May 20, 2011
How to Waterproof / Tips on Waterproofing
Ever wondered what is the right way to waterproof? Well the folks at Master Buildings Association explain how.
Flashings-The Forgotten Building Component
Where did you learn about flashings and damp proof course installation ?. It probably wasn’t from a book or a publication because there isn’t any to my knowledge. The BCA has some details – but they are sketchy and in part wrong, and AS 3700-2001 Masonry Structures covers the issue in six paragraphs. So it must have been like me, a crusty site foreman telling you this is the way you do it and don’t forget !.
For some reason bad flashing practices are become more prevalent in the building industry and the cost of rectification can be as high as $450.00 per meter to rectify.
Architects and designers have tried to keep flashings out of sight by keeping them back 10mm from the outside face of the walls and omitting weepholes in favour of the ‘flush look’. The problem with this detailing is that it results in rising damp and inadequate drainage and ventilation of the wall cavity.
Modern balconies now have the render and no skirting tiles. So there is nothing to conceal the overlap of the cavity flashing and turn-up waterproof membrane on the balcony.
Clause 18.104.22.168 of AS 4654.2-2009 goes a little way in resolving the anomalies with the overflashing of membranes by requiring overflashing that overlaps the upstand with a minimum gap of 10mm and a pressure seal with an applicable sealant, a reglet or an overflashing constructed of a liquid applied membrane.
From a membrane application stand point, the last thing that a manufacturer of a membrane will permit is water dropping off a flashing behind a vertical termination of a membrane. This detailing causes water to flow behind the membrane causing delamination of the membrane from the substrate.
Flashing materials have changed. Now you can get ribbed plastic damp proof material. But you try to box the end of it at the junction of a door or windows, or even worse try to seal the lap joint with silicone- it simply can’t be done.
Go to the hardware store and ask for some sealant to seal the lap joint in a bitumen coated flashing and you’ll probably be given an acrylic based sealant that will do everything but effectively seal the flashing. The old bitumen paste is yesterdays technology, but surprisingly it works. Some of the larger sealant manufacturers are now selling bitumen based sealants, but they are not readily available and the product names are not well known or promoted.
There are some simple steps a Builder can take to ensure that flashings are installed correctly and are available for correct overflashing of membranes:
• Install all flashings so that they project a minimum of 50mm out the face of the wall. It is easy to trim them off later with a knife if you don’t need them, but extending them latter means removing bricks.
• Where you have to box the end of a flashing use a flashing material that will hold its folded shape.
• Where a cavity flashing meets a door reveal, leave the cavity flashing protruding 50mm into the opening so it can be folded up.
• Seal all laps in flashings and damp proof courses with bitumen modified sealant. Call the larger manufacturer’s of sealant and ask for bitumen sealant. Test the sealant by applying it to a piece of flashing and then try to pull it apart – if it delaminates easily or does not seal try another sealant.
• Water test all your flashings before the internal linings are fixed or before rendering the internal walls.
• Leave bricks out of the wall at 1200cts for cleaning the cavities. This is the where the weepholes have to go anyway so they can be installed just before rendering.
• Get the apprentice to clean the cavities with a hose at smoko, lunch and after work.
• Do not close the top of a cavity until you have sighted the cavity and the flashings.
• Don’t let following trades pierce the flashings with conduits, pipes and cables.
Jack Horsfall from the Office of Fair Trading in conjunction with members of the MBA Waterproofing Technical Committee are in the final stages of putting together a “Industry Practice Guide” on the “Installation of damp proof courses and flashings”. This will end the problem of the lack of literature and provide a useful how to guide for the building industry.
Bloggers who are looking for more information about the niche of suspended ceiling accessories, make sure to check out the web page which is quoted right in this passage.