it can take some time to clear out as the smoke builds up as the smoke bomb burns away , i think the main things is watching the air flow smoke escapes to see if any improvements can be made , when i last did my loft neighbours thought it was on fire lol , once theres no visable smoke leaving the loft re enter , there should be no smoke in the loft , just a nice smell i put my smoke bomb on top of an old cooker grill tray so no damage can be done