35cm wide flat boards along inside of ridge - is this normal?

[SenileGit]

Hi, I have been repairing the battens on my roof, because I had a tile split across in half, halfway up the roof, on the edge, and the wet verge had slid down almost an inch, along with all the tiles below the split tile. As I started removing tiles on the right hand edge, I found that all of the battens were heavily rotted.
There are also two 35cm wide boards of wood, less than a centimetre thick, that are under the battens. They go above the inner and outer brick walls of the house (I presume it's a cavity wall, I'm not sure.) Do I need to replace these boards, or can I just remove the heavily rotted (lower) one and put new membrane across?
I asked in a roofer's supply place, and they said they don't sell anything like that, and that probably putting the breathable membrane all the way across it would be fine. The house is about 45 years old, so I don't know if this is a roofing method that is no longer used.

 

[SenileGit]

Also, you can see I've cut out parts of the old membrane, which was sort of 'tar like' and I'm pretty sure it's not breathable - the guys in the roofing supplies said it was ancient - I've cut out new breathable membrane rectangles to put across - I presume I can just put part of it (about 20cm) over the existing membrane on the left - by just lifting up the battens on the left and sliding it under, nailing it onto the right hand side with short clout nails, and then putting new battens on top?
I know that the whole roof's membrane probably needs replacing, but the weather is so hot at the moment, I can only face doing three hours a day - I'll get a proper roofer in some time in the future. I wasn't expecting it all to turn into such a big job. (Never done a roof before. It probably shows!)

 

[datarebal]

Pull the ply off and ditch it .
Doing more harm than good .
The old felt is 1f and none breathable .
Replace with breathable .

 

[SenileGit]

Many thanks for your help, I'll remove the ply tomorrow and put the breathable across it.

 

[^woody^]

Is that a timber framed house/gable?

If so, that ply may be stiffening the gable apex.

 

[SenileGit]

I don't think it's a timber framed house, there are two brick walls underneath the ply.

 

[^woody^]

What are these? And it looks like sheathing on the back.

 

[SenileGit]

Hi Woody, I don't know if the ply was stiffening the gable apex. The top piece of ply is still there, it hadn't gone rotten. I am hoping to get the whole roof redone (new felt and battens) some time next year, I presume all the battens going across will help stiffen the lower part?