These are only required if the wall has been partly rendered or pebbledashed (called harled in Scotland) creating a step in the surface. It is possible to bond several layers of packers on top of each other to build up thickness to the closest 5mm. For example if the render were 25mm thick you would buy one RP500 - 20 and one RP500 - 5.
Approximately half a tube is needed per Snorkel Vent - perhaps slightly more for a very rough wall.Please note that we do not advise you to use regular silicone, which is only a sealant. The modified silicone that we sell is a hi-tack adhesive as well as a sealant. It is ideal for bonding Snorkel Vent's to the wall.