Can I move the horizontal joists/beams in the attic space?

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Ok

This is some DIYing which is stretching my maths/knowledge.
(hopefully I am describing this well).

So I'm converting my attic into a living space (yes aware of regs, etc.)
It is 1930's Semi with, all ceilings down (the house is a shell).

New Steels installed at the front and back, under the perlin's
The problem is, the steels are sitting under the existing beams/joists, but too low and I want to move them up by a few inches. This would mean chopping out all beams to allow.
Which I believe would lead to the roof spreading/slipping.

So the thoughts are .
The diagonal struts in the roof (yep getting technical terms now :) rest on the outside wall (which is higher by 1 brick on the inner skin, due to the angle).
The horizontal beams sit on the inner skin and are nailed to the diagonals.

Are the horizontals stopping the diagonals from spread side wards AND skidding down?

My plans are:
1) Move each horizontal up by a few feet, one at a a time (to stop the sideways spread). This will change weights and add more on to the purlins.
2) Jack up the steel beams 4 inches and use large padstones
3) Install the new joists
4) Where the joists come close to the wall/diagonals, cut triangle shapes out of hardboard to screw to the joists and the diagonals to both horizontal and diagonal spreading. (I can't attach the joists straight to the diagonals due to the steels being in the way).
I'm hoping that everything is done in a weekend (steels are already in place, just need jacking up) so the diagonals will not be without horizontal support for more than 48hrs.


So if you managed to follow this, what do you think?

If it doesn't make much sense I can try to draw something.
 
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The 'triangular pieces' are gussets, and you would not make them out of hardboard but plywood or OSB instead (plywood far too flimsy).

In principle what you are doing is ok, BUT, your second drawing seems to show the timber joists running into the steel beams. You will not be
able to get a good-enough fixing between the steel and the joists to ensure that the joists themselves are secure enough to resist any tendency to spread

If you can screw and glue plywood flooring connecting the joists both sides of the steels, that would give you a rigid diaphragm and should be ok.
 
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Hi tony

Thanks for the reply.
I was thinking of something like Marine Ply http://www.travisperkins.co.uk/c/timber-sheet-materials/plywood/893617 as I presumed that to be strong, but will look as OSB too.

I forgot to add that I have bolted some joists running into the steels web, which I will attach joist hangers to.

The main sticking point is the inital moving of the beam further up the truss.
Can imagine having a sqeaky bum moment during this.

I've taken off the top 4 rows of internal bricks, re-motar'd. Putting on a wall plate and setting wall ties every Mtr to hold this in place.
Then I'll try to use a truss clip to attach the diagonals to the wall plate for temporary holding whilst I do everything.
using something like this, but a but taller to catch the diagonal properly

http://www.google.co.uk/imgres?sa=X...bnh=182&tbnw=151&start=30&ndsp=45&tx=66&ty=54
(the wall plate on the exterior wall doesn't look so solid)
 

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