Extra course of aerated block

Yes, it is generally possible on a small outbuilding like this, but it depends on the condition of the existing walls and how the new roof loads are transferred. With an outbuilding measuring approximately 2.1 m × 3.6 m and a lightweight timber pent roof finished with EPDM, the structural loads are relatively low. A single course of 100 mm aerated blocks laid on a 75 mm wall, leaving a 12.5 mm overhang each side (or even 25 mm to one side), is unlikely to cause structural issues provided: * The existing wall is sound, level and free from movement. * The blocks are fully bedded in mortar with no voids. * The timber wall plate bears centrally on the new blockwork so the roof load is transferred through the wall. * The overhang isn’t carrying any significant load by itself. Ideally, the wall plate should be positioned so the roof load sits over the centre of the original wall rather than over the overhanging edge. If you were building a new structural wall or supporting heavy masonry or floor loads, matching the wall thickness would be best practice. However, for a single additional course supporting a lightweight timber roof on a small utility outbuilding, many builders would be comfortable with this detail. If in doubt, or if the existing walls show any signs of cracking or instability, it would be worth asking your local Building Control officer for their view before proceeding.

Get someone who knows what they are doing

Yes it’s the way to do it .

75mm is best, cut them down with a wet saw.