Purlin to joist 2x1's - name and purpose?

[Dereekoo]

Yeah that's why I referenced it, thanks for the link - I've saved the sketch tony1851 posted. If I'm honest I'm quite happy to do that myself, I just need to clear it all out first and take up the old insulation to see what I'm dealing with in full. Not looking forward to that bit as it's like a Saharan sandstorm up there as a chimney stack was taken out years ago and everything you can imagine is still circling around up there. Henry the vacuum to the rescue!

Thanks again for the help. Might post back when it's done...

Tony's method is as good a monolithic 7 x 2 joist as you are going to get with out actually using a new joist, you will recall from your youtube video that the neutral axis is in the centre of the beam meaning no stresses and whilst your joint is not in the exact centre it is not far out and friction/compression stresses in that area will be small compared with the outer faces.
Running the calcs (hopefully with no mistakes) in theory with a 7 x 2 beam you can actually load the beams so the floor is imposing a load of 150kg/m2 (as opposed to just the ceiling load of 10kg/m2) and the deflection would be in line with what it is now i.e.<1mm but I would suggest improving the single nail fixing of the hangers

 

[Dereekoo]

Tony's method is as good a monolithic 7 x 2 joist as you are going to get with out actually using a new joist, you will recall from your youtube video that the neutral axis is in the centre of the beam meaning no stresses and whilst your joint is not in the exact centre it is not far out and friction/compression stresses in that area will be small compared with the outer faces.
Running the calcs (hopefully with no mistakes) in theory with a 7 x 2 beam you can actually load the beams so the floor is imposing a load of 150kg/m2 (as opposed to just the ceiling load of 10kg/m2) and the deflection would be in line with what it is now i.e.<1mm but I would suggest improving the single nail fixing of the hangers

STOP Ignore the above I cannot see the wood for the trees, Whilst my analysis is OK for the actual joist themselves I have omitted to take account of the effect the extra loads acting on the hangers would impart on the purlins/roof. Whilst a lone ceiling gives a downward force on each hanger of approx 20kg (assuming a hanger every 2nd joist) a floor load of 150kg would give a force of approx 300kg i.e. 15 times more force than present. Whist I would not expect you to be contemplating a new load to be anywhere near 150kg/m2 the effect of any extra loads to be taken by the hangers/purlins needs some consideration.

 

[Twilo]

This is excellent, thank you both so much. Pre-drilling the holes and using coach bolts is a great idea thanks @RandomGrinch, I'm a stickler for precision and this will be much easier in good light and space. I have a relative with a garage full of woodworking toys so I can use his pillar drill for this.

@Dereekoo, good suggestion on the hangers, thanks.

I've almost finished a complete kitchen renovation (tough job on your own with only evenings and weekends, especially with all the surprises that come with a Victorian terrace), just wall tiles to complete now. I plan a little rest then will likely crack on with this next project in the coming months. I'm in Yorkshire so still plenty of freezing cold weather to get through yet!

Looking forward to this one, I'll be sure to let you know how I get on, no matter how long it takes me to get around to it

Thanks again

 

[Twilo]

STOP Ignore the above I cannot see the wood for the trees, Whilst my analysis is OK for the actual joist themselves I have omitted to take account of the effect the extra loads acting on the hangers would impart on the purlins/roof. Whilst a lone ceiling gives a downward force on each hanger of approx 20kg (assuming a hanger every 2nd joist) a floor load of 150kg would give a force of approx 300kg i.e. 15 times more force than present. Whist I would not expect you to be contemplating a new load to be anywhere near 150kg/m2 the effect of any extra loads to be taken by the hangers/purlins needs some consideration.

Oops missed this before I replied. Thanks for this, it's definitely prudent to take all of this into consideration. I'm not planning on storing much weight, it will be things like empty suitcases, some camping and fishing gear etc. Most will be stored around the supporting wall and closer to the eaves and spread out as equally as possible.