A half-round gutter is a rainwater gutter with a semicircular section, hung on brackets from the eaves or the fascia board. The Dutch name mastgoot recalls the days when such gutters were hollowed out of a split mast or softwood trunk; today they are made in zinc, copper, galvanised steel, aluminium or PVC.
Why semicircular?
For the same capacity, a round trough has a smaller wetted perimeter than a square box gutter. Water moves through it faster and less leaf litter settles. It is also a hanging gutter: it sits on the roof rather than in it. If it leaks, the water runs down the outside of the wall instead of into the roof deck — which is why the form is so common on houses, outbuildings and listed properties.
The front edge is normally rolled into a bead. That keeps the gutter in shape so a leaning ladder does not crush it.
Points to watch when fitting
- Bracket spacing. Set brackets roughly 500 to 800 mm apart, closer where snow loads are high. Spaced too far apart, the gutter sags
- Fall. Lay about 1 to 3 mm of fall per metre towards the outlet. With no fall, water stands in the gutter and speeds up corrosion
- Thermal movement. Zinc moves considerably with temperature. On long runs fit an expansion piece roughly every 8 to 12 metres and never solder a full-length run rigid
- Material pairing. Do not run copper above zinc: copper-laden runoff attacks zinc. Avoid direct contact between zinc and bare steel or wet oak as well
- Ladders. Lean the ladder against the wall or use a ridge hook rather than resting it on the gutter — a dented gutter no longer holds its fall
- Leaf protection. A leaf guard over the outlet prevents most blockages but must stay accessible for cleaning
A well-laid zinc gutter with adequate fall will normally last for decades; PVC versions distort more readily and suit small structures such as a shed or garden building.
Related terms
- Gutter
- Box gutter
- Gutter bracket
- Rainwater downpipe
- Zinc
- Fascia board
- Eaves
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