1st Floor Timber Frame Extension

[Peter Hodgson]

Hi,

I wonder if anyone has a detail for the following situation and may be able to comment on a couple of points below.

Our property is a standard construction 1980s bungalow with brick outer skin/ 100mm cavity / insulated block inner skin.

I've had a timber manufacturer produce a bobtail attic truss design for a first floor extension and need to get my head around the best way to install it.

To minimise disruption below I plan on leaving the existing joists in place and install the new trusses above them.

The existing ceiling joists are already bearing on a wallplate over the inner leaf, with the majority running 90 degrees to the proposed new trusses.
There is however a small section shown in red where the joists will run in then same direction, would it be acceptable to pack between the existing joists with 4x2 to bring upto the top of existing joist, then install a new continuous wallplate above?


The truss walls will eventually have composite cladding on the outside, therefore I was hoping to support it on the inter leaf, but overshoot the trusses to the outer skin to allow the cladding to sit slightly proud of the brick face.
I plan to close the cavity somewhere just below existing wallplate level, then bring the cladding slightly below to conceal it. What would be the most suitable way to close the cavity off?

Also with the gables would it be acceptable to mount the last truss on the external skin with a dpc, if so what would happen with the cavity at this location?

Any thoughts / comments would be appreciated.

Thanks

Pete

 

[^woody^]

You could potentially do what you propose with the joist sand plate.

If the end truss was on a wall, you would need a double truss to form a spandrel panel with sheathing and membrane.