I am building the flat roof for my extension and have been specified 75 x 100mm tanalised for the timber wall plate. My local timber merchants doesn't stock this so can I use 50 x 100mm timber. the obvious answer would be to go to another timber merchants but I have already bought the 50x100mm. Also does the eaves details have to have a soffit or can I end the roof at the outter leaf blockwork edge and just have a facia board?