Essential items which need to be covered by the project team when specifying natural stone.
It is important at the design stage to understand the nature and characteristics of the preferred stone types and how they can be used to their best advantage. When deciding which stone to use, as well as choosing the required colour and finish of the material, other important considerations are its strength, durability, porosity, water absorption and expected weathering characteristics. It is also important to have assessed such things as the potential effects of salt contamination, frost action and nearby vegetation growth.
To assist with selection, it is always necessary to obtain the following information on each stone under consideration:
This is an essential starting point for the evaluation of the stone, and it can determine which other tests might be appropriate. It will, (a) identify the presence of any potentially unstable minerals, such as clay pockets or clay-like inclusions that may undergo volume changes upon saturation and drying, thus adversely affecting the stone’s durability and performance, (b) show the presence of any iron-bearing minerals which are always prone to oxidisation and can cause discolouration of the stone over time, (c) identify potentially frost susceptible stones.
Initially it is important to see, and analyse, any existing technical data and test results for the stone and to establish the date when this information was obtained and from whom.
All natural stone testing must now be to British / European Standards (BS EN) and be undertaken by an independent and accredited testing laboratory. These test results must be included in a CE Marking Certificate which identifies the geological name of the stone, full location details of the quarry and a petrographic description (denomination), all as required by reference standard EN 1469 (Natural stone slabs for cladding). The certificate also requires specific test results, so it is essential that the following tests are commissioned and undertaken on all the stones being considered for use on the project.
- Density and porosity (BS EN 1936)
- Flexural strength – wet and dry (BS EN 12372)
- Freeze-thaw testing / frost resistance (BS EN 12371)
- Water absorption and saturation coefficient (BS EN 13755)
- Compressive strength (BS EN 1926)
- Breaking load at dowel hole / resistance to fixings (BS EN 13364)
The CE certificate also makes reference to the stone’s resistance to fire and its thermal shock resistance which are dealt with separately by me, as stone consultant.
As you will no doubt be aware, as from 1st July 2013 CE marking is mandatory and it is the manufacturer’s responsibility to have a CE certificate in place for all products supplied into the UK market, together with a signed Declaration of Performance (DoP) for each product. Whilst some UK quarries are fully compliant and will be able to submit the required information to the design team, some still need assistance to conform to the regulations.
Alongside testing and viewing existing information and data, it is important to consider the stone’s history of use by arranging visits to a few similar projects, in similar locations. This allows an assessment of the colour characteristics and general weathering. Ideally five reference buildings should be viewed.
Once the above is done and everyone is happy with the stone, then it’s time to move onto the assessment of the quarry and production facilities, which can, to some degree, dictate what and what isn’t going to be possible, in terms of stone design and detailing. This will usually include an understanding of (a) block sizes and general availability, (b) variations in colour and texture of the stone, (c) finishes available, (d) weekly expected output, (e) quality control procedures and manufacturing tolerances, (f) quarry storage facilities, and (g) packaging and delivery processes.
It will also be necessary to assess the quarry’s environmental policy and its commitment towards sustainability, responsible sourcing and protection of the environment generally. Such things as effective water recycling, reducing CO² emissions, land restoration etc. are extremely important if the design team wish to create a BREEAM certified building.
It is essential that key-marked masonry working drawings are produced, using the architect’s drawings and details, together with a set of production schedules and templates which will be required by the stone supplier for manufacturing. These detail each individual stone on the project and may be accompanied by pallet tickets identifying which stones are to be packed together for a given area of stonework. This can greatly assist the stone masons on site when fixing.
The masonry drawings will take account of the production capabilities of the quarry and any limitations of the selected stone, with good detailing to ensure effective water run-off. Other important aspects include the design and positioning of stainless-steel fixings and support angles, positions of DPC’s, cavity trays, weepholes, cavity closers, fire-stop barriers, insulation and the like, together with details showing the interfaces between the stone and the backing structures. In particular it is now essential that stone designers provide construction details which comply with Part L of the Building Regulations, in respect of thermal insulation requirements and the need to eliminate cold bridging. This regulation came into force on 6th April 2014. The drawings will also make provision for differential movement (thermal and moisture), by the inclusion of movement joints, as appropriate.
The success of every job depends on the quality of this information and it is therefore essential that it is undertaken by a skilled and experienced company who specialise in masonry detailing and design.
It is always recommended that a reasonably sized sample panel (say 2m x 1.5m minimum) is built out of each stone being considered – preferably side-by-side on site. These should be designed to include examples of all the finishes required on the building and also include feature stones, such as quoins, window surrounds, profiled string courses and coping. The quarry must ensure that the panel is representative of its current extraction, in terms of colour and texture of the stone, and special finishes must also be an accurate representation of what can, and will, be achieved throughout the supply period.
The panel can also be used to set and agree the quality of workmanship required and, very importantly, the requirements for jointing and pointing. Ashlar buildings will typically have 5mm wide, flush pointed mortar joints throughout and the panel should be used to ensure the colour, texture and finish of the joints is to the required standard. Once approved by the client and architect, the main contractor and masonry contractor will use the panel as a control sample throughout the project. As such, it should remain in place on site until completion and final hand-over of the finished stonework has taken place.
Invariably once the stone is selected and specified, one of the first questions a client asks is ‘what is natural stone going to cost.’ An easy question, but not that easy to answer. It takes considerable skill and experience to ascertain a meaningful and accurate cost. This can only be achieved when the design is finalised and stone choices and finishes have been made. We specialise in providing professional assistance to the cost planning team to establish accurate costs for all the stonework elements. This may be just the supply-only price, or a comprehensive design, supply and installation figure, including stainless steel fixings and all essential ancillary/sundry items. It may also extend to a variety of different stones for both external and internal use, in several applications, utilising different fixing methods e.g. walling, ashlar cladding, traditional masonry, internal stairs, wall cladding and flooring.
This includes planning and undertaking the following operations.
- Site logistics – storage and accommodation requirements
- Access and scaffolding requirements
- Health & safety documentation, including risk assessments, method statement and COSHH data (RAMS)
- CDM regulation compliance and special lifting requirements
- Construction programming
- Supplier assessments and ordering of materials
- Stone fixing and installation operations
- Final inspection and hand-over procedures
- Preparation of an Operation & Maintenance manual
To achieve high quality workmanship, it is vital that the stone fixing is undertaken by skilled and experienced masons, rather than bricklayers or those who deal with stone ‘now and again’. Bespoke stonework needs to be carefully handled and is fixed using proprietary stainless-steel restraint and support fixings (cramps, dowels, angles, and the like), and when specifying 5mm mortar joints it is essential that the stone is bedded and pointed correctly, with careful cleaning off of the joints as the work proceeds.
Needless to say therefore, it is important to appoint a main contractor who has undertaken similar projects using natural stone and is happy to work alongside/engage the services of a specialist masonry sub-contractor. As the stonework is post-fixed, generally the internal block walls and structural steelwork can be constructed in advance of the external stone walls and features.
A detailed programme will need to be agreed which incorporates time for design, the lead-in period required by the quarry, the production period for the stone and the on-site fixing/installation. It is impossible to estimate timescales for any project until the design work is completed, but typically working drawings and cutting schedules may take upwards of 3 to 4 months to complete; the quarry is likely to require a minimum 10 to 12 week lead-in period and the overall production period will be determined by the size and nature of the project and how many cubic metres of stonework is required. It is not unreasonable for a quarry to require a 12-month production period for a substantial house. Clearly there can be a degree of overlap between the various processes, but professional input is required to ensure timescales are properly estimated once the design and costs are finalised, leading to the preparation of a realistic construction programme which, in turn, can be agreed and incorporated into the contract documents in a suitable form.
This completes the summary of recommended processes, and we would be more than happy to elaborate on the kind of services Graham Goldthorpe Stonemasonry Ltd can offer, if you feel this might be of benefit to your project. If necessary, we can obtain references from clients, architects, private quantity surveyors, contractors and suppliers with whom we’ve worked in the past or give you their details to enable contact to be made direct.