how many vent tiles do I need ?

[diydad888]

Help!

Up here in Edinburgh Scotland, doing a couple of lean to single storey extensions in traditional build, sizes of extensions are 5mlong and 3 m wide to rear an d 2.45 m long by 2m wide, I am using plywood sarking with breatheable roofing membrane, redland 50 double roman tiles, I have bought 5 plastic double roman style vent tiles.
My question is..how many of these vent tiles do I need to use? (vent soffit board fitted to both eaves)
My plans are unclear and a crap job but they still passed it so I am stuck for info.
I bought 5 tiles but looking around the area most people use less on bigger extensions.

Thanks,
Alex

 

[Alastairreid]

3 at high level on the larger lean- to, 2 on the smaller in the same place, both in conjunction with vented soffits.

Another option for another time. http://www.cemex.co.uk/bp/pdf/Russell_Abutment_Ventilation.pdf