If the wall plate of the flat roof is level with the wall plate of the existing house can you make the two wall plates the same height then you will need to strip off some roof tiles and run the new joists through butting and fixing them up to either the roof rafter or the ceiling joist on top of the house wall plate.
Flat roof wall plate fixing.
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Where a new flat roof abuts an existing external wall the joists can be hung on metal joist hangers which are fixed to a horizontal timber plate wall bolted to the wall.
The roofs size is 2 8mx1 6m.
Generally wall plates are in lengths of not less than 3 m.
To fix the joists and rafters down on to the wall plates they should be toe or skew nailed to the top plate like below.
Fixing joists to an existing wall.
These are typically 1 2 m long with a cranked end and are fixed to the wall plate at 2 m centres.
Where trussed rafter roofs are not subject to uplift a minimum of two 4 5mm x 100mm galvanized round wire nails skew nailed one on each side of the trussed rafter or truss clips fixed in accordance with the manufacturer s instructions are acceptable.
By way of metal flat roof joist hangers.
Wall plate for a flat porch roof.
As the mortar does not bond the wall plate to the wall steel straps are used to ensure that the roof structure remains secure.
Set out the ceiling joists onto the wall plates the ceiling joists are installed before the rafters as they will stop the roof spreading and also give something for you to lay planks across to stand on when building the roof.
Creating a fall on a flat roof.
The plate straps at least should be in your spec also yes to the half laps.
Fixings used to connect the roof structure to the wall plate should be specified according to the roof construction and exposure of the site.
Using timber wall plates.
A cavity closer should be provided to the top of the walls to prevent fire spread and to stop damp air entering the roof space.
Also for your wall plates fix them down about every 500mm with concrete screws and then every 1500 with plate straps.