What is considered the RPZ for a tree with a TPO?

[doobin]

As above. Mature oak, client wishes to protect it (and it also has TPO). However, he also wants a large concrete base in fairly close proxmity...

 

Is there anyone with the relevant letters after their name who would like me to put their number forward to visit the site and provide a specification for works? I'm just a groundworker. Job is in Cranleigh.

[monkeybusiness]

RPZ radius is 12x stem diameter as a general rule of thumb. It can be offset upto 20% (depending on ground conditions) and doesn’t necessarily have to be circular. 

[Chris at eden]

There is no such thing as an RPZ, it should be either:

• RPA - root protection area or,
• CEZ - construction exclusion zone. 

RPA is as described above but there is no standard 20% offset anymore. That was from the 2005 standard.  And there is a different calculation for multi stem trees.  RPA should be circular unless ground conditions dictate otherwise in which case it ca be amended to reflect likely root morphology. 
 

CEZ is the area you fence off. It’s usually based on the RPA as RPAs tend to be bigger than canopy spreads. But if you have and RPA of say 10m  and a canopy spread of 10 - 12m you would extend the protective barriers around the bits of the canopy that are bigger than the RPA. So the CEZ can sometimes be bigger than the RPA. 
 

Any hard surface in the RPA will need to be no dig and ideally porous and should not cover more than 20% of any part of the RPA that as previously soft landscaped. 
 

It’s too far for me but you probably need someone to write a method statement and spec for the surface. 
 

Cheers

 

Chris. 

[daltontrees]

On 07/12/2021 at 19:29, monkeybusiness said:

RPZ radius is 12x stem diameter as a general rule of thumb. It can be offset upto 20% (depending on ground conditions) and doesn’t necessarily have to be circular. 

It can't be offset by 20%. That was in the previous version of the British Standard.

[Jon Heuch]

3 hours ago, daltontrees said:

That was in the previous version of the British Standard.

I always find it interesting to read reports & decisions referring to standards and laws that are woefully out of date. A sure sign someone or organisation has got lazy & is in a bit of a rutt. Council tree work decisions referring to the 1999 Regs & even the 1969 regs once, being an example. Text in the decision notice reflecting those old regs including reference to Article 5 certificates and compensation reflecting what is in the particular TPO being another. I had an expert witness report on my desk the other day in which the expert had used the Helliwell method.....except they had used the method from about 20 years ago, predating several revisions & updates. It did provide something of an open goal when it came to questioning the competence of the expert.

BS5837: 2012 & the 2012 Regs in England will be approaching their 10th birthday next year. Plenty of time for people to have read them and forgotten history I hope!

[daltontrees]

1 hour ago, Jon Heuch said:

I always find it interesting to read reports & decisions referring to standards and laws that are woefully out of date. A sure sign someone or organisation has got lazy & is in a bit of a rutt. Council tree work decisions referring to the 1999 Regs & even the 1969 regs once, being an example. Text in the decision notice reflecting those old regs including reference to Article 5 certificates and compensation reflecting what is in the particular TPO being another. I had an expert witness report on my desk the other day in which the expert had used the Helliwell method.....except they had used the method from about 20 years ago, predating several revisions & updates. It did provide something of an open goal when it came to questioning the competence of the expert.

BS5837: 2012 & the 2012 Regs in England will be approaching their 10th birthday next year. Plenty of time for people to have read them and forgotten history I hope!

I got planning conditions on a site this year that required compliance with the 1989 version of BS5837. Chespale fencing 2m beyond the dripline, or something like that. Last month a Council cast up a 1957 TPO in a new town that pre-dated the existence of the whole town. The TPO is reliant on the 1947 Planning Act which was repealed in 1972. Sigh....

[doobin]

Well I had a  look to day (and thanks to Kev for the advice on the phone). Goreous mature oak, and he was wanting to put the cabin and shed right under it. He readily agree that in this case it couldn't be on concrete, and is seeking a spec from the log cabin company for low disturbance base. The shed can go on sleepers.

 

Looks like I will be picking up a bit of regular type groundworks for other works there anyhow, so not a wasted trip at all.

[monkeybusiness]

On 09/12/2021 at 08:14, daltontrees said:

It can't be offset by 20%. That was in the previous version of the British Standard.

So it can no longer be offset? I’m pretty sure with justification it can, and 20% is  still a good rule of thumb to go by.

[Khriss]

Would also agree there, on offsetting if soil or structure conditions allow  . Plus shed on sleepers so its not a bad result for the tree . Currently converting a contractor to BS3998  -2010 , so there is a long way to go K 

[daltontrees]

I suppose this merits a more detialed explanation.

 

The 2005 version said that the RPA is to be calculated (of radius 12 x DBH as it is in the 2012 version too) then plotted. It could be offset if the tree could tolerate the damage. This was elective. It said "For individual open grown trees only, it may be acceptable to offset the distance by up to 20 % in one direction." We might call this 'bearable offset'. It was a deliberate removal of rooting volume if it was decided that the tree could stand it and that other compensatory soil was available on the other side.

 

What that means is that the circle or other shape could only be shifted to one side by up to 20%, not that the area could be shifted by 20%. In fact, a 20 % shift in distance away from a linear feature will only affect the outmost 5% of the area.

 

That said it is common to find situations where there is no possibility whatsoever of roots or even soil to one side of a tree. In such situations it is inevitable that the RPA should be non circular and greatly offset, sometimes by over 80% of its calculated radius. But the onus then is to provide a greater protected volume in other directions. This was acknowledged in a separate subsection of the BS. We might call this 'natural offset'.

 

What the BS was and is trying to do I think is to avoid abuse. It could be abused by offsetting RPAs to make way for development. So it is restricted to 20% by distance. If an existing building foundation is restricting it by 80 or 90%, that's a  different matter, there's no good in not offsetting it. But if it's a kid-on that there are underground barriers, offsetting away from them could be disastrous for the tree.   

 

Much later in the BS it said that "New impermeable surfacing should not cover more than 20 % of the root protection area." and a maximum of 3m distance. This would have allowed surfacing within the RPA, but not excavation or structures or the removal of rooting volume. This to me seems OK since roots will be able to get water and breathe sideways by diffusion over short distances.

 

So I withdraw my previous comment and replace it with "It can't be offset by 20%. Up to 20% by distance (subject to the likely tolerance of the tree to root disturbance or damage) was in the previous version of the British Standard.

 

The 2012 version doesn't make a distinction between (elective) bearable  and natural offset, it just applies the tolerability test and the natural asymmetry requirement. There's no mention of 20%. It does say though that "New permanent hard surfacing should not exceed 20% of any existing unsurfaced ground within the RPA." This is to be laid on top of soil, so it doesn't change the RPA.

Edited December 11, 2021 by daltontrees