Vent tile placement

[pasty-man]

Since having the pitched roof on our cottage re felted (or rather breathable membrane) and tiled 3 years ago it has become apparent that there is now insufficient ventilation. (small areas of white mould on timbers and only a small air vent on 1 gable)
There is no overhang to install soffit vents in combo with ridge vents and I dont really want to go to the expense of engineering this so was looking just to do 2?low level/2 high level vent tiles on each side.
Does this sound fair enough? Can anyone guide me on placement ie how far between vents, how far from Gable, how far from top/ bottom.

 

[catlad]

You can get strips that sit on your facia's they are about a meter long and they are about £1 each with vents at ridge that should be enough.

 

[pasty-man]

Yeah, those over fascia vent strips. I know what you mean. I didn't really want to touch the fascia boards at the mo as they need replacing anyway and then the guttering will need doing as well ( it's all faded brown plastic). Saying that by the time I've got 8 vent tiles and a poorer result it's prob worth doing it your way.

 

[catlad]

Its an ideal time to do it all in one go, you can put felt support trays in while you are at it.