What Went Wrong? Paving Levels and Falls
This article is the eighth in a series of articles written under the heading of ‘What Went Wrong?’ involving landscape projects that have failed for one reason or another. All cases are genuine court claims, examined using notes taken from expert witness reports commissioned to produce court compliant statements for use in the claims system.
Locations are anonymised to prevent conflict. All cases have been settled through court claims, or by insurance companies.
Using the same methodology throughout the series, to ensure the same standard of information is followed, with an introduction to the claim, the problems identified, and the outcome of the case.
All expert witness comments are made using either Industry Best Practice, or British Standards where applicable, in the absence of any specification provided by the contractor.
PAVING LEVELS AND FALLS – All matters of design and installation of paving are covered by BS7533:101 and 102. As the British Standards are only advisory, on occasion, the Defective Premises Act 1974 may be invoked. In any event, the Consumer Rights Act 2015 will apply to all claims for poor workmanship.
The most common causes for complaints are:
1. Water build-up within the area of a path or (especially) a patio caused by a lack of crossfalls.
2. Discolouration of the paved surface, especially natural stone paving such as limestone.
3. Build-up of algae caused by lack of cross-falls, allowing moisture to collect.
4. Slipperiness on the surface (especially porcelain projects) due to water collecting and surface tension creating slip planes underfoot.
5. These problems may be especially prevalent around swimming pools, jacuzzis and hot-tubs, where the central feature must remain level, whilst water requires a slope to prevent puddling and surface tension issues.
All paving slabs are rated according to slipperiness, using a ramp test, on a scale between R9 (least abrasive) and R13 (most abrasive), with R11 being the most popular rating in products sold into the domestic market. Rating measures friction, with R10 suited to covered, low-moisture areas, and R12 providing high slip resistance for sloping or wet sites.
These ratings apply to newly supplied paving, with some of the natural stones liable to changes in slip rating due to weathering and contaminants such as moss, leaf detritus and the effects of certain plant saps or even bird droppings, and maintenance is essential in ensuring the slabs remain slip resistant.
It is important to note that many customers do not appreciate the need for falls across paving, and it is incumbent on the specifier to explain the requirements of British Standards, which are designed to meet safety on site.
INCORPORATING FALLS IN LANDSCAPE SCHEMES.
BS7533 calls for crossfalls of at least 1:60, with a recommendation of up to 1:50 for porcelain, which can be difficult to work into a landscape scheme without careful forward planning.
Every site survey should identify the highest point (inflow of water entering the site) and lowest point, where surface water will naturally exit and move on beyond the property. These individual datum points cannot be ignored, as they will dictate all fixed points within a scheme.
For example, a paved area within a garden plan must have a cross-fall of at least 1:60, which means two different level points; one to the higher end, and the other to the lower end.
If there are two areas of hard landscaping, each high and low point should be marked on the site drawing plan, and fixed before any construction work takes place. As the areas of hard or fixed landscaping features are decreed, the rest of the construction works are established across the project, ensuring that surface water can traverse without hindrance.
Case Study – Property in Hertfordshire
This property is a new-build large scale ‘barn’ clad in timber, with wrap-around paving, with paths to three sides, and a reasonable sized patio, totalling around four hundred square metres of porcelain paving.
Designed by an architect, including cross-sectional drawings showing the paving level to be set at 150mm below the timber cladding, there were no levels provided for the paving, only a general indication of the finished level under the damp proof course (cladding) in the design.
The customer became aware of potential problems with the paving works, and commissioned an expert witness report on the condition of the project. There were no specific issues raised, beyond a general feeling that all was not well.
This single photograph illustrates two of the problems encountered during the site survey;
1. The paving has been laid too high against the timber cladding, with the gulley between the paving and the building filled with decorative aggregate, which has acted as a wick, drawing water from the paved surface up into the timber (board ends) causing discolouration and likely future problems with rotting to the boards.
2. The area of paving seen in the photograph is the rear footpath, clearly showing the issue with water wicking up into the timber boards. The weather was dry during the site inspection, although the timbers were noticeably damp at the bottom
3. The paving has been laid without any measurable cross-fall, indeed, tests proved that the main body of the patio was ‘dished’ not allowing any surface water to progress across the plane and discharge beyond the paving.
4. The second image shows the slight dishing in the surface, proved by gently rolling marbles or ball bearings across the paving, and recording where they cease moving. In this instance, the area was relatively large, showing the extent of pooling that would occur during wet weather, causing problems with aquaplaning.
5. Not shown in these images – none of the slabs had been laid using slurry bridge bonding, and they were all liftable by fingertip once the first slab had been raised. This lack of a suitable key would have probably caused de-bonding during frosty weather as water penetrated the grout.
6. As this point was not raised by the owner – and none of the slabs was obviously loose or a trip hazard, and the whole paved project was recommended to by lowered by 150mm, it was not considered to be an important factor in the complaint.
NOTE: Although the most accurate method of measuring differences of height and fall would be by laser, it is more common, when producing court compliant reports that do not call for a scientific standard, to use either a spirit level (which will show the ‘bubble’ but is difficult to relate to a particular area in a photograph) the visual impact of using marbles clearly proves the point that the report text is making when describing a situation.
The results of the expert witness report were sent to the customer, who successfully challenged the builder (who had ignored the architect’s instructions) under the Defective Premises Act of 1974, which states that a dwelling must be fit for habitation free from damp, mould or structural issues.
Section One is called Fitness for Habitation, and covers new builds, conversions and renovations. It is not widely used, although I suspect in this case, as a developer and architect were involved, the use of the Act may have been a compromise or device used by a solicitor.
WATER MOVEMENT ACROSS A SITE IS PARAMOUNT TO A SUCCESSFUL PROJECT. IT REQUIRES ACCURACY IN PLANNING, OR ADHERENCE TO WRITTEN INSTRUCTIONS PROVIDED IN THE CONTRACT DOCUMENTATION.
