A gravel guard roof outlet (Dutch “kiezelbak”) is a collection box used on a flat roof with a gravel ballast layer, at the point where the rainwater drains into the downpipe. The box collects the roof’s rainwater and directs it to the vertical outlet, while a toothed rim prevents gravel from washing into the downpipe and blocking it.
How it works
- The box sits embedded in the gravel layer around the drainage point
- A rim with teeth or slots lets water through but holds the gravel back
- Many models also have a leaf guard (grating) to keep out leaves and debris
- Materials: aluminium, plastic (PVC), zinc, stainless steel, copper or lead
Application
- Gravel-ballasted flat roofs — the classic place for such an outlet, at the roof edge or, on large roofs, within the roof surface
- Maintenance — a removable leaf guard makes cleaning easy
Points to note
- The need has decreased because many modern roof coverings no longer have to be weighted with gravel
- Keep the box and its teeth free of leaves and silt; a blocked outlet leaves water standing on the roof
- Always provide an emergency overflow, so that if it blocks the water can escape safely instead of loading the roof
Related terms
- Flat roof
- Ballast layer
- Leaf guard
- Rainwater drainage
- Roof gutter
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