Lower joists and or thinner insulation for more head room?

[TKH]

I have got planning for a roof lift / loft conversion on a small 2 bed bungalow but the planning office is being rather stingy on roof line height. I am not allowed to go as high as I originally would have liked and in so I am looking to eke out some more head room inside.

The floor joists we were planning on using are 57x170mm C16 Softwood at 400mm centers. Are there any thinner options I could look at that would not be crazy money? I have seen talk of laminated joists but am struggling to find more information on such small sizes.

The roof insulation is 25mm air space, then 100mm recticel urethane boards between rafters, then 50mm urethane under rafters. I did look at aerogel under rafter but its getting extremely expensive very quickly. Is there any other options to save a cm or two with out spening quite so much?

Any advice would be most appreciated.

 

[tony1851]

Yes, for the floor joists, you can have shallower joists if you double them up, or space them closer - don't be tied by the TRADA tables.
What size you can get away with depends on the span - deflection/'bounce' of the joists will be the critical aspect, but often it's possible to save at least 25mm.

For the roof, consider thinner insulation than you would normally have (the regs say it must be 'reasonable' and if lack of decent headroom is an issue.....?).

PS; some inspectors are more reasonable on insulation than others - i recently did a job where they allowed 25 between the rafters and IIRC 40 below - but they were private inspectors. LABC bods are more likely to demand the full gamut.