Masonry external wall to timber frame external wall junction

[Moonshine]

My build is a split level house, and there is a stepped flat roof, as shown in the red bubbles area below left. The tricky part is that it straddles a room on first floor, below right with the red hashed lines

 

The structural engineer's thoughts are to have the stepped roof wall as masonry, but to do that it will need some significant steels (2 x 5.4m long + 2 x 4m) at roof level to support the cavity walls.

 

My thoughts are different in terms of buildability and to have the stepped wall as a timber external wall, which would be supported by the roof joists running left to right across the master suite to the structural wall between the master suite and the bathroom. I have talked this though with the joist designers who have indicated that is should be fine with some beefed up joists, and if the SE can confirm that the steel (at ground / first floor) can take the extra loading, which may need a bigger steel which is fine.

 

Below is how this looks in section, with the stepped wall circa 1.5m in high.

 

 

 

I am sure that this can work, but my concern is the junctions between the the masonry and timber wall constructions, indicated in the blue bubbles in the first picture.

 

This is the junction detail i have come up with at the areas marked in the blue bubbles, but my main concern is the potential difference in movement between the two construction types, the render cracking and moisture / rain getting in. Basically once water gets though the render and carrier board, its got a pretty free path down into the master suite.

 

I would be interested to hear what are peoples thoughts on using timber in this area rather than masonry and the potential weaknesses of this junction and how it could be designed out.

Edited March 4, 2021 by Moonshine

[Mr Punter]

I think you will need a movement joint between the materials.  If this is a parapet type wall it may be good to bring the flat roof waterproofing layer up and over, or certainly run it up 200mm behind the cladding.  Finish with a quality aluminium coping.

[Moonshine]

20 minutes ago, Mr Punter said:

I think you will need a movement joint between the materials.  If this is a parapet type wall it may be good to bring the flat roof waterproofing layer up and over, or certainly run it up 200mm behind the cladding.  Finish with a quality aluminium coping.

 

Good shout on the movement joint, and I think that will be the solution, a bit of googling gives this

 

https://nhbc-standards.co.uk/6-superstructure-excluding-roofs/6-11-render/6-11-5-accommodation-of-movement/

 

Rather than bolting the timber to the masonry use some kind of slip tie for timber, and have a vertical break in the render at the joints (weather tight seal over the movement joint).

 

It is a parapet flat roof below, and I will be looking to use a drip trim (as below in another location) but for a timber wall, with the EPDM raised up min 150mm up the rendered wall

 

Edited March 4, 2021 by Moonshine

[Mr Punter]

If you wanted to build something more complicated you would have to really try hard!