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.
Fixing flat roof joists.
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.
This is a completion video of a flat roof framing.
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.
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.
While it is not entirely impossible to take on a diy roof joist repair replacing or repairing a damaged roof joist is a complicated procedure that requires precision.
Code of practice for permissible stress design materials and workmanship and bs 5268 7 2 1989 structural use of timber section 7 2.
When it comes to flat roof repair options simple patch repairs for flat roofs such as those on garages or extensions can cost a few hundred pounds.
This is a long video but it will show you how to do it.
However given the nature of flat roof repair costs it can sometimes make sense to replace the entire roof.
Rotten roof joists are a severe problem and commonly lead to underlying issues and damage to opposing structures.
And tie it into your hip roof.
Make sure you offset yo.
Repairing roof joists yourself.
How to remove repair and install a flat roof.
This is a completion video of a flat roof framing.