Some advice from the experts, please:

 

We have been having a dormer window constructed - no plans as such, though builders seem reasonably competent (mostly!)

 

I didn’t know what to expect, and all looked reasonable to me - until got home to this anonymous note through the door, saying that the builders should have used roofing felt in the construction, and that we were storing up problems.

 

So my question to this forum is: is this letter correct? Or is a waterproof (breathable?) membrane under the cladding considered sufficient?  
 

I have attached a few photos of the construction - thoughts welcome!

 

 

….And the letter

I would have included it and run it over the lead to catch condensation etc.

Cladding/batten/breather membrane is a pretty standard detail for a wall construction. I've never seen a wall specified with an extra layer of felt. Not looked at dormer construction in detail but can't think why it would be different.

 

From your first picture it looks like they started with a line of breather parallel to the roof. I can't see in the photos but from the angle/cut of the pieces I assume they are L-shaped and come out from the wall a bit at the bottom & overlap either your tiles or the roof membrane below the lead.

 

If so, that should catch any driving rain/condensation that gets behind the cladding and take it out down the roof.

 

Potentially your neighbour is familiar with older construction methods & not aware of what the breather membrane is & does.

I wouldn’t have added felt but I would have a corner tray (not really a tray but term often used to describe a flashing under the cladding) to take any rain blown behind the corner flashing away from those corner battens. 

I used Tyvek Housewrap for my dormers. This is classed W1 - extremely water resistant. It looks like they used Permavent for your dormer, this is also classified W1 - extremely water resistant. 
 

https://www.permavent.co.uk/products/permavent-max-all-zones/

 

Also your choice of cladding looks pretty robust to driving rain. I’m sure there is more that could have been done. But I don’t think felt was necessary. 

Out of interest, where and what is the insulation in the dormer?
 

(I have crappy leaky+drafty dormers on my cottage and they need rebuilding so I am trying to find out as much as possible)

3 hours ago, Spreadsheetman said:

Out of interest, where and what is the insulation in the dormer?
 

(I have crappy leaky+drafty dormers on my cottage and they need rebuilding so I am trying to find out as much as possible)

it is insulated all round (roof, sides and front below / above window) with solid foam insulation (kingspan or similar). Think mostly 150 mm total thickness -  all done on inside of structure  (I.e a cold roof) before plaster boarding. This feels plenty….

One more question:

 

 this photo shows the drainage arrangement for the new dormer as built - it drains forward into a down pipe - this then exits over the last 3 rows of original roof tiles into the original roof gutter at the front. Just wondering if people feel this is appropriate - Is there potentially an erosion issue channeling the water over the tiles in this way?

 

 Any thoughts / advice welcome…thanks!