joist thickness for loft conversion

[DavidWP]

What is going on here?
If you haven't got the knowledge do this job employ somebody who has.

An estate agent friend of mine tells me the biggest cause of altered/extended house sales falling through is non-acceptance of unsuitable/uncertified work.

 

[loftwood]

Used a box girder on a job in Manchester. The internal walls were made of crap, so a neighbour, an engineer drew up the design. Used 9x3 spliced glued & bolted together as a base plate let into both gables, a 6x3 as a top plate, with 4x3s as verticals & diagonals. The structure resembles a "Bailey Bridge" which then has ply glued & screwed to it both sides. The joists are then attached using hangers.
This is an immensely strong structure which can be pre-fabricated below then assembled in the loft. If you doubt the efficacy of this, then try to bend a sheet of ply edge on, it's virtually impossible. Beats steel -has less deflection easier to get into a confined space & creates a wall either end.
(The span was over 20')

Hi how was the box beam constructed,i intend using the same method for a span of 4.500,height of beam 1.200.Where was the 9x3 spliced together was it bolted?

Is there anywhere i can look at some designs, of box beams?


Thanks

Loftwood

 

[lostinfens]

Basically I just made it up in the back yard in kit form & then put it together in situ. The uprights & diagonals were just "cut in" to the top & bottom plates with the diagonals tight against the uprights. I really don't remember if or how they were fixed, the cutting in & ply would do that anyway ( this was about 20 yrs ago). If you can envisage the top braced against the roof & the bottom against the wall it's sitting on, there is little room for movement.
The 9x3 base was spliced halfway, glued & screwed together. It was fairly solid on its own. I think that its best to leave a little room beneath this to jack up with slate or steel to bring the whole thing together before fixing the ply. As for design, I don't have a drawing now, but if you imagine using the height between the top & bottom as the short sides of a rectangle ( not much more than a square), then fitting diagonals -top to bottom, bottom to top from one end to other. Obviously lay out dry to even out before deciding what goes where.
I hope this makes some sense, I am a bricklayer after all and am probably explaining it badly, but it really isn't that difficult to do with a little planning, patience & a decent saw & it really does work.