The discharge
Water leaves through a 125 mm pipe in the gable bay, set against the inside face of the column line at the low end of the gutter. Keeping the downpipe inside the column line puts it out of the path of machinery working the perimeter and keeps the outside face of the gable clean.
Rain and condensate share the same route. Both arrive in the same gutter, and both are worth keeping.
Condensation
In certain weather conditions condensate will form on the roof beam, the ridge, the gutter and other cold surfaces, and it will run. That is not a defect; it is what happens when a warm, saturated interior meets a cold single-glazed roof on a still night.
It matters because dripping water on a crop is a disease vector — free water on leaf tissue is what botrytis and downy mildew need. The whole glazing detail is built around directing that condensate: the PVC seat under each pane forms a channel, the roof bar leads water down to the gutter, and the gutter takes it to the downpipe. A roof that does not manage its own condensate is a roof that irrigates the crop with pathogens on a schedule set by the weather.
-
Managed The PVC seat under each pane forms a channel; the roof bar leads the water down to the gutter, and the gutter to the downpipe.
-
Unmanaged Water that drips instead of running is a disease vector: free water on leaf tissue is what botrytis and downy mildew need.
The glazing detail decides whether the water that forms on the inside of the glass reaches the gutter or the leaves.
Storage and reuse
The downpipe is the first element of the water system, not the last. Rainwater harvesting and storage, UV disinfection, drain-water recovery and recirculation, filtration and desalination where the source water needs it, are all designed on the same project as the roof that feeds them.
Sizing the storage is a climate question rather than a plumbing one, and it is answered from the thirty-year rainfall record for the site: the amount of precipitation, snow and rain, over a thirty-year period, is one of the studies carried out before the structure is laid out at all.
From roof to root
Collecting the water is the easy half. What decides whether it is usable is what happens between the tank and the dripper, and on an irrigation system running a whole house it is not optional.
-
From roof to root Storage is sized to the longest dry run in the record, not the annual rainfall. Filtering protects the drippers, UV makes recirculation safe, and the blend with a second source is set at design stage.
Each stage is sized from the site's own record and the crop's demand — and the blend ratio is set at design stage, not decided daily.
Storage sizes to the gap, not to the rainfall. The number that matters is not annual rainfall; it is the longest dry run the thirty-year record shows, set against the crop's daily demand. A reservoir sized on an annual average empties in the season it was needed. The record is read for the worst gap, not the mean.
Filtration comes before anything else. Roof water carries dust, organic debris and whatever the wind delivered, and drip irrigation does not tolerate particles — a blocked emitter is an unwatered plant that nobody notices until the row behind it is visibly behind.
Disinfection is what makes recirculation safe. Any system that returns drain water to the crop returns whatever pathogens are in it, and root diseases move through a recirculating system faster than by any other route in a greenhouse. UV disinfection is the usual answer: it uses no chemicals, leaves no residue in the water, and treats continuously in line.
Blending, because rainwater is almost too pure. Rainwater has an electrical conductivity close to zero, which is exactly why it is valuable — there is no sodium and no bicarbonate to correct before it reaches the crop. A fertigation system dissolves nutrients into water to a target EC, and starting from zero is not a problem; relying on rainwater alone and running out mid-season is. Most houses blend collected rainwater with a treated second source, and the ratio is a design decision, not a daily one.
Desalination where the second source needs it. Where the alternative supply is well water with high sodium, treating it is usually cheaper over twenty years than living with the yield penalty sodium imposes.
In arid regions — the Gulf, central Iran, much of Iraq — this is the argument for the whole roof. A hectare of glass is a hectare of catchment producing the best water the grower will ever have, and where water rather than land or capital is the binding constraint, that is often what makes the project viable at all.