Boarding loft with planks

[janehowell]

I have a small loft area above a first floor extension, it's only about 8 square metres (just above a kitchen) in small terraced house.
I want to put boards on the rafters but the access hatch is very small - 30cm wide and 90cm high.
So I can't get large loft boards into it.
I am planning to use 100mm wide timber to form a floor. Main reason for creating a floor is so I can put insulation (PIR) in between rafters.
Question: Will 12mm thick planks be sufficient, or should I go for 18mm?
I have read many threads on MDF vs Plywood loft boards, but nothing on 100mm pieces of timber.
Thanks for any advice!

 

[Tigercubrider]

Just buy loft boards at B&Q.
The only thing that you may need to check is the rafter spacing .
My 50’s house as ceiling joists that don’t coincide with the board lengths.

Also, screw the boards down, don’t nail