joists

Totally depends on the building. Can be anything from 2.0" (50mm) to 100mm (4"). Most are around 2.5" (65mm) Either get above the ceiling if possible and have a look/measure The next method is to knock/tap against the ceiling with a knuckle. If it sounds hollow there is no joist but if there is a solid sound you have found the joist. You will quickly see where the joist is form the sound. Or if you are prepared to fill and re-paint then get a very fine drill and drill outwards, 1/4" (5mm) at a time, from where you think the center of the joist is. The you will see if you have a plasterboard ceiling that the drill will meet resistance when it hits wood (the joist) and wood shavings will come out of the drill or it will go through the plasterboard quickly if there is no wood. If all else fails get a stud finder - that will electronically find the joist. Normally when it beeps that is the edge of the joist which you can mark lightly with a pencil. Hope this helps X-Pole Tech Team