Submitted by Onholidaybymistake22 t3_11zy3w3 in explainlikeimfive
Saw a video recently looking at houses in Bermuda and rain collection. They claimed that the ridges on the roof increase the surface area in order to increase rain capture. Surely rainfall is even over a surface regardless of terrain and therefore a ridged roof would collect the same amount as a smooth slopes roof?
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…
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
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.
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.
Granted; I was assuming a sustained downpour.
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.
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
I think there is a healthy amount of tradition and folklore at work here.
> Surely rainfall is even over a surface regardless of terrain and therefore a ridged roof would collect the same amount as a smooth slopes roof?
Of course. It's the cross sectional area of the roof that matters, not the surface area. Imagine that there is no wind, and the rain is falling straight down. It's not going to make any difference whether the rain falls on a flat roof, a tall A-frame roof, a mansard roof, a hip roof, a roof with ridges, or a smooth roof.
Bermuda is primarily a limestone island. The original roofs on houses were made from limestone slabs quarried on the spot. The pit that the limestone slabs were quarried from became the cistern in which the rainwater was collected. Being somewhat thick, these limestone slabs gave the roof a stepped appearance. Over the years, this stepped appearance became the norm for what roofs were supposed to look like. Modern Bermuda roof systems are made to look like the old limestone tile systems because that's what people expect.
Long story short, the ridges are aesthetic, not functional.
P.S. Bermuda needs to collect rainwater because there are very few other sources of fresh water on the island. The government mandates that at least 80% of the roof needs to be dedicated to harvesting rainwater.
Bermuda is a rock in the Atlantic Ocean 600 miles east of North Carolina.
The roofs are made of limestone and the ridges channel the water in a pattern across the limestone in an effort to clean it, then it lands in a collection barrel.
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.