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Jeremy Kohlitz gravatar image

Have you looked at the manual "Roofwater Harvesting" by T.H Thomas and D.B. Martinson? It can be downloaded here:

There is a chapter on guttering and using galvanised sheet metal as a material for guttering is discussed on page 109. It states the following:

"Galvanised iron roof sheeting can be cut and bent unto a ‘U’ to form a gutter. A more popular shape, though not one to make best use of the metal area, is a cut-strip folded to give a trapezoidal gutter with a vertical back face to fit against the fascia board and an outwards-angled front face intended to intercept even the most intense run-off.

Unfortunately, the cutting of both curved and of folded gutters leaves a dangerously sharp edge, which is also prone to rusting. Normal tinsmith practice would be to fold over such edges and this is widely done. However, roofing-grade GI sheeting is too hard for this operation and softer metal (milder steel) sheet must be used, which is normally more expensive. Metal gutters made in this way are not easy to support or to join. The gutter-to-gutter joint often leaks unless sealed with some sort of mastic (e.g. bitumen) and taped firmly together (e.g. with rubber strips cut from car or lorry inner tubes). Soldering is another widely used process to join GI gutter lengths, attach end plates or, more commonly, attach metal downpipes to gutters. Such rigid gutter-to-downpipe connections are a source of weakness and one often sees gutters badly twisted because the attached downpipe has moved."