Taking movement out of rafters

[m0t]

I have a 1930s semi and keep having a problem where the cement under the ridge tiles keeps falling out and pigeons come in and poo all over my stuff

The bottom of the rafters are nailed to the top of the wall plate. About 1/3 of the way up they are bolted to the top of our dormer roof. 1/2 way up they sit on a beam and I think this is where the problem starts.

In the front half of the house the beam is supported by two trusses. The beam is straight and the rafters are fine. The back half of the beam has no support and it has bent quite a bit, the rafters have dropped and a gap has opened at the top under the ridge tiles.

The tops of the rafters butt up to a wooden plate bolted to the wall. The rafters are nailed to this plate. The ones on the bent bit of beam have slid down this plate and the nails have pulled out. The ends have quite a bit of flex and I think the movement is causing the cement to crack.

I'd like to take the movement out and stop the rafters sliding any further. What's the best way to do this? Are there any angled joist hangers I could retrofit? I've also got lots of c24 floor joists that I could bolt to the wall underneath the bottom of the rafters so they can't slide further if that would work?

 

[noseall]

The ones on the bent bit of beam have slid down this plate and the nails have pulled out. The ends have quite a bit of flex and I think the movement is causing the cement to crack.

I can't imagine how poorly fixed a rafter must be, to be able to slide down...? Are there no roof elements fixed to these rafters, i.e. tile batten etc? A regular hanger will still work on a pitched rafter. Even a pair of long galv angles each side would be better than nowt.

Surely the issue here is the lower purlin deflection that needs addressing?

 

[^woody^]

I struggled last night with the tablet in one hand, but I don't know if it was the drink in the other or all that text. I'm no better off this morning, so it was not the drink.

Something sliding or something causing something else above the roof to crack ..... or something?

Pictures?

 

[noseall]

Something sliding or something causing something else above the roof to crack ..... or something?

Sounds like a poorly constructed dormer whereby an unrestrained, rafter supporting purlin is bending sideways and dragging the rafters down with it - or summat.

 

[m0t]

Sorry, I've hurt my leg and can't get up the loft ladder to take photos, that's why I tried to describe it. I've added a drawing from the dormer plans with an arrow pointing to where the issue is. The party wall is brick and there's a 2x4 that runs along the face of the wall, it's fixed to wood that has been embedded in the wall. The ends have moved down this plate.

The roof has been like this since we moved in 10 years ago and predates the dormer. The guys that built that said the ends couldn't go anywhere and to stop worrying about it. I'm just trying to sort the slight movement at the top that keeps cracking the cement between the top row of tiles and bottom of the ridge tiles.

I actually think the root cause was a double glazing company replacing a structural timber framed door and sidelights (2.5m wide in total) without putting in any supporting lintels before we owned the house. The bottom of the rafters originally sat on a wall plate 3 courses above the top of the door and it had dropped an inch by the time we found it and had it put right.

There are battens and tiles on the rafters, I was surprised that I could wiggle the ends with that sort of weight on there.

 

[m0t]

Got some photos now.

Photo of the rafters that have slipped.



The supporting beam.


The rafters that havent slipped. The beam here is supported by two angled pieces whereas the other end isn't