Uneven roof

[amateurroofer]

Hi all, please help if you can!

I'm replacing all of the slates and felt on my roof whilst adding an extension to the back that follows the existing roofline. In essence the roof is eactly the same but a bit longer. It's a pitch roof, quite simple with nothing awkward sticking in or out anywhere. So far I have stripped back the old slates and battens (about 80 years old!) and put down a breather membrane and re-battened to the specified gauge. However, when I ran a string line from front to back yesterday, I noticed that the old part of the roof dips down around an inch or so in the middle. It must have done before (wouldn't have gone like it overnight!), so will I notice it when I install the new slates? Or, should I build up the batten where required with another batten on top or half a batten on top? I'm using natural spanish slate.

Any help or ideas greatly appreciated, thanks.

[acsimpson]

I'm no expert so I don't know the correct way to fix this but assuming that the roof is sound do you notice the dip as it currently is when standing on the ground? if not chances are you wont notice it when completed.

[amateurroofer]

Not too much but the new part of the roof (the extension) is perfect and the new roof is costing so much, I just want to make sure that it matches and is spot on. I'd rather spend a little more time on the preparation to ensure all goes well when the new slates are finally laid.

I have since read that you can pack the battens up/out with laths, and I think this is what I'll end up doing. Anyone else done this before?

[TheDoctor4]

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