What is a rainwater offset (swan neck)?
A rainwater offset, often called a swan neck, is an angled connecting piece in the rainwater system between the roof and the downpipe. It bridges the gap between where water leaves the roof, such as a gutter outlet, a gravel guard outlet or a spout through the parapet, and the downpipe fixed to the wall. Without this offset, the downpipe would have to hang away from the wall or run through the eaves overhang. In Dutch it is called a stadsuitloop.
Types
Offsets come in PVC, zinc and aluminium, round or square, and angled to the left or right. Some versions have a rubber or EPDM flange that is bonded into the roof covering or parapet to make the connection watertight. For siphonic systems, in which the pipe can run completely full, there are versions with extra seals.
An offset changes the direction of flow twice. Every bend slows the water slightly and is a place where leaves and dirt can catch.
Application
- Choose the right diameter. Use the same size as the downpipe so there is no restriction.
- Keep the slope generous. The steeper the angled section, the better water and debris flow through. A nearly horizontal run blocks more quickly.
- Fit in the direction of flow. Always push the upper piece into the socket of the lower piece, so water runs over the joint instead of leaking out.
- Support with brackets. Fix a pipe clip just below the offset so the weight of the downpipe does not hang on the roof connection.
- Fit a leaf guard in the gutter or outlet to prevent blockages in the bends, and check it after autumn.
Common mistake: not bonding the flange properly into the roof covering. Water then runs underneath the covering, through the parapet and inside, often only noticed as a damp patch on the ceiling.
Related terms
- Rainwater drainage
- Gutter
- Box gutter
- Leaf guard
- Emergency roof drain
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