Kentish Ragstone in Modern Construction
Kentish Ragstone was one of the traditional stones used for the construction of walls in London and the southern counties of England for thousands of years. The traditional methods and skills were well understood by all involved in the design and construction processes of buildings. However, the introduction of new cheaper materials and methods, mostly after the Second World War, has meant that the use of stone in new build has markedly declined.
Currently, natural stone is used predominantly as thin cladding to load-bearing structures and as flooring, paving and other decorative surfaces.
Where once natural stone was used in a load-bearing context, it has been replaced by concrete block, brick and timber frame construction, with cast stone features to replicate the appearance of natural stone. As a result, building in natural stone has become a specialist area of expertise for both design and construction and the traditional knowledge base has been eroded.
In addition, there is an increasing awareness that the modern substitute materials, while well covered by standards and tests, do not always weather in the same way as natural stone, with longer-term detrimental aesthetic results.
As a result, the use of natural stone to give a traditional appearance to buildings is increasingly a requirement of the planning process for new developments in sensitive areas.
However, there are examples of new buildings in which natural stone has been used in a way that has not reflected traditional design methods, resulting in premature soiling of the façade and potentially longer-term deterioration of the stone. This merging of traditional methods and materials with modern methods and current structural design has resulted in some failures.
Stone can contribute to sustainable development. When used well it is a durable and long-lasting material.
Codes of Practice
Understanding the relevant British and European Standards is a vital ingredient in the competent design and construction of stonework in new build construction, more so than for repair and maintenance work.
The move towards the harmonisation of standards throughout Europe has had, and increasingly will have, implications for the specification, selection and testing of natural stone and related components and materials.
Lead Times & Availability
The total lead-time for the delivery of stone to a project should not be underestimated and the need for timely forward planning is an essential feature of the process. While many projects operate on a very tight timescale, a fact that project professionals are well aware of when planning a project, there is sometimes a tendency to overlook or underestimate the time required for stone to be available on site to facilitate the project programme.
Kentish Ragstone as a building material demonstrates a number of important characteristics; it has a long life, it is highly durable and requires low maintenance over its life and additionally has a high thermal capacity.
In sustainability terms when compared with, for example concrete and concrete block or brick construction, natural stone for new-build construction shows an advantage in that fewer energy resources are consumed during the material production, transportation and construction processes.
Ashlar stone generally has fine joints thus reducing the volume of mortar used. In addition when natural hydraulic lime is used as a binder, the energy used in mortar production is less than that of cement mortar.
In terms of embodied energy and embodied carbon, it is possible to compare natural stone with other related building materials. Hammond and Jones (2006) have conducted an inventory of embodied carbon and embodied energy within the boundaries of Cradle to Site. For example, the embodied energy and embodied carbon values for facing bricks are 8.2 MJ/Kg and 0.3 – 1.4 KgCO2/Kg respectively, autoclaved aerated concrete blocks are 3.5 MJ/Kg and 0.28 – 0.375 KgCO2/Kg and for limestone 0.24 MJ/Kg and 0.012 KgCO2/Kg.
Special requirements that could affect stone selection can be the heights of beds/lanes of stone in the quarry - this will impact on the size of stones that can be extracted and cut to produce dimensioned stone. For example, shallow beds and natural vents in the quarry will restrict the amount of large masonry blocks available to produce elements such as mullions, lintels and sills where the direction of the bedding planes must be correctly oriented.
Provenance, denomination and test certification
Because much of the stone currently available is imported from many different parts of the world, the origins of the stone may be obscure. It is necessary, as a result, to ensure that the stone specified and delivered to the site is of known provenance and the denomination is clearly stated on the accompanying certification. Denomination of the stone as defined in BS EN 771-6:2005 shall be in accordance with EN 12440, which means that the traditional name, petrological family, typical colour and place of origin shall be declared.
Stone where there is no defined denomination should not be used as its performance in use may be unsatisfactory. This is essential to ensure consistency of material and its future performance.
All stone specified in accordance with BS EN 771-6:2005 should be accompanied by CE marking and labelling that confirms the details of the relevant essential characteristics.
Construction (Design and Management) Regulations 2007 (CDM2007)
The designer should be aware of the implications of CDM when designing individual stone units. Because of the mass of stone there will be handling requirements to be considered, which will require appropriate scaffolding, support and lifting equipment to be in place.
This may influence the design of individual stone units so that handling costs are controlled. It is recommended that the advice of the stone supplier is sought at an early stage in the design process.
Types of Finish
Stone selection will be influenced by the type of finishes planned for the stone: a smooth-faced ashlar finish, typically with tight joints, will require a stone that can be produced with fine tolerances, whereas a rough-textured finish on rubble stonework is less demanding in terms of joint thicknesses and tolerances.
A wide range of surface finishes is available, some of which are produced by machine and others are finished by hand tooling. In some cases a finish may be required to match existing stonework.
It is important to ensure that samples of the finish required are agreed with the quarry or supplier before the specification is prepared, so that both the quarry and masonry contractor fully understand and can accurately cost the stonework element.
The choice of walling type is influenced by:
- the design of the building,
- technical considerations (whether the wall is load bearing or non-load bearing),
- its location,
- site exposure, and
- aesthetic considerations.
While it is possible to replicate the appearance of a modern stone facade with that of a traditional building, the use of natural stone in a new building or structure today will generally utilise wall construction materials and techniques that reflect current design criteria, as contained in British Standards and British European Standards (particularly BS EN 771-6:2005 and BS EN 1996-2:2006) and in the Building Standards.
Most new-built stone walls are therefore not of traditional construction but are a combination of stone prepared using modern tools and machinery with backing structures of concrete, blockwork or brickwork or timber or metal frames. The incorporation of appropriate levels of thermal insulation within the wall construction also impacts on the design of the wall.
For the stonework itself, the choice often reflects the degree of formality and appearance required of the facade, which can be influenced by its location; more formality may be required in an urban facade than one in a rural setting.
The choice is between ashlar (more formal) and random rubble (less formal).
The rough stone blocks from the quarry are passed through sawing processes which produce the dimensioned ashlar for normal work. The stone is then supplied to the site in pallets or bulk bags. Alternatively, the stone may be further worked by hand or machine to produce special finishes and shapes for mouldings, cornices and the like.
Ashlar blocks can be supplied in a range of finishes but not all stone types are suitable for every finish. A hard dense stone such as Kentish Ragstone can have a highly polished finish, along with smooth, fine-rubbed and honed finishes. It is recommended that the designer discusses the stone selection and finishes required with the stone supplier at an early stage in the design process and before detailed specifications are drawn up.
The course heights of ashlar stonework may be equal or random. A 300 mm course height is typical for high-quality ashlar and provides a traditional appearance to the stonework. However, courses of this height will not coincide with the course height of a concrete block backing and will mean that standard wall ties will not be appropriate: bolted fixings will be necessary.
The stonework may be designed with 225 mm course heights so that the coursing coincides with the coursing of the backing material, which permits the use of standard wall ties. This will facilitate the building process and the positioning of wall ties between the facing and backing structures. The coursing is further complicated by the provision of fine joints (nominal 5 mm) in the ashlar and wider joints in the backing blockwork or brickwork. Course heights are also dependent on bed thickness from the quarry.
There is a variety of rubble walling types found in traditional buildings, most of which can be reproduced in new buildings to suit the aesthetic qualities required.
The varieties of walling are determined by the type of stone and the way in which it is extracted from the quarry (i.e. the sizes and shapes available), which influences the way it can be dressed.
Normally, the stone used should be small enough to be placed by hand. The advice on size of stones contained in the old British Standard Code of Practice CP.202: 1951 is still relevant:
‘For general purposes, the length of the stone should not exceed three times its height, and the breadth on bed should be not less than 6 in. [150 mm] and not greater than three-quarters of the thickness of the wall.’
The above quotation applies to traditionally-built rubble walls where the stone wall carried all the structural loads. However, today, in new build the rubble element may be used as a facing only, being supported and backed by an internal loadbearing structure (or an external structure when the rubble forms an internal finish). The stones, therefore, may have to be cut to a thickness to make them suitable for incorporation into modern new-build construction.
It is common in modern practice for the rubble stone to be manufactured from six-sided sawn blocks, the exposed face is then ‘pinched’ by machine to produce a rough face, which is then hand-trimmed round the edges to give a rough protruding face to the block. However, the bed and, sometimes, perpend faces are left as sawn.
Random Rubble Walling
The following commentary is a summary of fuller descriptions contained in BS 5390. The types described are indicative only as there are many other regional variations that may be adapted to modern construction.
a) Un-coursed. - Blocks of all shapes and sizes are used and placed in a random manner but in such a way that a bond is achieved, often levelled up with small stones or pinnings. The mason will roughly trim and shape the stone by knocking off protruding edges.
b) Brought to courses. - Similar to un-coursed but the work is roughly levelled to courses with heights of between 600 mm and 900 mm.
Squared Rubble Walling
Un-coursed. - The stones are roughly squared of varying heights, and are laid un-coursed. If small stones are introduced to assist bonding they are referred to as ‘snecks’.
Galleted rubble. - The walling consists of roughly squared rubble but with no limitations to size as is the case with un-coursed squared rubble. It is specifically designed to include gallets.
Brought to courses. - The stones are similar to those used in galleted rubble, but the work is levelled to courses varying in depth from 300 mm to 900 mm, usually to coincide with quoin or jamb stones.
d) Coursed. - Walling built in courses which may vary in height from 100 mm to 300 mm (225 mm average) but the stones in any one course are roughly squared to the same height. Stone faces may be finished as rock-faced or smooth. This type of rubble walling is more readily accommodated where there is a brick or block inner leaf or backing structure, which makes for easier insertion of wall ties.
The traditional form of external stone wall construction was invariably a thick wall (around 600 mm or more in thickness), carried the structural loads from the building (floor, roof and wind loads), was permeable to water and relied on its thickness to prevent moisture transfer to the inside surface. The external stone finish was either ashlar or rubble, of various types, or a rubble wall, often with stone dressings around openings and corners (jambs and quoins). A dressed stone base plinth course is common with all wall types.
While walls of this type can still be constructed, their construction is labour intensive and requires large quantities of stone and mortar. For this reason the traditional forms of wall construction are now employed only in special circumstances, such as for the reconstruction of an existing wall or in an extension to a historic building where the maintenance of authenticity and character is important.
STONE WALLING SYSTEMS FOR MODERN CONSTRUCTION
When built using mortar as the jointing material, the use of natural stone in modern walling systems requires knowledge and understanding of both materials and structural design that are not fully defined within the current British Standards and British European Standards. In a modern context, natural stone, whether ashlar or rubble types, is used either as:
- part of a composite loadbearing wall where the stone carries part of the imposed loads; or
- as a facing to the main supporting structure, carrying wind loads only.
Ashlar-faced cavity wall
In cavity wall situations the ashlar outer leaf does not carry structural loads from the building, apart from its self-weight and resistance to wind loads. For practical purposes, the width of the stone used in an outer leaf should be as follows:
i. A minimum of 100 mm for single-storey-height walls (3 m maximum height) to allow an adequate mortar joint to be formed.
ii. The width of stone outer leaf should be increased to a minimum of 150 mm for walls up to two storeys or 6 m high.
These thicknesses are derived from BS 5628-3: 2005 and take into account the influence of lime mortars and the standard production tolerances for stone block thicknesses allowed in BS EN 771-6.
Polished and smooth faced ashlar is traditionally designed with nominal 5 mm wide joints and, when the appearance of traditional ashlar is required, this joint width should be maintained. However, the use of 3mm diameter wall ties, or other modern wall ties, cannot be accommodated within these fine joints. For this reason it is usually necessary to provide rebates in the bed faces to accommodate the wall ties. Also, the smooth saw-cut bedding faces of the stone provide limited bonding with the jointing mortar. This means that it is usual to consider wider joints for those situations where such joints can be tolerated and which can support standard wall ties without the need for rebates.
Because of the generally fine joints used with polished and smooth-faced ashlar, accommodation of wall ties may require the use of joggle bed joints, with the wall ties set into rebates and turned down into the joggles, or dowels may be used as an alternative fixing. Other types of finishes are available and may be used with wider joints which permit full bedding of the wall ties.
Rubble-faced cavity wall
The rubble facing to cavity walls can be formed in two ways (Figures 14(c) and (d)).
i) In Figure 14 (c) the cavity wall is constructed using two leaves of brick or blockwork; the rubble stonework is then built as a facing against the outer leaf and tied with wall ties to the outer brick/block leaf.
ii) In Figure 14 (d) the rubble stonework is the outer leaf of the cavity wall and tied back to the inner leaf in the normal way.
Ashlar-faced solid wall
With this wall type, (Figures 13 (a) and (b)) the dressed stone is attached using stainless-steel wall ties or brick reinforcement to a concrete block or brickwork background structure. Whether or not the stone facing acts compositely with the background structure and carries structural loads depends on the way in which the stone facing and backing structure are bonded or tied together. Normal cavity wall ties will not be sufficient to enable a full transfer of load to take place between the two leaves of a solid wall. The stone is essentially designed to carry its own weight and to resist wind loads. To ensure composite structural action it is necessary to build the wall with bonder stones built into the backing structure.
Gallagher is able to provide a number of different surface finishes and textures to dressed stone to suit a range of joint widths. It is advisable to consult the stone supplier or specialist on the most appropriates stone finishes for particular joint widths.
Rubble-faced solid wall
In general the issues discussed above apply equally to rubble faced solid walls (Figures 14(a), (b)). Unless the stone is bonded into the backing structure, the stonework acts only as a decorative finish and is designed to carry its own weight and to resist wind loads.
Because the stone is only roughly dressed the joints will generally be wider than for ashlar work. The consequences of this are that the minimum width of the stone is increased to 150 mm to allow an effective mortar joint to be formed: wall ties can be easily accommodated within the joints.
A fully load-bearing ‘traditional’ rubble wall is shown in Figure 14(e).
Brick or block wall with dressed stone features
A common use of natural stone is as exposed features incorporated within brick or block walls with cement-based rendered external wall finishes. Typically the stone is featured as dressed quoins, margins around openings, mullions, lintels and sills, base courses, wall-head courses, string courses and the like.
When used in this context the following points should be noted -
a) At openings, ties between outer and inner leaves in cavity walls, and between a stone facing and backing structure, should be positioned at centres not exceeding 300 mm vertically and within a maximum distance of 225 mm from the edge of openings.
b) Stones forming margins at openings and quoin stones should either be bonded or tied at every course, using stainless-steel ties, into the brickwork or blockwork of the adjoining outer leaf.
c) Where the surfaces of stone comes into contact with concrete blocks or bricks, the block or brick backing should be coated with lime-mortar slurry to prevent staining of the stone.
Stone facing to retaining wall
Stone should not be used as a facing to a retaining structure where the stonework is in direct contact with the main structural element. This is to prevent staining of the stonework and water penetration (with accompanying salts) from the retained soil. Similarly, stone should not be used as permanent shuttering to in-situ concrete. Natural stone is thus used as a facing with a drainage cavity behind to avoid saturation of the stone and to relieve the stone from any hydrostatic pressure.
SUMMARY OF STRUCTURAL DESIGN ISSUES
Ashlar-faced solid walls (Figures 13(a) and (b))
Stone and wall thicknesses
The minimum thickness of an external wall given in BS 8103-2:2005.
Code of practice for masonry walls for housing is 190 mm, which could be increased if the ground floor does not provide effective lateral support and where the top storey height gable lateral support is only along the slope. This BS is limited to the structural design of low-rise buildings. For larger and taller buildings minimum thicknesses should be determined by the application of BS EN 1996-1-1:2005 Eurocode 6 – Design of masonry structures or BS 5628-3:2005 Code of Practice for the use of masonry.
In the case of ashlar-faced solid walls, it is recommended that the minimum thickness of the stone facing is 100 mm for stone supported at every storey (3 m maximum), backed by 215 mm brickwork or blockwork to give a minimum thickness of 325 mm.
Ashlar joints typically joints in polished or smooth-faced ashlar are nominal 5 mm, which means that normal 3 mm diameter wall ties can be accommodated within the mortar bed. Rebates in the stones can be provided to accommodate the ties. Other types of surface finish, such as rock or split faced, may be provided with wider joints that allow full bedding of wall ties.
Bonding or ties between facing and backing -
For a solid stone-faced loadbearing wall to act as a single structural unit there must be an effective bond or tie between the facing and backing structures. Because of the relative slenderness of this form of wall relative to traditional walls and, typically, the lack of bonder stones, wall ties should be positioned in every course of stonework and at each stone (maximum spacing 900 mm).
Water penetration -
100 mm thick Ragstone facing will normally resist water penetrating directly through the stone from the outside face. Water penetration through both porous and non-porous stone types typically occurs at the joints by capillary action through the mortar or through shrinkage cracks in the mortar and can then penetrate to the rear face of the backing structure if it is not dense concrete. The insertion of a waterproof membrane into the wall, for example, painting the outside face of the backing structure or the inside face of the stone with bitumastic paint is not recommended.
Direct penetration of moisture may occur through poorly detailed and/or executed features such as window mullions, window jambs and at horizontal surfaces of sills, copings, cornices and string courses.
A gap should be left between any internal finishes and the inside face of the structural wall to allow for dissipation of any penetrating moisture.
Staining of stone -
This can occur on a porous stone when the backing structure is in contact with the stone. It is a particular problem with concrete blocks built in cement mortar. The problem can be mitigated by coating the front face of the background with lime-mortar slurry.
Rubble-faced solid walls (Figures 14 (a) and 14 (b))
Stone and wall thicknesses
The minimum thickness of a random rubble faced external wall given in BS 8103-2:2005 Code of practice for masonry walls for housing is 250 mm, which could be increased if the ground floor does not provide effective lateral support and where the top storey-height gable lateral support is only along the slope. This BS is limited to the structural design of low-rise buildings. For larger and taller buildings minimum thicknesses will be determined by the application of BS EN 1996-1-1:2005 Eurocode 6 – Design of masonry structures or BS 5628-3:2005 Code of Practice for the use of masonry.
In the case of rubble-faced solid walls, it is recommended that the minimum thickness of the stone facing is 150 mm for stone supported at every storey (3 m maximum), backed by 250 mm brickwork or blockwork to give a minimum wall thickness of 375 mm.
In the case of a traditionally-built rubble wall (Figure 14(e)) using only stone and mortar the minimum thickness of the wall, excluding internal lining, should be 400 mm. This is because there will be difficulties in bonding within the wall if the thickness is less than 400 mm.
Rubble joints - because of the uneven nature of the stone, joints in rubble work are of variable thickness, typically in excess of 10 mm. This permits full bedding of the stone and accommodation of wall ties.
Bonding or ties between facing and backing -
In the case of rubble walls the stone outer facing does not typically perform a structural function but must, nevertheless, be tied back to and supported by the structural background. In general the minimum spacing of wall ties should in accordance with BS 8103-2:2005 for low rise domestic buildings at a density of 2.5 ties/m2 placed in a staggered pattern; increased at openings, roof verges and adjacent to vertical movement joints. For other situations and conditions the density may have to be increased as required by the structural design.
For walls where the rubble stone element is bonded to a backing structure (Figure 14(b)) and is thus loadbearing, the minimum depth of stone on bed should be 150 mm and bonding stones, with a spacing of 1/m2 in random work and at one metre horizontal and vertical centres in coursed work (BS 5390).
Water penetration -
As with ashlar, water penetration will typically be through the joints.
However, the increased thickness and volume of mortar may cope better with small rainfall events than ashlar work. This is especially the case with granite and other non-absorbent stones. Shrinkage cracks within the mortar joint will permit ready transfer of moisture to the background structure.
Ashlar-faced cavity wall (Figures 13(c), (d) and (e))
Stone and wall thicknesses
The minimum thickness of an external cavity wall given in BS 8103- 2:2005 Code of practice for masonry walls for housing is 230 mm (two leaves each 90 mm and a 50 mm minimum cavity), which could be increased if the ground floor does not provide effective lateral support and where the top storey height gable lateral support is only along the slope. This BS is limited to the structural design of low-rise buildings.
For larger and taller buildings minimum thicknesses will be determined by the application of BS EN 1996-1-1:2005 Eurocode 6 – Design of masonry structures or BS 5628-3:2005 Code of Practice for the use of masonry.
For practical purposes it is recommended that the minimum thickness of natural stone in ashlar-faced cavity walls should be 100 mm. The thickness of the stone outer leaf may have to be increased to suit the type of stone used or to create a finish to coincide with the appearance of existing work.
Ashlar joints -
Typically joints in polished or smooth-faced ashlar are nominal 5 mm, which means that normal 3 mm diameter wall ties can be accommodated within the mortar bed. Rebates in the stones can be provided to accommodate the ties, with the ties turned down into a joggle bed joint. Other types of surface finish, such as rock or split faced, may be provided with wider joints that allow full bedding of wall ties.
Bonding or ties between facing and backing
Stainless-steel wall ties appropriate for user category to BS 5628-1 and EN 845-1. Note: BS 1243 for butterfly wall ties was withdrawn in 2005. It is advisable to ensure a mechanical fix between the tie and the stone.
This could be by the use of a dowel into both the stone above and below or by a joggle in both bed faces with the tie turned down into the lower stone.
For a cavity wall over 9 m or three storeys high the stone outer leaf should be supported on a stainless steel angle fixed to the structure at each floor (Figure 14(e)). A movement joint is required below the support angle.
Water penetration - where water penetration occurs it is typically through the joints (joggles can help in this respect) and at poorly detailed openings. Water may then run down the inside face of the stone. Provision should be made for cavity drainage at the base of walls and where the cavity is bridged by support angles and other features.
Cavity ventilation -
Where cavity insulation is provided it is advisable to control dampness in the cavity (either from external penetration or interstitial condensation) by introducing cavity ventilation.
Rubble faced cavity wall (Figures 14(c) and (d))
Stone and wall thicknesses
There are two main options available for the construction of the outer leaf:
a) A 150 mm minimum thickness of rubble facing tied to a 100 mm or 150 mm brick or block outer leaf to provide a composite outer leaf of 250 mm minimum thickness: a total minimum masonry wall thickness of 400 mm depending on the width of the cavity (Figure 14)
(c)). This method allows the brick or block cavity wall to be built before the stonework, which makes the construction of the stone element less critical in terms of time.
b) The 150 mm minimum rubble stonework is not backed by a brick or block outer leaf and is tied back directly to the inner structural leaf.
(Figure 14(d)). The rubble leaf is normally built in parallel with the inner leaf. All stones in the rubble outer leaf must extend to the full width of the leaf, unless the stonework is at least 200 mm in thickness.
Rubble joints -
Because of the uneven nature random rubble, joints in rubble work are of variable thickness, typically in excess of 10 mm. This permits full bedding of the stone and accommodation of wall ties. Squared rubble also has joints of at least 10 mm and allows full bedding of wall ties.
Bonding or ties between facing and backing
In the case of rubble-faced walls the stone outer facing does not typically perform a structural function but must, nevertheless, be tied back to and supported by the structural background. In general the minimum spacing of stainless-steel wall ties should in accordance with BS 8103-2:2005 for low rise domestic buildings at a density of 2.5 ties/m2 placed in a staggered pattern; increased at openings, roof verges and adjacent to vertical movement joints. For other situations and conditions the density may have to be increased as required by the structural design.
As with ashlar work, for a cavity wall over 9 m or three storeys high the stone outer leaf should be supported on a stainless steel angle fixed to the structure at each floor (Figure 14(e)). A movement joint is required below the support angle.
Water penetration -
Water penetration in the wall type shown in Figure 14 (c) is unlikely to be an issue, even in severe conditions. However, even if it is still likely that water will be able to penetrate to the inside face of the composite outer leaf. This means that cavity drainage would only be necessary in the most severe exposure conditions. Where cavity insulation is incorporated, cavity ventilation should be provided.
In the case of the wall type shown in Figure 14(d), water can more easily reach the inner face of the rubble leaf. Cavity drainage and ventilation, where appropriate, should be included.
Staining of stone –
For the wall type in Figure 14(c) the face of the concrete block or brick backing should be coated with lime-mortar slurry.