Sharp slopes are not inhabitable?

A bioclimatic answer


Fausto Simões






It is somewhat easy to understand and modify microclimatic conditions, as we can see and feel in an air-conditioned, artificial-lighted room. But what are the side-effects on environment, of our mechanized buildings and towns?

It seems very dificult indeed, to understand antropogenic climate change at the global and at regional scales… But it is likely that the antropogenic signal will emerge more and more convincingly with time, in model simulations, as says the IPCC [1].  So it seems well justified to design, build and use our setlements WITH CLIMATE, and not against it, applying scientific research.

Sharp slopes are not inhabitable? This is perhaps a provocative question to those who intend to design with climate, and broadly, with nature.

Design with nature, that´s to say: WHERE and HOW to build our setlements following local liabilities and assets, within a holistic approach. All this, in order to satisfy our needs and to do NO HARM to man and to NATURAL ENVIRONMENT. This is precisely what is NOT done, when hills and valleys are covered by the same concret-and-asphalt receipt, be it the high-rise or the detached house one. Natural diversity and complexity are reduced to a boring sameness (see Figure 1).

Figure 1: Hills and valleys covered by the same concret and asphalt receipt


Side effects of this current pratice can imply high costs, even life costs, not accountable in the financial system.

So, intending to design with nature, one should not, of course, build on poorly drained, flooded, unhealthy or rich topsoil valleys, but  on well drained plain land and on slopes…even on steep slopes?


Building on slopes


Even on steep slopes? That´s a good question to answer, intending to enhance the relationship between  WHERE and HOW, and to show that rigid, stringent rules should be rejected.

So, what are the suitable slopes for residential areas?

In the sunny Mediterranean fringe, solar space heating and cooling, solar water heating and daylighting can give more comfort with less pollution by energy plants.

Passive and active solar require a free solar access in a South exposure, more or less 15 degree, for maximum efficiency.

Microclimatic conditions related to solar radiation have a better equilibrium between winter and summer on steep slopes up to   fifty-sixty degree, with a south exposure, in the case of Lisbon.

This can be seen in the graphic of solar radiation against slope for Eastward slopes and Sowthward slopes, blue and red curves, in winter and summer, lower and upper curves (see Figure 2).

Figure 2: Solar radiation against slope in the case of Lisbon [2]




How to build on steep slopes?


Building on steep slopes is an old story. Hilltop villages were a strategic rule, during the primeval age of “castros”, in what is now Portugal.

This, long  time before they came downslope to the valleys, clearing the primeval forest.

The URBAN MORFOLOGY suited to steep slopes is LOW RISE GROUP HOUSING descending the hillside (see Figure 3). The hilltop must be THERE, not built.

 With Low Rise Group Housing can be attained the Human scale in buildings and in a positive open space and also, a low-energy profile considering transport, comfort and construction demands.

Figure 3: Low rise group housing descendig the hillside, after Schwagenscheidt [3]


The ARCHITECTURAL TYPOLOGY  suited to steep slopes is TERRACED HOUSING, following a natural slope. Terraced housing is the urban counterpart of rural terracing. The terrace of one house is over the roof of the other downwards. With terraced or diagonal housing one can attain a free solar access, large views, privacy staying outdoors in secluded terraces and a desirable urban density. This main requirements and its relationships can be translated in a terraced-housing set of equations.

In the graphic form of these equations, one can see that the Floor Space Index (IC) can be a little over 1.5, allowing a density higher then 500 p.p.h., suitable for a good balance between community and privacy (see Figure 4). FSI is represented by the lower red curve in the graphic.

  Figure 4: Graphic form of the terraced housing equation set


Sensible creativity is needed, in order to give form to this terraced-housing formulary. The terraced housing landscape asset of a modern example on a 35 degree slope is shown in Figure 5.

Figure 5: Landscaped terraced housing




1.    Non speculative terraced housing on steep, brownfield, stable southward slopes, from 10º up to 40º (nearly) can enhance life, dealing with the problems of energy use, flood control and erosion. Portuguese recommendations based on the UN convention against desertification consider terraced housing on steep slopes [4], but current interpretation of the Portuguese environmental regulation (REN) forbids residential areas on slopes greater than 17º [5].

2.    Conception and evaluation methods of urban design following local geo-climatic suitabilities, should be supported by scientific modelling.





1.     Intergovernmental Panel on Climate Change 1996. Climate Change 1995 – The science   of Climate Change. Cambridge University Press (Cambridge 1996).

2.     Mata, L.; Marques, C.; Totais Diários da Radiação Global Recebida por Superfícies com Diversas Orientações e para Diferentes Ângulos de Inclinação. INMG (Lisboa 1981).

3.     Schwagenscheidt, W.; Ein Mensch Wandert Durch die Stadt. H. Müller- Wellborn. (Bad Godsberg – Mehlen, 1957).

4.     Ministério da Agricultura, do Desenvolvimento Rural e das Pescas. Convenção das Nações Unidas de Combate à Desertificação. Organização Nacional para a Aplicação da CCD (Lisboa 1997).

5.     Pereira,A.; Bento, J.; Gomes,M.; e N. Cerejeira. Metodologias de Avaliação da Reserva Ecológica Nacional (MAREN). Ambiente, Ordenamento, Gestão do Território e Sistemas de Informação Geográfica. Faculdade de Letras da Universidade de Lisboa, (33-41).   (Lisboa 1995).