Help with render problem onto timber bay window

[focaltucker]

I have an 18 year old barratt house which has some external render at the front. One area which is between the downstairs and upstairs bay window is blown and has been steadily getting worse.

It now needs removing and re-doing. However, I noticed on the inside of the same window (behind a fitted set of drawers, that the internal plaster board is molding and shows signs of damp.

I took the fitted drawers out for a better look and found the board and plaster was moldy. I removed some of the board and it seems the render in the bay window is onto ply board and they have put a damp proof membrane between the ply and the wooden frame. However, they have also nailed through the membrane all over the place!

It looks like water has got behind the render and wet the ply, which is allowing damp in and causing the plasterboard to soak up some moisture.

Is it common to render onto ply like this?

What's the correct way for it to be done and for me to make sure it's rectified so it's fully waterproof?

I intend to remove the render myself and re-board with ply and get a plasterer to then do the render.

 

[legs-akimbo]

Its pretty much the standard route to render the cheeks of Dormersas has been done previously, Yours has perished to the extent that it is allowing moisture to penetrate internally.
If the ply is still sound leave in place
Sheath the Dormer cheeks in ordinary blizzard felt using staples and expanded metal sheet with felt studs, allow for a bellcast drip and lead flashing, best here is half inch two coat pvc married to any other pvc two coat beads at external angles for clean lines.
Scratch coat render with added waterproofer followed by finish coat render with added waterproofer, should be good for the next 25 years if done correctly.

 

[flexcrend]

Yes the correct way would be to strip back to the existing studwork replace or install/upgrade any insulation, fix new membrane then ply then building paper or other breathable membrane then s/steel eml apply pricking up coat, scratch coat and then finish render coat , in sand/cement or colour through polymer modified (remember to add re-enforcing alkaline resistant mesh in base coat off polymer modified renders). Internally replace all affected plasterboard and membranes.
Regards