from the front of the house to the back and appear to be the only thing supporting the floor for the last 2' of the house.

I'm not sure what my options are for sistering a new joist. Is there anyway for us to have a spacer so that the new joist can be on the other side of the joist running to the front of the house? Or will I need to build a temporary wall under the short 2x8s to support the short 2x8s while I put a new joist between the old joist and the short 2x8s? Bonus points if you can tell me what the short 2x8s are called so I can call them something other than the short 2x8s.

I plan to call a structural engineer in a couple weeks but would like to get an idea of what we're getting ourselves into.

Answer 1

Yes, you can use spacers. Even vertical 2x4 blocks every 12-16" would do. Just fasten it all together very well with 16d nails, construction screws, or bolts run through.

You can also probably trim back the interfering joist to the center of the bearing wall, giving you half the depth for your new joist.

Or, replace the joist completely. A temporary wall under the cantilevered joists would let you slip a new joist in place. Just watch for uplift when you pull the nails in the first one. Sometimes cantilevers were badly done and there's actually more weight outside the exterior wall.

The fact that there's a cantilever and the fact that there are no joist hangers and just two face nails per tell us that there's no great load here. Everything's in fairly static balance.