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Rain Intensity As Critical Factor

imgsifonikaIs your roof manhole protected?:  there are several key elements in the design of a proper roof drainage system but one in particular stands out: The rainfall intensity to use when deciding how much protection you want to give to the roof. This is a critical factor for the system to successfully accomplish its mission of drainage and for us to avoid unpleasant problems caused by water in buildings.

Considering what the Spanish Technical Building Code (CTE) indicates to the area where the project is located we will be fulfilling a "legal minimum" but we must ask ourselves whether meeting the minimum is enough in our building in view of heavy rains experienced with increasing frequency (climate change). In principle, the rainfall intensity must be considered in terms of two parameters according to the so-called rational method: One is the return period and the other one, much more sensitive and important, the duration of rain. The duration of rain should be in line with the time of concentration which in turn is marked by our roof pending. As a general rule, we can establish the following simple rules based on the experience gained in this field over the years: Flat roof → 10 minutes Roofs with pending between 2% - 4% → 5 minutes Roofs with pending greater than 4% → 2 minutes A helpful tip relating to the return period is to match at least the expected life of the building. This ensures that the calculation intensity will be faced at least once. Choosing 25 or 50 years should be usual in Construction. In general, you should consider using the rational method in the following cases: . 1 - If the building has a public function or serves to hold valuable content. As an example, we will say here that the UK regulations apply mandatory enhancement factors for such cases. . 2 - If the building roof pending is greater than 5%. . 3 - If we are already aware that the "peak intensity" of storms are truly what test your drainage system. . 4 - If we think about the role that the so-called climate change may be playing in the previous point both in regard to higher intensities as their greater frequency pattern. . 5 - If our building is located in a "heavy rain" prone area.

imgsifonika2Those buildings with low rainfall protection face the threat of water ingress by gutter overflowing or, in the worse cases, a partial or total collapse of the roof if it has not been consciously designed to withstand unexpected surges of water due to not drained flow. As a comparative exercise, we have to state that the French legislation requires the use of an intensity of 180 mm/h for the entire country which is twice as indicated in the CTE for areas such as Madrid and Zaragoza. And that is given the known evidence that the higher southern latitude means the greater likelihood of experiencing torrential rainfall intensities. The use of secondary siphonic systems that provide extra safety, that get the primary systems to start working faster (shorter priming) and that discharge directly to ground level (or tanks for recycling) to not interfere with saturated sewerage networks are an excellent solution, unfortunately, it is almost always used post having already suffered the problems of a low level of initial protection, either using classical gravitational systems or siphonic systems. In both cases, a bad choice of parameter implies a high probability of facing an unwanted incident. From Sifonika we want to make designers and owners aware not to fall short in the intensity parameter, because the consequences can be truly disastrous and this item on the execution budget often means no more than 0.3% of the total amount. We will talk about secondary and emergency systems in the next entry of our blog.

 

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