Stairway Enlargement: Cutting First Floor Timbers

[EvilDonut]

As part of an extension I need to move my stairway 'forward' from where it currently is. I don't currently have enough headroom for this, so need to modify the first floor joists.

So I need to cut into two main load bearing timbers, with an acrow prop taking the load. I will need to install a few new trimmers and somehow 'extend' the one parallel with the stairway.

How should I go about this? I have attached an annotated diagram.

The labeled spans are (in mm):

A: existing: 2700, new: 3150
B: existing: 1800, new: 850 (920 to be removed)
C: 1800 (to be added)
D: 850​

I spoke to my builder about this. (Due to the split of work that I agreed to, this job falls into my hands, not his!). The cutting at A/B sounds easy. Installation of C sounds easy. He said that trimmer 'A' would need sistering for its full length. This is where I see difficulty arising. Which side of A did he mean to run the new trimmer? It can't be on the stairway side as there won't be room. And putting it on the other side would surely mean I'd have to cut every single joist D.

Note: the image is just a stock one from google that looks almost exactly the same as my first floor joist plan. Handy coincidence!

 

[Doggit]

If the new stairs are the same width, then yes, you can't sister the the two trimmers. You need to support the joists at C, and at D as well (using 2 acrows at least) before you take out A, cut back D, and Put in C, and then A. If you can remove B, then you should be able to reuse it at C, as long as it will handle the extra loading.

 

[EvilDonut]

Actually, I might be able to get narrower stairs. Since writing the initial post I checked existing width (840mm) and the regulation for minimum stair width (there is none! only a recommendation of 800-900). So I could possibly get away with a 47x175 or even 63x175 depending what I'd need to cover a span of 3.15m.

I wasn't going to completely remove 'B', just cut it down to size and leave it as if it were a normal joist like D. C will be a whole new piece of timber (if I re-use B I cannot rely on any strength grade, its 60 years old).

Thanks!

 

[Doggit]

It's up to you how you do it, but at 60 years old, it will have been grown more slowly, and will be stronger. I'd be inclined to sister B to C, and if you can get away a narrower stairs, then sister the 63x175 to A, but double check with the BCO, as he may want 2x47 along the length of A, rather than just part of it. What's the sizing of A at the moment.

 

[EvilDonut]

Not had the boards up for a while but seem to remember it being anywhere between (2” - 3”) x 7”

I know if I make my stairs narrower then it’ll bug me forever Will think on it, good to know the overall strategy is correct though, cheers.

 

[^woody^]

Why bodge it with narrower stairs? Do it properly.

Replace the trimmer with a suitable one in one piece (or double joist), or fit a plate or angle to the existing if there is room. Either way you will need to agree sizes with the inspector or involve an engineer.

 

[EvilDonut]

Ok, I guess replacing the existing trimmer (A) with a new one would be much cleaner.

Just contacted my building inspector for input too, he's been pretty helpful so far.