I need to install an air brick under the front entrance steps to ventilate the space under the stairs. The wall is approx 270mm thick including existing render etc. Will I need to install 2 or 3 air bricks (in the place of the old bricks) to maintain the thickness of the wall (approx 270mm) or will 1 air brick on the outside face suffice? Will the wall be structurally sound enough or will the old bricks above the air brick space need support below for the full width of the wall? Thanks for your help.