38 Degree Rule

[benedmonds]

I have never heard of a set "38 degree rule."  Some local planners have, is this something I have missed, I have scanned NHBC chapter 4.2 and can't see how any of that would be relevant. 

 

Planners have stated:

Any dwellings should be at 38 degrees from the mature height of the Common limes and therefore any dwellings have to be located further away i.e. not just outside the RPAs but in context of site working space, compatibility and shading.

 

The garden for unit 5 is inadequate and would be overshadowed by the retained trees to the site frontage. Much of the internal accommodation to unit 5 would also be impacted upon in terms of natural light levels. The Tree Officer has confirmed that any dwellings, if 38 degrees away from the mature height of the limes,  i.e. 22m. (ref. NHBC chapter 4.2) would be 28m. away from the trees.    

[Khriss]

Sounds a bit arbitrary ? K

 

Tree shading is a valuable component in cooling a house in summer and winter wind buffering. K

Edited October 9, 2020 by Khriss
Its late.

[daltontrees]

21 hours ago, benedmonds said:

I have never heard of a set "38 degree rule."  Some local planners have, is this something I have missed, I have scanned NHBC chapter 4.2 and can't see how any of that would be relevant. 

 

Planners have stated:

Any dwellings should be at 38 degrees from the mature height of the Common limes and therefore any dwellings have to be located further away i.e. not just outside the RPAs but in context of site working space, compatibility and shading.

 

The garden for unit 5 is inadequate and would be overshadowed by the retained trees to the site frontage. Much of the internal accommodation to unit 5 would also be impacted upon in terms of natural light levels. The Tree Officer has confirmed that any dwellings, if 38 degrees away from the mature height of the limes,  i.e. 22m. (ref. NHBC chapter 4.2) would be 28m. away from the trees.    

There is a common rule of thumb in daylighting calculations called the 25 degree rule. It is derived from the British Standard for internal daylighting BS8206-2, essentially it means that if the vertical angle between the mid point of any window facing a solid continuous obstacle to light and the top of the object would be greater than 25 degrees the internal daylighting may be below the minimum standard, and a more detailed analysis would be required. I'll not get into the detail of it, but that's the basic 'rule'. I dont know where the 38 degrees came from. Hopefully it's clear that the further way the window is, the smaller the angle becomes for any fixed height of obstacle.

 

It would be possible (and I would say it should always be the case) that the mature rather than current height of the trees that are being treated as the obstacle could be used. If so, this would give a distance which could be insetted into the angle calculation in reverse to give a current equivalent angle for the current height of the trees. If so, it would be much much better just to state the distance.   So, ATan 22/28 = 38 degrees.

 

The rule if it is being used in this way would only work on level ground and only if the trees are of uniform height for most of the panorama left and right of the window positions. Plus if the height of window has been ignored the buildings could in teory be almost 2 metres closer to the trees.  Plus account needs to be taken of the window design, room sizeds and most importantly room uses. Kitchens require twice the daylight of bedrooms. Living rooms are half way inbetween.

 

It's possible to do a full analysis using measured and calculatee Vertical Sky Components and Average Daylighting Factors for rooms. It's time-consuming, though. Rules are no good if the rationale and limitations behind them is not understood so that the user knows when the rule cannot be safely used.