Minimum depth for floor in loft conversion

[willinthehill]

I'm looking to do a loft conversion with very limited head height so I need to reclaim every bit of space I can. What are the thinnest alternatives to regular floor joists that could help reclaim a bit of height?

I'm already looking at lowering the ceiling in the floor below to gain some space but just wondering what other tricks are available. Raising the roof height is not an option unfortunately as it's a listed building.

 

[freddiemercurystwin]

Adding some extra steels and reducing the spans and consequent depth of the timber joists. Depending on the listing you may have problems lowering the ceiling etc. Hope you're talking with your Conservation Department?

 

[big-all]

are you remembering the space required for insulation overhead