Solid walls may be built of brick, dense block or thermal (lightweight) block. Identify which type you have by drilling a test hole and examining the drill dust:
Reddish or yellowish dust shows you have brick walls
Dark grey dust (and slow drill progress) shows you have concrete block walls
Light grey dust (and easy drill progress) shows you have thermal block walls
Sandy dust means you have hit a mortar joint. Drill a second hole about 50mm away from the first and 45° above it to confirm the wall type
