Rafter support beam, is this useable?

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Hi there, this forum has been such a help but I can find an answer to this!

The purlin I have in place halfway up the rafter length is supported by one brace on either side of the roof face onto a supporting wallnin middle of loftspace (terrace house). On one side the brace has slipped and cracked at its birdmouth join onto the purlin and the purlin is clearly bowed a bit but no cracks. I want to support the rafters on this side.

This beam at 'floor' level goes all the way across into the brick work at both sides.

My question is, can I setup a rafter strut onto this lower beam? And can I 'sister' this beam to strengthen it, without going into the brick? It's an old beam so I'd like to sister it if possible. And I'd use a 2x4 length horizontally across the rafters and strut up onto that length from this beam below if use able. Any help appreciated
 

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(I would not be looking to remove the purlin at all, just to add struts to support the rafters)
 
Needs a couple more pictures, I think.
Suggest you need a structural bod to advise, for when you come to sell, but :
I needed to stiffen a ceiling so it bowed less when I walked around in the loft above it. I sistered a (laid flat :rolleyes: ) ceiling binder with a new beam of 50 x 125 sawn, with long screws. Stiffness is proportional to width x (depth CUBED), so an on-edge 50 x 125 is double the stiffness, half the deflection, of 50 x 100. It worked for me. In your position I'd use 50 x 150, over 3x as stiff as 50 x 100. No harder (?) and price difference is insignificant.
In inches,
4³ = 64____*
5³ = 125___**
6³ = 216___***+
 
Unless I'm misunderstanding, you need to replace the broken purlin prop in the existing position, otherwise the one on the other side will push your spine wall over.
 
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Also, unrelated, move all that junk that is compressing your insulation and roll out another 170mm
 

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