Phage0070 t1_jdew9hl wrote
The idea isn't that it is going to somehow get more rain to fall on the roof than if it was evenly sloped, which as you rightly guessed would be impossible.
Instead what the steps are intended to do is slow down the flow of the rain as it runs off the roof so that it can be caught by the gutters and stored in a tank under the house. If the roof was just a straight slope then the water would hit the gutter going quickly and some portion of it could splash over. Instead by increasing the surface area while not changing the overall slope the friction of the water flowing is increased and its speed reduced.
Onholidaybymistake22 OP t1_jdf9lmp wrote
But surely then they’d save resources by just making wider gutters? I suppose this is presuming that the Bermudans have calculated the perfect way to capture rain with a minimal use of resources instead of going off tradition…
MidnightAdventurer t1_jdfgo8o wrote
It's not just larger gutters, its all the pipes to the tanks and even then, there's a limit to how fast water will flow through the pipes for a given level difference between the gutter and the tank. By slowing the water down earlier, the peak flow rate is reduced.
Also, from a quick read, the traditional design design is made of stone so increasing the gutter size may not be as easy. I suspect the shape is also partly a result of the construction technique and as with many local styles, once they become characteristic of the area, they tend to get carried on just out of tradition
Toger t1_jdfkrji wrote
All the rain hitting the roof has to drain off it at the same rate it is arriving though, since the roof isn't a deep pool. Hence all the piping etc has to handle the full volume of rain -- the ridges aren't holding the water back except to absorb some of its kinetic energy.
MrShotson t1_jdfvxg7 wrote
I think that's incorrect, to a certain extent. Increasing the overall surface area of the roof increases the volume of water that can be on it at one time. Each of those ridges would act as a cistern capable of holding a volume of water that would have otherwise overran a simple slope. Instead of a straightforward through-rate, you need to take into account many small pockets of internal storage. Depending on the frequency and intensity of rain in the area, that could be significant.
Toger t1_jdg71kg wrote
Granted; I was assuming a sustained downpour.
MrShotson t1_jdgem8z wrote
Just did a quick Google of weather patterns in Bermuda. Apparently, they average 5 to 7 inches of rain per month. With very little fluctuation. Rain is sudden, heavy, and short-lived. That may kind of make sense for the roofing strategy then.
MidnightAdventurer t1_jdh5b8q wrote
Yes and no - water on a flat surface with an open edge doesn't drain at the same speed as water on a smooth slope. You can build up a shallow pool on each ledge that will spilling over the open side. With enough rain, the effect will be overcome and it will basically end up the same and with light enough rain, some of the water will get trapped on the roof and evaporate off but in the middle it should work.
I don't really feel like digging out my old fluid dynamics notes to try to calculate it out but the basic principle makes sense
Phage0070 t1_jdfagmj wrote
I think there is a healthy amount of tradition and folklore at work here.
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