BS5837 modification of an RPA

[SelfBuildSawyer]

Hi all, I'm not from your arboriculture realm, but I have recently taken a very keen interest, read this forum to death and value your experienced views. I'm a chartered structural engineer, trying to design and build my forever home, close to an existing 17m tall conifer (in a neighbouring garden). Other planning constrains and site dimensions are going to dictate that if this build is feasible, some level of standard circular RPA encroachment will be inevitable. Historic land use and conditions (boggy ground, old hard-standing and old brick wall footings on boundary) lead me to question if the stock 12x stem dia. circle RPA was indeed realistic in this location. I have hydro-vacuumed a root investigation trench (heavy clay soil so an air spade was not going to "cut" it) to see if the roots do indeed extend into where I am proposing to locate a new building. 

The trench has revealed 5 singular roots, all smaller than 25mm in diameter.

The building will be on pile foundations with RC slab in any case due to ground conditions, but I'm trying to avoid the LA tree officer making me raise the building, with a ventilated air gap and rain water re-directed to existing ground under slab.

BS5837 guidance says that I can safely cut these sub 25mm roots.. If I were to then place a root barrier in the vacuumed trench before back-filling, would you guys see it as acceptable to redraw the RPA as a polygon of equivalent area, taking into account the new root barrier and likely real rooting spread? Obviously assuming the new RPA was reasonable with good rooting soil etc. An arb consultant will be on-board (initial site visit conducted only at the moment), but I just want to know how your community would react to such a suggestion. Will reply with more info if required.

Hopefully an interesting topic in any case, and I'd welcome your views on this with regard to the incoming BS update.

Thanks in advance, John

 

Edited Monday at 11:48 by SelfBuildSawyer

[Ledburyjosh]

In this scenario, I would state something like...

The proposed footprint impedes within the RPA of tree X by X meters. However, further investigations and study of the land's previous use and composition have revealed... 3 roots, etc... therefore, it is highly unlikely that the proposed plans will have a detrimental effect on the health and stability of tree X. 
Then, display on a map how much of the RPA will be protected and how, ideally, the total m2 should exceed the minimum. You can also state the remaining RPA exceeds the minimum required area of Xm2 to show the tree being considered and you not pinching bits on of its RPA in multiple sides.

State what you see, and give reasoning for what your doing if it's outside of the recommendations of BS5837.
The circle RPA's are in relatity to black and white.

[Dan Maynard]

How far away is the tree, you've said 17m tall. Also, what kind of conifer? Cedar is just getting going at 17m, Leylandii may be just about to fall over.

[Kylus Sylvestris]

Hi John,

This is an interesting post and one which I can't reply to as fully as I'd like. Yes, its perfectly reasonable to deviate from the arbitrary default circular RPA with justification. It's not something that can be addressed without the particulars as there are many unknowns here:

• How deep was your trench?
• What is the condition of the tree... species/age/health etc? (will it tolerate the RPA infringement)?
• What are the existing constraints to root growth?
• and many more...

By way of a starting point....

The default RPA is intended to protect a volume of soil - sufficient to sustain the tree. So soil type/depth is important. The x12 RPA doesn't protect all the roots - typically about 1/3 to 1/2 - so arguing about encroachment and size/shape can be exceedingly tedious if the LPA are fixated on arbitrary circles. The attached pic gives you an idea of typical root spread vs minimum RPA. Logically if you have a minimum RPA there must be a maximum. And so if you've pinched a small amount of RPA then maybe consider retaining additional area outside RPA.... such that the actual impact on the tree is lessened in any event.

Knowing how, what, where, and when any deviation is justified will take professional assessment. Get this assessment done early. I'm actually on the BSI panel that has just issued the (draft) revised BS5837, and I'd be happy to have a chat with you about your particular issues.

ROOTS RPA.pdf

[SelfBuildSawyer]

11 hours ago, Dan Maynard said:

How far away is the tree, you've said 17m tall. Also, what kind of conifer? Cedar is just getting going at

17m, Leylandii may be just about to fall over.

 

Thanks for your reply Dan.

The tree is circa 6m from the proposed building (a chalet bungalow). I believe it is a Monterey Cyprus, but there is some debate about this.

8 hours ago, Kylus Sylvestris said:

• How deep was your trench?
• What is the condition of the tree... species/age/health etc? (will it tolerate the RPA infringement)?
• What are the existing constraints to root growth?

Thanks Kylus.

600mm deep trench and it bottomed onto heavy clay of density 1.94g/cm3 (soil report conducted for foundation design)... The soil make-up is this.... Stoney clay soil from 0 - 400mm deep, then a layer of very stoney clay to 550mm deep, then pure heavy clay to at least 2m deep.

Tree condition is good to my untrained eye. Age estimated to be circa 40 years based on satellite images and talking to neighbours.