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"Tree Root Damage to Buildings" by P.G. Biddle strongly advocates the advantages of precision level monitoring and level distortion surveys as the most useful and cost-effective part of site investigations.   The draft of the forthcoming revision of the Institution of Structural Engineer’s report on “Subsidence of low-rise buildings” also emphasises the advantages of both these types of investigation, and that level monitoring, rather than crack monitoring, should be the preferred option when investigating suspected tree root damage.

The “Bidpod” has been specifically developed for these purposes, and for easy single-handed operation, thereby avoiding the need for extra operatives.  It consists of an aluminium ‘U’ section staff, 1.95m long which is supported against a wall by twin aluminium legs on an adjustable slider.  The staff supports a 3.0m long flexible steel Stanley tape measure with a “Power Lock”.  A precision optic level is used to take the appropriate measurements off this tape.  The staff has two main uses.

Level distortion surveys.  (see “Tree Root Damage to Buildings”, pages 190-194, including Figures 12.10 and 12.11)

For measuring the level of a brickcourse around a building, the “Bidpod” has a slider which is attached to the end of the tape.  When measurements are taken, the staff is leant against the wall and the top edge of the slider is placed against the top of the brick, with the powerlock on the tape providing a convenient friction device to allow the slider to be adjusted.  After setting up the staff, the observer returns to the level to take the reading off the tape, recording to the nearest millimetre.  This operation is then repeated for each reading.  The sliding bracket and the support for the legs each project by 60mm at right angles to the staff on either side.  This allows the staff to be set up either against the building or facing along the building, and to be held clear of the wall to make it easier to view around downpipes or other obstructions.  For distortion surveys with the “Bidpod” there is, of course, no need to place nails or other markers into the brickcourse.

For measurements off a floor, the slider is removed and the end of the tape attached near the bottom of the staff with the powerlock released.  This keeps the tape under tension.  The staff is also used in this mode at transfer stations when moving the level to a new observation position around the building.

Precision level monitoring (see “Tree Root Damage to Buildings”, pages 239-246, including Figures 15.1 and 15.2)

Precise measurement of the pattern and distribution of foundation movement is the most effective way to determine whether vegetation (trees or shrubs) are involved as a cause of movement and damage, and to identify the culprit.  Unlike crack monitoring, which oftens requires a year-long cycle of measurements just to confirm whether movements are seasonal, even a brief spell of level monitoring is usually sufficient both for diagnosis and to enable remedial action to be correctly targeted.  After action has been taken to remove or prune the offending vegetation, level monitoring can be continued to check that the action has been effective.

The "Bidpod" allows measurements to be taken off fixed markers, consisting of roundhead brass screws, inserted nearly flush to the wall at appropriate locations around the building.  The “Bidpod” tape has now been modified, and includes a special end fitting which slips over the screw, with the tape held taut against the shank of the screw by the tension in the powerlock tape.  (This is a recent modification - see “Staffs for level monitoring” in the “News and Update” section of the website).

When used for level monitoring the slider is removed, the staff is supported flat against the wall with the base well above the marker, and the end fitting is slipped over the screw. 

Unless abnormally long sightlines are involved, readings can be taken to the nearest 0.1mm by interpolating between the millimetre markings on the tape.  Provided the precautions advocated on page 241 of "Tree Root Damage to Buildings” are followed, and using the modified end fitting, it is usually possible to achieve a closing error of less than 0.5mm in a circuit around a typical house.  This provides the level of accuracy which is essential for this work.

Price

The Bidpod is available exclusively from P.G. Biddle, price £225.00 (inc. VAT).

Modified Steel rule

“Staffs for level monitoring” in the “News and Update” section of the website also describes the use of a 1.0 metre long steel rule, with an egg-shaped hole which can be hung off a marker screw.  These can be used where there is no low-level horizontal cracking, and are ideal in some awkward situations.

Suitable non-reflective rules with engraved markings (at 1.0 and 0.5mm intervals, and imperial units on the reverse side), and with a modified hole suitable for slipping over a No. 8 screw, are available from P.G. Biddle, price £25.00 (inc. VAT).  Carriage is free if ordered with the "Bidpod", or £5.20 if ordered separately.

Modified Durabeam torch.

A small Durabeam torch, riveted onto a bull-dog stationery clip, provides a convenient method for illuminating the Bidpod under poor light conditions (see Tree Root Damage to Buildings, Figure 12.12).  Modified torches are also available from P.G. Biddle, price £8.00 (inc. VAT).

Precision Optic Level

We do not supply levels or tripods, but can supply details of suitable contacts on request.