Chimney removal and joist need replacing/sistering

[moe]

Hello everyone,

I hope you are all doing well.

 

I find myself in a bit of a bind and would greatly appreciate some assistance regarding my previous post.

If any of you could lend a hand, it would really ease my mind.

 

I need guidance on a joist that measures 7x3, spanning 5 meters, made of old timber. I've taken out the chimney breast from top to bottom, and the stack has been removed all the way down to the cellar floor.

 

The chimney breast is on my side and isn't attached to the neighboring wall.

The issue I'm facing is that when I took down the chimney breast wall, the old joist wasn't fully embedded in the wall; it was short and rested on the chimney wall.

Currently, it is properly supported by an acro prop.

 

My question is, should I sister the new timber alongside the original one?

 

I can extend the new timber from wall to wall. Additionally, for added security, I could double the new timber, effectively creating a 7 x 6 timber connected to the old 7 x 3 timber using m12 nuts and carriage bolts.

 

Is this the best approach? Or do any of you have a better suggestion?

 

I look forward to hearing from all of you.

 

I can provide a diagram of the drawing if that would be helpful.

 

Thank you.

 

Kind regards,

Moeen.

[Nickfromwales]

3 hours ago, moe said:

I can provide a diagram of the drawing if that would be helpful.

You just answered your own question.

 

Pics please!!

[moe]

Hello! I've created this layout plan to the best of my skills. I hope you find it helpful.

[Temp]

So this is the joist that is now 7" from the wall?  

 

If this was a normal joist (eg one of a dozen similar joists crossing a room) then yes I would cut a 7x3 to fill the gap and extend into the wall. Then either sister josts both sides  or use metal joist repair plates extending perhaps 2 feet either side of the joint. 

 

 

However I hesitate to suggest this in your case as the floor structure looks somewhat unconventional. Is this joist is supporting a wall separating the stairwell from the landing? 

Edited August 21 by Temp

[moe]

Hi temp,

 

I had to rewrite this again, as I'm not computer literate. Lol

 

The joist that is 7 inches from the wall is currently being supported by an acro prop, and there is no wall beneath it.

 

On the other hand, the joist that is 9 inches from the wall is supported by a wall that is load-bearing, which extends all the way down to the cellar.

 

I've attached a picture to clarify this. As you can see, the stack and chimney breast have been completely removed. The area where the joists are located was previously supported by a 'U' shaped chimney breast, which then split into two chimney breasts leading down to the cellar.

 

In the picture, highlighted in red, you can see that the joist spans 5 meters in length—one end is anchored in the wall while the other end is not. The end that is not anchored is 7 inches away from the wall.

 

Conversely, the joist highlighted in yellow is on a load-bearing wall, but it is 9 inches away from the wall.

 

I apologize if I am going into too much detail; I just want to ensure everything is clear and straightforward.

 

I hope this information is helpful, as I've done my best to explain it clearly.

 

Thanks,

Moeen.

Edited August 21 by moe

[Nickfromwales]

34 minutes ago, moe said:

I apologize if I am going into too much detail; I just want to ensure everything is clear and straightforward

No need! Details are what we need to be able to help you best.

 

For the one 9" away, over the LB wall, you can simply add a section on the end, supported by say a 1800mm long new section sistered alongside, making as you sat a 6x7 there.

 

For the other side you can do the same again, but this would ideally need to be the full length, and would be subject to the condition of the timber (at the end opposite the chimneys) being in good order. If in any doubt there, you'd need to cut a new pocket alongside the existing one for the new timber to slide into; if it were my own place I'd do this first as last to be sure. You probably cannot get a full new timber in as the new timber would be longer than the width of the room.

 

The piece 3" from the wall is a simple swap out for a new piece afaic?

[moe]

55 minutes ago, Nickfromwales said:

No need! Details are what we need to be able to help you best.

 

For the one 9" away, over the LB wall, you can simply add a section on the end, supported by say a 1800mm long new section sistered alongside, making as you sat a 6x7 there.

 

For the other side you can do the same again, but this would ideally need to be the full length, and would be subject to the condition of the timber (at the end opposite the chimneys) being in good order. If in any doubt there, you'd need to cut a new pocket alongside the existing one for the new timber to slide into; if it were my own place I'd do this first as last to be sure. You probably cannot get a full new timber in as the new timber would be longer than the width of the room.

 

The piece 3" from the wall is a simple swap out for a new piece afaic?

 

 

Hello, I really appreciate your response.

I can obtain a new full-length timber and wall-to-wall on both sides while sistering. I can double it, and it's feasible since I started early today on the walls, having opened them up from my side. I've done this very carefully, and quite a bit of dust came out, haha.😁😅

 

So, in theory, I can sister the old joist to create a '6 x 7' inch joist (C24 graded) using nuts and M12 carriage bolts. However, the new joist will be an 8 x 3, trimmed down to a 7 x 3 timber to match the original 7 x 3 timber. Since the new timber is smaller than the original, it makes sense to match them closely for like for likeness.

 

I hope this information is helpful.

 

Thanks,
Moeen.

[Nickfromwales]

7 hours ago, moe said:

 

 

Hello, I really appreciate your response.

I can obtain a new full-length timber and wall-to-wall on both sides while sistering. I can double it, and it's feasible since I started early today on the walls, having opened them up from my side. I've done this very carefully, and quite a bit of dust came out, haha.😁😅

 

So, in theory, I can sister the old joist to create a '6 x 7' inch joist (C24 graded) using nuts and M12 carriage bolts. However, the new joist will be an 8 x 3, trimmed down to a 7 x 3 timber to match the original 7 x 3 timber. Since the new timber is smaller than the original, it makes sense to match them closely for like for likeness.

 

I hope this information is helpful.

 

Thanks,
Moeen.

Yup. Steam on 👍👌

[moe]

On 21/08/2025 at 19:37, Nickfromwales said:

Yup. Steam on 👍👌

I forgot to inquire whether it is necessary to utilize a 'dog tooth-washer' between sistering joists?

 

Thank you,

Moeen.

[Nickfromwales]

1 hour ago, moe said:

I forgot to inquire whether it is necessary to utilize a 'dog tooth-washer' between sistering joists?

 

Thank you,

Moeen.

It won't hurt that's for sure.

[Temp]

Sometimes you need some big washers or temporary plates to stop the heads and nuts can pulling into wood while tightening them enough to get the dog/spike washer embedded.  

[Nickfromwales]

On 23/08/2025 at 23:17, Temp said:

Sometimes you need some big washers or temporary plates to stop the heads and nuts can pulling into wood while tightening them enough to get the dog/spike washer embedded.  

Yup. There are thick square washers which are typically used for this job, and you just size them for the threaded bar you’re using.

 

Prob M12 or M14 for this size joist. 
 

 

[moe]

Dear All,

I forgot to raise this question earlier and would appreciate your input.

For sistering the joist, would it be more effective to use carriage bolts or a threaded stainless steel rod?

 

Which option would provide superior structural strength and stability?

 

Thank you for your guidance,
Moeen.

[Temp]

Won't make much difference. Bolts are generally made to a higher standard than threaded rod but threaded rod might work out cheaper?

[Nickfromwales]

You tend to want a softer metal vs say a hardened bolt.

 

Threaded bar is plenty good enough, maybe better go M16 even as it’s 150mm of timber to pull together. 

[saveasteading]

I like threaded rod. Buy 1m and you get to cut any length that suits. Then do it again when you see that a different length is better.

TIP  put 2 nuts on the rod before cutting. then when yo take one off, it straightens out the burrs made when cutting.

Agreed that you must add big washers to spread the load.

The loads in doing your task are tiny compared to the rod capacity.