With varying advice, the ILP recently approached us to help provide clarity to this question; having received an enquiry where the manufacturer had advised a + or – 200mm.
Which got us thinking, how much guidance is available for planting columns?
Not the most riveting of subjects, we know, but! one of those subjects you really do need to know the answer to (as a column installer).
So, what guidelines are contractors using, and what are manufacturers recommending? Here are our thoughts...
What is tolerance?
For planting depth tolerance, let's define the hole being too deep and the column being too low as +ve tolerance and the column being too high out of the ground as -ve tolerance.
Where to look for guidance?
An obvious place would be BS EN 40. However, there are no specific tolerances stated on planting depth, because this is a product standard and believe it or not - it's not allowed (to discuss the installation tolerances of a column).
However, Table 7 in BS EN 40-2 does refer to minimum planting depth; automatically defining the tolerance of the root being planted too shallow as zero.
In terms of planting too deep, there are no tolerance limits specified, but a requirement which states that the bottom edge of the door cut-out must remain 300mm or more above the ground level.
All clear so far?
We also have the manufacturer's instructions.
The manufacturer may or may not guide tolerance. If they haven't, you should always approach them for advice. Guidance is case-specific, and in practice, the allowable tolerance of the planting depth before the column becomes unsafe, is dependent on the column load, the foundation material and ground conditions.
Some considerations
Levelling material
It seems reasonable that a hole can be slightly over dug, then a paving stone or anti-sink plate placed in the bottom to level to the correct depth. This depth can be measured easily to existing ground level where there are no paving finishes.
Settlement
However, on a verge consisting of made ground that is non-compacted, you can expect some settlement of the ground level over time, and so while the graded level relative to the column may be ok initially, the ground will recede slightly leaving the ground level mark exposed.
Finishes
Where paving finishes are required, you need to take into account the depth of the finishes placed around the column and plant accordingly. Ideally, the finished level should be clearer, so the allowable tolerance on planting depth could be less.
In summary,
For hard paving, it is reasonable to have a column with a planting depth tolerance of +50 / -50 mm.
For grass or made ground finish, a planting depth of +100/-0mm at installation is acceptable, but after one year, this would be +100/-100mm to allow for settlement.
In answer to the original question, it is our view that a 200mm tolerance is too large, unless the column manufacturer has provided specific permission. The key message here is that there are general guidelines, but for your particular case, always check with your manufacturer.
