Installing Loft Floor

[TherealJB21]

Hi,
New to the forum after spending many hours reading up on other people loft projects. We have recently moved into a 1950s semi with a decent sized loft that i want to make into a decent storage area.
The current loft floor joists are just the 2x3 ceiling joists holding up a plasterboard ceiling. Obviously these aren't going to be strong enough (the joists above the front room span over 4 meters) My initial plan was to run thicker joists sister-ed to the ceiling joists from the front and rear walls to the load bearing wall in between the front and back room, however I'm not able to get thick enough joists under the purlins next to the walls (please see pics).
I've had 3 thoughts on how to do this but am not sure if these are viable options so any advice would be greatly appreciated.
1 - can i attache hangars to the purlins and run joists to the central load bearing wall?
2 - could i bolt a length of wood to the party wall/chimney and attached hangars to this running perpendicular to the ceiling joists and sit these on a length sat on the other internal wall (im not sure if this is load bearing but assume so)
3 - Add 2x3 joists on top of the existing ceiling joists 3-4 meters in length with the load bearing wall the central point of these, then sister joists to these for added strength.
Thanks!

 

[Doggit]

No,No, and um, no. You want to set the new joists in between the existing ones, and keep them above the existing plasterboard. What is the size of the piece of wood down at the junction of the joists and the rafters (brown in picture one), as this is the point you'd place the joist hangers and set them to the central wall, but this needs to be a 4x3 set vertically, and nothing less than that.

 

[TherealJB21]

Hi Doggit, thanks for the reply. That's what i meant for option 1 - I thought both the long wooden beams were purlins, I'm still learning though!

The wood at the junction is 3x9 and spans just shy of 3.5 meters. I have attached pics of how they are held in place.

I have realised there isn't one of these beams at the opposite end, any suggestions of how i would combat this? Would it work to hang the joists on hangers from the wood at the junction over the central load bearing wall extending about a meter over this?

 

[Doggit]

I can't see what what purpose that 3x9 is doing there, but it'll be quite a way forward, whereas a 4x3 would sit further back down into the corner, and gain more support from the wall plate. Under normal circumstances, the BCO would tell you if he was happy for you to use it though, but I doubt if you've got one on this job. You can put a 4x3 (setting the 4" vertically) at the other end of the loft, and then fix the joist hangers to that, and then take them to the central wall. I think you'll need a 2x9 C24 at 450mm spacings for 4.5m If you were doing a proper loft conversion, but I'm not sure if I'd want to go less than 2x6 even for storage. If you sistered the old and new joist together, or added extra onto the old ones (although I do recommend glue and screw at times) then as you have a plasterboard ceiling below, then you risk the plastered ceiling cracking as the floor above flexes

 

[TherealJB21]

Thanks Again Doggit, i appreciate your help. You think 6x2 would suffice - the actual span from the 3x9 to the central wall is more like 4.2 meters. if these joists were joined by noggins and attached to the rafters in the same way the ceiling joists currently are?

 

[Doggit]

They won't attach to the rafters because they'll be hanging off the beam you put in, but they'll want noggins to stop them twisting, and provide rigidity. As long as it's just for storage, then 6x2 should be okay, but somehow, I feel 7x2 would be a bit more sensible. Work out the costs, and go for the best you can afford. C24 is structurally stronger than C16 grade. What's the distance between the joists.

 

[TherealJB21]

Distance between the joists is 400mm. I’ve been up and re-measured the distance from the 3x9 to the central wall and the span is 3.6 metres.

I’m not sure how I would get a 4x3 onto the opposite wall plate as the rafters seems to cane down to close to it to fit anything in. Any suggestions or alternatives for the other side? Thanks

 

[Doggit]

You just wedge the 4x3 back as far as it will go; it's as simple as that. The 3x9 obviously sits further out from the wall plate than a 4x3 would, but it'll never sit over the wall plate itself.

 

[TherealJB21]

What does the 4x3 rest on or is attached to?

 

[Doggit]

It rests on the ceiling joists, but is close enough to the wall plate to provide adequate support. You nail/screw it the the ceiling joists, and the rafters.