Replacing a wallplate with a RSJ during an extension

[tpb1975]

I appreciate this is a question that comes up from time to time, just wanted to ask it again in the context of my house...

My house has the gable ends at the front and back, with wall plates at the side. I'm having an extension built over the attached garage, that will run the full length of the house. This will create a 'T' shaped roof, with the majority of the roof space within the original property.

I was planning/hoping on knocking through a room into the extension over the garage but wanted a flat ceiling. This would mean the RSJ/Steel replacing the wall plate for about 40% of the length of the house.

The builder suggests that most rafters will be cut out, therefore it will mostly be joists connecting to the RSJ. Any major problems with replacing the wall plate in a roof like this?

The builder also said that the joists offer little, if any lateral support. So he's just going to sit the joists within the web of the beam and keep them in place with noggins. I assumed that it was all about triangles with a roof and not attaching one side just made it three sticks of wood As it's T shaped roof, the rafters on that side get cut out so the load appears to spread to the wall plates on the extension. Any thoughts on this? Would joist hangers offer enough strength against the lateral pull? The existing roof has a large purlin on both sides, I guess this helps prevent the rafters from moving/dropping.

 

[noseall]

STRUCTURAL ENGINEER ANYONE?

Why are the rafters on that side being cut out?

You would normally extend the roof via a lay roof whereby the new roof sits on the existing roof.

If this is the case then the rafters and the joists need to remain. It sounds as though your flat ceiling idea needs a re-think.

Don't rely solely on your builder for advice.

 

[tpb1975]

Yeah when I got the SE to size the beam I said I'd like it in the loft. But the builder and the SE fell out with each other over something else and won't talk to each other (seriously) and this particular detail is proving to be a bit of a pain.

The SE said that ideally it should be placed below the wall plate and the wall plate shot bolted on. But if it's placed above it needs to be designed right or you can have major problems. Not really much more than that to work off. Other than mechanically attach the joists to the beam.

Looking at pictures of T shaped roofs, it looks like the rafters cut off at the boards that make the valleys. So I don't see how many rafters will make it down to the RSJ.

I must admit I had assumed that the new roof would overlay the existing and they wouldn't really be looking to cut anything out. But I guess you need to form the valley.