How to stop spread if you lower the loft floor

I've got a Victorian terrace house with an original loft room. I'm intending splitting it into 2 flats as there's a big, ground floor extension and so it should work well. I'd really like to lower the loft floor as headroom isn't great up there, and will be even worse once I upgrade the insulation. However, when I considered doing something similar a few years ago I was told that the existing rafters tie the house together and that the weight of the roof could cause the front and rear walls to spread.

Ideally I'd like to lower the floor somewhere between 20 and 30cm, is this a possibility, and , if so, how could it be done? I know I'll need to get a structural engineer in, but I just want to try to get an idea of what's possible and what isn't, bearing in mind the solution has to involve ensuring there's no spread.

Put a steel ridge beam in with high level collars.

Many thanks for the reply

Sorry, I should have included that I think the party walls are only 4 inches thick so I'm not sure I can do any work that involves changes to them or beams put into them.

We had to do repair something similar.

In place of a ridge beam, our SE said to bolt angle iron to every other rather (about 1m up) then join to the floor joists/noggins (depending which way they run), these ran at about 45degree into the room. Then plywood over three joists top and bottom to make a box.

You loose some room space but stopped the spreading!

Sorry if not explain well, was quick and easy to do.

You may need to fit some kind of fabricated girder truss at each end to support the ridge beam or look at ways to connect the new lower floor joists to the existing rafter ends.

This is how we've done it occasionally.

Thanks for all the replies.

Tony

Do you mind me asking if that's your own solution or is it on the instructions of a structural engineer? It's exactly the sort of thing that would work in my loft as there's no dormer so the area under the eaves would be behind a dwarf wall anyway.