Roof Truss Fixing Query

[unclebobby]

I am building a 2 storey 10x5m agricultural shed with general purpose storage room/office over.
It is built with 100mm concrete blocks (dense outer/Durox inner) with 100mm empty cavity (3 sides) and 150mm Durox/wood clad on other.
I have a question relating to eaves details and roof design, which will be pre-fabricated raised tie trusses, half hip ends and ultimately covered with slate.
Can I support the trusses on the outer leaf ? Why are they normally on the inner leaf ?
Should I close the open cavity, if so how - block on flat or just leave open ?
What size wall plate should I use (trussed rafters are 9x2 at 400 centres due to very high collar to increase ceiling height) and should joints be lapped ?
Any advice much appreciated.

 

[noseall]

I have seen roofs have been pitched on the outer course (225mm 1 brick thick walls) but it is rare.

For one thing, it is far easier to accommodate the holding down straps on the inner leaf than on the outer.

With standard trusses and cut roofs, setting the joist height would become a pain also when trying to achieve max height.

But as you have pointed out, your roof has raised ties (i.e. no problem with ceiling joists) and you are rendering the outside. I would point out though standard cavity lintels have the greater load capacity on the inner leaf only. Stability may be an issue also.

Wall plate can be as small a section as 100mm x 50mm, though a majority are 100mm x 75mm. Wall plate joints are usually lapped and screwed.

 

[unclebobby]

Lintels aren't an issue for me, but I suppose straps on the outside could also suffer corrosion ?
I thought about stability but couldn't think how it could be an issue if leaves are tied together properly ?
Any thoughts on cavity closure ?

 

[Static]

Outer brickwork is normally just cladding.. so can remove the whole lot and the building will still be standing.. making it into a load bearing element isnt impossible but would cause future issues with work on the brickwork..

Waterproofing the wall plate would be fun too.. in the end traditional methods are done because centuries of construction has ironed out all the potential problems and you will be left with a building that will last.. if you want to pioneer new construction methods, expect them to be expensive and has limited lifespan..

 

[unclebobby]

Unless I'm missing something, cavity walls haven't been around that long and stability relies on the fact that inner and outer leaves are connected with ties on a permanent basis (thats not to say they will fall down if they weren't). I wouldn't be surprised if in 200 years time, all the 200 year old houses today are still standing, and all todays new ones have fallen down or been replaced.