Cantilever roofs and Wood in the cavity

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My house, as well as others in the street, immediately stick out as being unusual.  They were built in the art deco style in 1930 but have been modified replacing original windows with uPVC and some have had their original flat roof replaced with a pitched roof. 
There are some clear remnants of the original ere however. One of the defining features of art deco houses is the use of cantilever overhangs over windows, flat roof sections and for crisp clean (often curved) balconies.

There are two such cantilever overhangs on my house.  The canopy over the front door (see photo below) and a wrap around flat roof over the bay window at the front of the house. 
Cantilever canopy over the front door( possibly over specified  as can easily take 2 people jumping  on it it!)
As part of my ongoing investigation into the construction of the house I decided to look how the overhangs were constructed.

The roof at the front is only a thin strip of roof just in front of the 1st floor bay which itself is stepped back from the ground floor bay. The flat roof then extends to the left (looking from the street) and wraps around into the corner. I've tried to illustrate below:



Sketch illustrating the front roof construction with the flat roof joist becoming the floor joists as you step back

With the soft removed, the joist can be seen penetrating through the cavity to provide the support for the roof.


The canopy structure is provided by joists which penetrate the cavity walls.

This means that there is structural wood inside the cavity as illustrated below. (apologies for my poor isometric drawing skills!)


There are two issues here which should have been picked up in a pre installation survey
  • multiple pre-existing cavity bridges
  •  Wood within the cavity needs ventilation to prevent rotting.
As part of the insulation process, the installers intentionally block off the cavity vents as part of the guidlines but they must make sure those providing ventilation to e.g sub floor are sleeved and in good working order.
Timber framed buildings are a contraindication to retrofit cavity wall insulation for the same reason.

One of the joists were completely rotten mid cavity. The photo below shows it was surrounded with wool insulation at the point where it has rotted.

Rotten joist in contact with the unfit for purpose insulation

It's very disappointing that a crew of unqualified installers can be let loose on an older property without any safeguards. Even the most up to date cavity assessment system wouldn't pick these problems. It takes hours and lots of invasive work to appreciate the build and characteristics of a house and how it has been modified over the years. 

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