Sagging Beam on Monopitch Extension Query

Hi folks,

Was wondering if any of you might be able to provide some opinions on the seriousness of a roofing issue I have.
Currently looking at buying a house in the UK, it’s a terraced timber-framed property from the 15th century, with a hallway and kitchen extension out the back.

I believe the extension was built sometime around the 1980s and replaced an older (possibly Victorian) outbuilding.

Anyways, just had a full building survey done, and whilst he has said that the main original building is in good condition throughout, the extension has a few issues. (Surprised since I thought the part that was almost 600 years old would be the problem!).

The main issue he has found with the kitchen extension is that there is a sagging beam running along the middle of the roof.
Essentially, it’s a simple ‘lean-to’ construction, single pitch roof, ground floor extension (nothing above it) with a beam that spans the length of the extension roof, and another central beam that seems to traverse the inside ceilings and wall running perpendicular to the other beam.
There is a walled off part that has a toilet on the other side.
I’ve attached a rather crude drawing to try and illustrate which part is sagging [Part 1]. I would say it’s possibly sagging by about 3cm at it’s lowest point. (Please ignore the general wonkiness of my drawing and assume that everything else is fine!)
There appears to be no other sagging of other beams, and the surveyor can’t find any evidence of water ingress.

Any questions, please fire away.
Just looking to get an idea of the seriousness of this…we really love the house otherwise and it’d be a shame to lose it over this!

Thanks for your help.

By your drawing, it appears that the beam is supporting the roof on its wide or weak side…? If that is true, you probably are lucky that the rest of them haven’t begun to sag (although the bath wall probably saves the other beam and shortens the span). You didn’t mention the span of the beam.

Thanks for your reply. Yes, I suppose since that part of the beam is the longest then it’s taking the most load. Span of the building is about 7.5 metres by 2.7 metres, so at a rough guess, the sagging beam is about 3 metres long.

Jack up the roof and double the beam. Joist hanger at both ends to keep both beams together.