Scorpion Roof
Forty years in the building industry produced enough negative evidence for me to label box gutters as being "not a good idea". Recently however I was asked to resolve a seemingly impossible situation of an unfortunately shaped building with an internal box gutter that clogged up with leaf mulch and flooded the ceiling space on a regular basis regardless of what the owners did to
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prevent this happening. In finding a solution I was alerted to the fact that directing the water from a roof to a single point is not such a bad idea after all.
A well designed well installed central box gutter allows storm water to to be directed to a single collection point thereby doing away with eaves gutters entirely. The principles are shown below in (fig 1) what I describe as a "Scorpion Roof".
In its simplest form it could be described as a flat roof with a unique looking central box gutter. At the other extreme, shown in (fig 2) in custom orb roofing, as a collection of three roofs, two with five degree pitch draining onto a central roof section with a seven degree pitch. The common and important element is that water is falling away from vertical surfaces.
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It is a simple idea that focusses on a more elaborate collection point, one that is designed specifically for the building. An elongated rain head with a functional leaf guard that can shed debris is one example. This could be made of a durable material, eliminating almost all ongoing maintenance and effectively reducing the cost of the gutter system.
Furthermore I would also do away with the externally mounted down pipe. Providing access to service these down pipes is a complex and expensive exercise on tall buildings. For example if this were the roof of a five storey residential apartment building, installing water tanks on each floor directly below the point of stormwater collection would also provide a storm water flushing system for the toilets throughout the building and water for the garden. The internal PVC drainage would be away from UV damage and serviceable from within the build.
This is just one solution with specific reduced maintenance advantages for a multi storey apartment building which is rectangular in shape. If one were building in a climate where it snowed, then a high pitched simply shaped roof would definitely be the way to go. If one was designing for an environment subject to excessively high winds, then there would be the destructive wind factor to consider.
Brick buildings often have a parapet wall running around the roof which effectively provides a solution to three issues. It breaks the wind effect (air flow) over the roof, it provides an effective balustrade for the safety of workmen that might be on the roof and it hides the visual impact of the roof. There are of course problems associated with parapet walls. They provide a vertical surface within the roof structure that may promote the accumulation of debris should there be a fall in the roof towards this parapet wall.
(fig 3) shows a scorpion roof with a parapet wall and the central section draining onto a small flat absorption roof covered with membrane, drainage cell, filter fabric and pebbles. Under the drainage cell there are four flat grated roof outlets with clamping rings for the membrane. To meet the Australian Standard AS 3500.3.2 there needs to be an over flow scupper on one of the three sides of the absorption roof. I recommend that these be no narrower than 250mm, be open at the top and 70mm deep, set 70mm above the roof drainage outlets and 100mm below the top of the membrane turn up. The scupper should extend 100mm clear of the wall and have a turn-down to prevent water tracking back along the underside. The scupper is there in case all four drainage points fail.
(fig 4) shows another variation on the scorpion and absorption roof.
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