What Went Wrong? Paving Base Failures and Their Consequences
This expert witness review examines paving base failures, including collapsed sub-bases, hollow slabs, voided laying beds, poor bonding and failed patio installations.
This is the third in a series analysing landscape project failures as recorded by expert witnesses commissioned to produce court compliant statements for use by the court’s system.
All locations and company names have been redacted, and all images anonymised to avoid any conflict. All cases shown have been settled through the courts.
Modus operandi – how the analyses are conducted.
In order to produce a series of inter-related articles, the same format will be used throughout.
All expert witness comments are made using either Industry Best Practice or British Standards where they apply when writing reports in the absence of any documentation or working practices mentioned as specification by a contractor.
There is no requirement to follow British Standards. They are a guide to good construction, but are not mandatory, unless the contractor has mentioned them in their quotation and contract, when they will be held to be the bench-mark against which work will be assessed.
In the same manner, unless a customer complains about a project, there is no requirement or need for a court claim to be made. Only once a scheme has failed does it become an issue.
Expert witnesses are only commissioned by one party or the other – Plaintiff or Defendant – or a third party acting on their behalf (solicitor, insurance company or bank) once a project has become subject to legal proceedings, with settlement and adjudication by the court the only recourse (sometimes mediators will be appointed in an attempt to come to a mutual settlement out of court).
Base Failures and their Consequences
An expert witness will conduct a series of tests during a survey of a paving project, beginning with an audio test, which involves gently striking each slab, and listening for the sounds (if any) that may be heard under the slabs – a hollow, ringing sound indicating that the slab is loose from its laying bed, or the bed has voids indicating a less than full mortar bed.
If the slabs appear to be solid and firm, with no signs of cracking joints or movement, and provided that the number of ‘ringers’ is less than ten percent, there should be no need for an invasive survey, requiring the lifting of any suspect slabs.
However, in most cases of hollow sounding slabs, there are serious defects in the laying bed, or quality of adhesion or bonding between the laying bed and the slab.
Using the photographs below, the importance of ensuring full adherence to British Standards becomes obvious.
One – Property in West Sussex
A twenty-year old detached house, with a narrow footpath across the back of the property. The project called for an extension to create a patio and wider footpath, with a step down onto the lawn of approximately 30cm. (with one external step)
The complaint involved several factors, mainly rocking and hollow sounding slabs (sawn granite) and cracking of the joints.
An audio test revealed that over ninety percent of the slabs appear to be ‘hollow’ or de-bonded, and a probe using a twenty-centimetre hunting knife revealed that the sub-base and base had collapsed under the slabs, allowing for the full depth insertion of the blade.
Further investigation by invasive means, showed that the contractor had extended the patio, which was raised by thirty centimetres from the lower ground, and failed to remove the grass from the old lawn before laying the sub-base and laying bed directly onto the soil.
The soil and grass had rotted and settled, undermining the whole of the extended patio. The project was a complete failure, and had to be taken up and rebuilt, beginning with proper foundations and full laying beds.
Two – Property in Essex
This was a detached house, around thirty years old, with a large patio laid to Indian sandstone, subject to a complaint by the owners of loose slabs and many undulations and trip hazards caused by uneven surfaces.
Walking the site, before conducting an audio test described at One, crunching noises could be heard from underneath the slabs, indicating that they were loose in their ‘sockets’.
Lifting slabs at random, with ease, it was discovered that none of them had been laid using a bridge bond, although attempts had been made to lift and relay some of the worst offenders.
The result of lifting, and applying various types of adhesive and silicone glues had not improved the problems, as the laying beds were a) too dry, b) soft builder’s sand, and c) almost 50% voids.
The only thing holding the slabs in place was the sand and cement jointing material.
Three – Property in Suffolk
A medium sized detached house, built in the 1960s, the owners decided to create a patio at the rear of the property, using 20mm porcelain slabs with all materials to be supplied by themselves, and using local labour.
The work was carried out in the past four years, therefore the works were assessed using Industry Best Practice as BS7533:101 had only recently been introduced at that time.
All of the slabs were ‘visibly’ loose in their beds, with no jointing material remaining in the gaps, having either washed away or removed by brushes over the years. (Probably single part resin permeable brush-in compound)
All of the slabs had been laid on ‘dot and dab’ sharp sand and cement mortar beds, with large voids throughout. A dozen or more slabs were easily lifted by fingertips, and it was obvious that none of them were secure on their laying bed.
The backs of the slabs had not been slurried, and all of the paving had been laid at the same level as the damp-proof course (covered in pine needles in the image).
The whole project was condemned, and subsequently lifted and replaced, paid for by the contractor after a court order was given.
Four – Property in Berkshire
A large relatively recent detached house, with a large rear patio surrounding a swimming pool and outdoor kitchen area with overhead canopy, 20mm porcelain paving had been laid throughout in a ‘half-bond’ pattern, and pointed in impermeable grout.
Many of the units had become loose from their laying bed, with areas of cracked grout and crunching sounds could be heard when walking on the slabs. The owner was concerned that the project had not been carried out in a professional manner by the house builder, and sought an independent professional opinion.
Whilst conducting an audio test by gently striking each slab with a thin wooden pole, it was evident that the slabs had either de-bonded from their laying bed, or the laying bed was full of voids.
Having lifted two of the slabs, it was discovered that they had been laid on a partial bed of sharp sand and cement, and laid using a PVA mortar mix, which had failed to adhere to the back of the slab. This may be seen as a white powder on the surface of the mortar bed.
The whole area – over two hundred square metres had to be lifted and re-laid at the builder’s expense. The matter was settled out of court by mutual agreement.
Over sixty percent of expert witness reports concern porcelain paving disputes. All complaints were caused by a failure to read and follow the instructions provided by the manufacturers.
