New loft floor joists - a bit / lot confused!

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Hi, Looking for a little help....

I'm in the middle of converting my loft and a lot of the hard work is done. Stairs and windows are in and extending my rafters tomorrow to accomodate 100mm kingspan.

My problem is the floor. It's a huge loft (the part I will be using as a room is 6mtrs x 5mtrs) but only has ceiling joists fitted. My house is a semi with a pitched roof and the current joists run from gutter to gutter (or front to back) but i think the new joists need to go the other way, wall to wall.

The reason for this is that I can only see one way of hanging the floor and thats from hangers attached to the walls (meaning the new floor will criss cross the ceiling joists, and have to be fitted higher to clear them). Does anyone have any other ideas for hanging the joists so they can go the same way?

What size joist will I need to span 5 metres? In the centre of the room there is the top of a brick wall which would be ideal for resting the joists on but it runs the wrong way (the existing ceiling joists sit on it)!!! It's a bit frustrating because i'm sure there is a simple solution.

I want to use the smallest joists possible (to save height) and still have the quickest/ easiest installation. Am i asking too much??

Thanks in advance for your help.
 
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This wall in the middle. Whats under it? and then whats under that, and whats under that?

What have you done with the roof supports/struts/purlins so far?
 
the wall in the middle is an internal bearing wall that runs all the way down. It ends at floor level in the loft (ie no courses go up into the loft)

I haven't done anything with the roof supports struts or purlings and shouldn't need to?
 
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I am struggling to figure out the best way to attack it. I don't really want to start introducing steel into the roof and was hoping to figure a timber solution if possible. I amy just have to use hangers to span between the two walls (about 4.2 metres) Do you think i would get away with 7 inch joists at that span?
 
Your going to need 8"x3" c16 timber at least for that sort of span with no or hardly any center support
 
I suspect you aren't doing an official conversion then? One with plans, engineers calcs, visits from the council building control?

If you want to continue with this, and are able to carry out your mickey mouse conversion without removing any components of the roof structure...

I would suggest that you fit new larger joists in between the current joists, resting on the outer wall, and again on the centre wall, doing this and using 8x2's at 400mm centres, you should be able to support a basic floor. You would have to raise them up slightly to clear the ceiling, use mortar/slate packing under the ends so that they are raised about 15-20mm from the bottom of the current ceiling joists.

You may encounter other problems on the way though ;)



binders, roof pitch at eaves, stairwell in the way etc
 

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