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Project Name: Thornwood Gardens

Site: Thornwood Gardens, Kensington, London


Thornwood Gardens is one of London’s top 5 most desirable residential addresses, this prestigious development was completed in 2006 with a gross value then of £358m.

This luxury 24 hr concierge facility comprises 35 flats and 8 town houses, current town house values are circa £18m.

The development, designed by KSR Architects, is beautifully complimented with classic materials of natural stone paving and cladding, brick and timber to recognise the architecture of the local historic environment and combine with a contemporary architectural design.

To combat environmental and organic discolouration on the horizontal and vertical elevations of natural stone, the board of directors decided to find the most suitable solution to regain and maintain the original cleanliness, vibrancy and beauty of all of the natural stone.

Despite intense competition from some of the industry’s best known stone maintenance companies, and based on the immense professional capability and experience, YFMG the won the contract for this prestigious, high-specification requirement.

The contract took 2 years to negotiate and win. This prized award was preceded by extensive test work to prove that the proposed methods of high temperature/low pressure cleansing, natural stone invisible protection and organic staining control were effective in achieving the desired appearance.

Since completion YFMG monitor progress every quarter, assess and provide maintenance as is required to ensure the natural stone reflects and complements Thornwood’s desirability.


The primary objective was to remove or improve the levels historic atmospheric/organic pollution soiling to help restore the colours and vibrancy of the effected natural stone and various areas of low level brickwork.

The secondary objective was to protect, as far as is possible, against future atmospheric/organic pollution and residue staining, and provide ongoing maintenance to the above treated areas so that they remain in premium condition year on year.


Cleaning Scope –

Use of low pressure, hot water applied with rotary cleaning heads aided with a PH neutral detergent.

Biocide Scope –

Apply slow release Qcide (YFMG biocide with unique slow release detergent capability) to prevent organic re- growth and assist in trying to gradually remove or improve post cleansing organic residues.

Protection Scope –

Apply Qstone SF solvent free invisible impregnating protection with 5 year guarantee to reduce future staining and ease future cleansing.

Maintenance Scope –

Quarterly inspection visits by YFMG.

Further applications of Qcide to all cleansed stone/low level brickwork; identify suitable treatments to attempt to reduce remaining post cleansing stains.


Prior to contract start, low pressure, hot, rotary head cleansing was satisfactorily carried out on the natural stone fountain perimeter.

Post cleansing, some “whitening” of the fountain perimeter top stone caused by embedded bleaching from water additives became more evident. To remove the effects of the bleaching may be achieved by extensive surface mechanical dressing or by replacing the effected stones BUT this may all be in vain as the water additives will only start the whitening process all over again.

Since cleansing, the fountain re-jointing has been completed by the stone contractor, the natural stone has now been treated with Qcide, further treatments of Qstone SF and Qcide will be applied to the fountain as soon as suitable weather arrives.

Treatments Contract

Normally, the three treatments (cleansing/biocide/protection) are carried out in rolling sections i.e. sections reachable without having to laboriously re-position the lift platform. By the time we reach the end of a section cleanse, the beginning is dry enough to apply the Qcide, same applies to the Qstone SF application. Given the persistent rain for the whole of the first week only cleansing was feasible necessitating laborious lift platform revisiting cleansed areas, especially on Campden Hill roadside where further delays and H&S risk were caused by regular concrete mixer trucks passing by (reversing!) to reach the adjoining construction site.

The required Z80 high lift platform takes circa half an hour to safely position, reach full height and lower to ground!

Findings during treatments:

  1. 1. Method.

From the outset it was evident that the covered rotary head appliance had to be regularly augmented with careful use of an uncovered rotary head to impact heavy staining without damaging the surfaces e.g. on window, ledges, top copings, other horizontal surfaces.

  1. 2. Construction residue.

As has been witnessed during and since cleansing, run-off water contained white particles which are mainly construction residue with some fine efflorescence resulting from atmospheric pollutants reacting overtime with the calcite calcium carbonate Anstrude limestone. Some of this residue is visible on some natural brick areas, this should wash away (by rain action) over time.

  1. 3. Discolouration.

Post cleansing, the following discolorations were visible.

It is important at this point to be clear that despite even our highly experienced cleansing methods, they are only effective up to a certain point, beyond that point the natural stone is at risk of being damaged.

To remedy and or understand the remaining staining (built up over circa 9 years), we arranged for the original stonemasons (Putney & Wood) to view on site to identify the causes and commercially viable processes to try and reduce these even further.

a) Thornwood (road side) marked improvement on dark stains but little change on lighter staining.

b) Horizontal Ledges/vertical stains.

Compare ledge over lower LH lower window to ledge over lower RH window, also note vertical stain.

c) Entrance Archways

A vast improvement, but still some horizontal to vertical run-off weathering/staining evident.

d) Brick Chimney staining

To identify ‘non cleansing’ stone related issues YFMG contracted Putney & Wood to give insight into the issues. However, to gain a more authoritative report they recommended we should contact Peter Harrison at the Stone Federation of Great Britain (SFGB) YFMG made contact to discuss the issue.

As this SFGB contact was an unknown to YFMG we gained two independent satisfactory references.

Peter Harrison has a reputation for straight talking and is probably one of the UK’s most experienced and revered natural stone experts; he is a past president of, and represents the SFGB in Europe with Euroc requirements.

We contacted Peter Harrison to arrange the visit to site. During the meeting, Peter provided some useful answers to various issues on the staining, stone delamination/erosion and efflorescence. He was requested to provide a quotation for an in-depth survey with recommendations, the proposal has now been provided.

Other Staining

In late 2015 there was an early return of green algae on most ground level paving, low level wall caps and low level property entrances.

In December 2015 YFMG applied a stronger solution of Qcide to various test areas which proved successful; consequently all the paving and low level areas were similarly treated on 21.1.2016.


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