Dull one - how best improve the ventilation/airflow in the garage?

[TheGoodGoodOldDays]

We have a detached, brick built garage which has recently been re-roofed (flat roof) rendered to match the house was fugly before, new UPVC side door fitted and a new sectional garage door fitted.

All looks wayyy better than it was before, and it actually watertight. However, it is also a bit too airtight. Very little airflow (garage door is a electronic one, with puffy seal at the bottom and actually fits the opening so there are no gaps). There is a teeny trickle vent in the side door and a couple of high up small vents in the soffit.

Just not enough airflow though. The small of petrol from the mowers hangs around - if the garage doors are not opened daily it just lingers.

How best should we get more airflowing?

an airbrick on two or more opposite sides would be good. Or you can get a Core Drill from a tool hire shop, as used for extractor fan ducts, and put a grill on (no need for a fan) so there is airflow. Examine your lawnmower for any signs of petrol leaks and check your fire insurance. The petrol evaporates but leaves a yellow or orange stain. Where it drips will look strangely clean as it washes away grease and dirt. Turn off the petrol tap just before you finish running the engine so the carburettor is empty when you put it away.

Thank you. I did thing the big extractor hole thing would probably be a good idea - big enough to get some proper flow going. Would you put them high up or low down the wall?

Oh and no leaks (have checked) though I did stupidly dribble some when filling it up the other day - I have now learnt my lesson and fill up outside

usually ventilators are put near the ceiling, because water vapour rises so this helps take it out.

Petrol vapour is heavier than air and pools on the floor.

If you hire a core drill, or get someone in, you might try an upper and a lower hole on each side, the hire cost will be the same as you will do it in an afternoon, and the labour will not be greatly more.

Be sure you get a cowl vent and take the flap out, this will prevent rain blowing in. On the inside you can have a plastic hit and miss vent, it has a fine insect grille.

Holes on opposite sides give airflow. The core is about 110mm and you can get plastic duct to fit, smudge a bit of mortar round it. Duct will prevent water soaking into the bricks if there is rain or condensation. Slope it slightly down outwards.