Loft Converion floor design

[glenlyn]

Hi

My house was build in 1898 and used to have a continuous load bearing wall running at mid span.

The original intention to create a new loft conversion floor was to run new joists in between the existing joists and bear them on the load bearing wall.

At some point in the 1960s the load bearing wall at ground level was demolished to create a larger room. The steel that has been inserted to support the remain wall at first floor level and ceiling joists is incapable of supporting a new loft floor.

My engineer has proposed the following:
plate stiffen the existing steel by welding or bolting, neither option is suitable as welding could be hazardous and there is not enough flange width to bolt
or
insert another beam beneath the existing beam. simple in theory but quite disruptive.

I am currently considering how to tackle the issue in the loft and divorce the new floor from this section of load bearing wall.

my current loft is 2300mm from top of existing joists to underside of rafter apex.

Taking into account insulation requirements etc, I have ascertained that I cannot afford to have anymore than a 170mm joist. Using this and considering all other regs this would result in a 2120mm floor to finished apex. this is also factoring in a 1" level tolerance for new joists to not rest on existing lath and plaster.

can i use 170mm steel flitch beams running in opposite direction from existing rafters to the suspend new 170mm joists running parallel to existing from joist hangers? If the flitch beams are any larger this would result in a step up in level?

 

[DSA]

What is the span of the proposed loft floor joist? This will determine the required depth (as well as width and grade), and your engineer will advise. If you are limiting to 170mm, you may end up with quite a wide joist, depending on span.

If you are considering the disruption of plating or installing a secondary beam below the existing (and conclude that neither are workable), why not needle, prop and replace the existing existing beam for a larger, designed section? Don't forget that the introduction of a secondary beam would likely reduce the headroom, which may be an issue for you. Alternatively you may be left with no option but to install additional steelwork as you suggest in the loft space to pick up the loft floor. Your architect/engineer should be able to advise on this.

The steel beams, presumed spanning between gables, would need to be designed by your engineer, along with any splices necessary to permit successful and safe installation. The beams can be deeper than the joists if they are located under a partition, for example (by matching bottom of steel and bottom of joist levels). You can support the joists using dropped joist hangers.

 

[ajrobb]

can i use 170mm steel flitch beams running in opposite direction from existing rafters to the suspend new 170mm joists running parallel to existing from joist hangers? If the flitch beams are any larger this would result in a step up in level?

As long as the beams run behind the attic walls, are you still restricted to 170mm for the beams?

I have another question. Is it possible to have steels/flitch beams running between gables above the ceiling joists and have the new floor joist hangers dropping down between the ceiling joists?

 

[Static]

If you are considering the disruption of plating or installing a secondary beam below the existing (and conclude that neither are workable), why not needle, prop and replace the existing existing beam for a larger, designed section?

As DSA said.. replace the old beam with a more suitable one..