It is true that taller buildings cast longer shadows but do tall buildings really create more overshadowing than their low and mid-rise counterparts?
Before diving straight into this question, it is important to recognise that we live in a cloudy climate and for this reason most of the natural light that reaches the ground comes directly from the sky and not from the sun. In the UK and Ireland, the sun is typically hidden behind clouds during 70% of daylight hours.
It is for this reason that the daylighting of our homes and workspaces has less to do with the degree to which neighbouring buildings can block light from the sun and more to do with how neighbouring buildings can block light from the sky.
So, do tall buildings block more skylight than their low-rise counterparts?
We investigated this question by testing the skylight access available to the primary elevations for three different building forms; all of which provided the same level of density (measured in dwellings per hectare). The three development forms tested included a terraced form of development, a perimeter block development and point tower development. When each development tile was tested it was assumed that it was fully surrounded by similar development forms. We repeated this testing at three different levels of density including 76 DPH, 114 DPH and 240 DPH. The results of our investigation are illustrated below.
The results of our investigation indicate that for a fixed level of density, it is generally easier to secure access to natural light with a point tower type of development than with either a terraced or perimeter block type development and that this becomes particularly appreciable at higher levels of density.
Stated more concisely higher buildings create less overshadowing than their low-rise counterparts, for a fixed level of urban density.
Reflecting on the findings from this study GV8 provides the following key takeaways:
- There are many reasons why low-rise and mid-rise forms should remain the staple building blocks of attractive urban development and renewal.
- In specific locations however, where higher levels of density are both needed and appropriate, it will be easier to provide higher levels of internal daylight amenity with a high-rise development rather than a perimeter block development. This is true both for the accommodation within the high-rise development and neighbouring accommodation.
- Where countervailing factors preclude high-rise as a possibility it is important for planning authorities to recognise that the perimeter block alternative is by nature less well equipped to secure access to natural light and that restricted lighting levels are likely to be encountered as an automatic consequence. Arising from this planning authorities may need to exercise additional levels of discretion when assessing the acceptability of daylight levels within this type of development proposal.
