The shading potential of trees on a proposed development is often cited as grounds for the refusal of planning permission. After all, we all appreciate the areas in our homes and offices which are flooded by natural light. So it is fair enough that developers and architects are asked to consider the effects of the surrounding environment, both manmade and natural, when designing these dwellings.
Because of this, you would be forgiven for thinking that arboricultural consultants follow a detailed and well-documented approach to assess the potential extent of tree shading on development sites. However, currently in the industry, it is not uncommon for opinion and hearsay to prevail over a more scientific method.
As arboricultural consultants, we deal with the requirements of BS5837 on a daily basis; and while it does provide a useful framework for outlining the many aspects we must consider, we find that it is often vague or outdated in its guidance. One such area is the calculation and interpretation of shading cast by trees. This can be problematic, seeing as planning departments often make their decisions based on the guidance laid out in this document.
Our approach covers the requirements outlined in BS5837, but uses the methods outlined in BRE 209, in conjunction with specialist shade analysis software. We find that this provides us with more scope for success when having to argue contentious matters.
BS5837 Guidance on Shading
So, cutting through all the "B S", what exactly are we asked to consider?
BS5837:2012 - Trees in relation to design, demolition and construction states that:
- Proposed buildings should be designed to take account of the lighting implications of:
- existing trees
- their ultimate size
- density of foliage
- Open spaces should be designed to meet the normal requirement for direct sunlight for at least a part of the day.
To indicate any potential sunlight obstruction, BS5837 asks us to plot a segment using two lines equal to the height of the tree; one due north west and the other due east. According to the guidance, this method indicates the tree's shading pattern throughout the main part of the day.
There are, however, two main issues with this method:
Firstly, by drawing the tree's shading pattern in this way, we are assuming that the sun is always at a 45 degree vertical angle. However, this clearly isn't the case. The sun's position in the sky varies from season to season.
For example, in Cambridge, in the course of a year, the sun's vertical angle varies between 14 degrees and 62 degrees (see below):
Secondly, the basic method described for use in BS5837 tree surveys provides no guidance on quantifying light obstruction. This is obviously an important aspect to consider. After all, trees do not prevent 100% of the light from penetrating their canopies. This varies drastically and is dependent on multiple features, such as:
- whether the tree is coniferous or deciduous (seasonal leaf-loss reduces the overall impact of tree shading).
- canopy density traits of differing species (Birch trees allow more light penetration than Yew trees).
- the shape of the tree (is the tree columnar or broad?).
- topography of the site (if the tree is situated lower than the development, the shading area will be reduced).
For these reasons, instead of BS5837, we often use an interpretation of BRE 209 as guidance...
BRE 209 - A More Enlightened Approach
Although originally aimed at analysing the shade cast by solid objects, BRE 209 gives us three particularly useful tools which can provide a detailed and accurate insight into the potential shading of trees. These quantify and consider availability of skylight, sunlight, and the sun's path.
Skylight / Daylight
Skylight is the ambient, reflected light from the sun that illuminates objects, even though direct sunlight is not present. The light provided by the sky on a cloudy day is a basic example of this.
To assess skylight we use a measure known as the Vertical Sky Component (VSC). This figure is expressed as a percentage. If measured above 27%, the building is likely to have adequate lighting; below this percentage, it is unlikely that daylight levels are acceptable.
Sunlight is measured as a percentage of the total direct sunlight time (annually) available for a given area of the proposed development.
This percentage should ideally allow for 25% of annual sunlight hours to be available at the point of reference. Furthermore, at least 5% of the available annual sunlight hours should be available in the winter months.
The sun-path, as the name suggests, describes the varying route the sun takes through the sky, varying from season to season.
By using this information, in conjunction with the other readings, we are able to predict the times at which any given reference point is exposed to direct sunlight.
Improving Sunlight Availability
In cases where the availability of light is on the threshold of what is acceptable, several options are available for consideration. These vary from pruning; such as canopy height reductions, or canopy thinning; to the complete removal of the problematic tree (not usually appropriate). Where this isn't suitable, adapting the proposed property's fenestration or layout often helps matters.
If you have a tree shading issue and would like to discuss your options, or would would like to find out more about our BS5837 Tree Surveys and Impact Assessments, please do get in touch...
Have A Question?
If you have a question, or would like to discuss your requirements, please get in touch for a free initial consultation...