So do you go for natural or reconstructed stone for your stone facade? Which will offer the best whole-life value? Peter Mayer of the Building Performance Group breaks down the options
Using natural stone cladding offers designers an unparalleled pallet of colours, textures and patterns. However, the dimensions of natural stone cladding are limited by the nature of the type of stone and the quarrying and processing techniques. Larger individual stone pieces are proportionally more expensive than small pieces. Prefabricated reconstructed stone and composite stone units provide an alternative to natural stone.
Natural stone cladding
Natural stone cladding design and installation is covered in BS 8298. Minimum thickness of the stone depends on its type, for example limestone and sandstone requires thicker sections than granite or metamorphic stones.
Reducing stone thickness reduces capital costs but increases the risk of premature failure with potentially higher whole-life costs. Reduction in thickness should be based on structural calculations or performance testing. Tests include freeze thaw, porosity, saturation co-efficient and strength tests coupled with evidence in the field.
Recent research into performance in use indicates some thin stone cladding is prone to fatigue failure. Repeated expansion and contraction because of temperature and moisture changes decrease the flexural strength of the stone. This can lead to unacceptable bowing.
Prefabricated stone cladding
Prefabricated or reconstructed stone panels can be manufactured as required to suit the project. See BS 8297, the code of practice for the design and installation of non-loadbearing precast concrete cladding.
Typically two processes are used to manufacture reconstructed stone: semi-dry (also known as cast stone) and wet-cast. Wet-cast reconstructed stone panels generally have higher strength, lower porosity and stronger edges so offer a better whole-life performance.
Reconstructed stone may be homogeneous or comprise a facing and backing material; homogeneous reconstructed stone may be more costly but there is less risk of the layers debonding. Larger panels are manufactured from reinforced concrete with a reconstructed stone surface appearance.
Alternative prefabricated options comprise thin stone veneers applied to a concrete or lightweight backing material such as aluminium honeycomb or steel laminated with epoxy resin. Performance is probably determined by the adhesion between the veneer and backing material.
A prefabricated solution may include the complete rainscreen assembly including the supporting frame, cavity, insulation, vapour control layer and stone cladding.
Whole-life performance and cost issues
Stone cladding should last the design life of a building, from 60 to more than 100 years. Whole-life costs take into account maintenance activities such as cleaning, repointing or sealant replacement. Unplanned costs can be minimised by planning. Issues to watch out for include:
Stone cladding performance assurances may be provided by tests prior to specification or assembly and confirmed in situ once built. Tests include: air permeability, resistance to water penetration, wind, and hard and soft impacts.
Fixings should be non-corrosive, such as austenitic stainless steel, with a grade suitable for the specific environment. The fixings should allow sufficient movement to allow accurate positioning and provide support and restraint without risk of spalling or cracking the stone units.
Moisture control should be detailed; methods include damp-proof courses, cavity trays and weep holes.
Allowances should be made for dimensional movements in response to drying, shrinkage and creep of the building structure combined with thermal and moisture induced movement of the cladding.
Use of sealants
Sealant joints should be primed to prevent plasticisers within certain sealants migrating into the surrounding porous stone. This causes unsightly stains that are expensive to remedy. Sealants have a relatively short life of 15 to 30 years. Sealant reapplication should be allowed for during the life of the building.
Protecting sealants from direct exposure to sunlight and weathering can extend their life. Alternatives to sealants include gaskets or mortar. Neither is maintenance-free – gaskets need replacing and mortar needs repointing. Designing the cladding with open joints avoids these maintenance costs – the cladding becomes a rainscreen relying on a weatherproof inner leaf to the cavity.
Atmospheric pollutants can stain stonework. Rainwater may produce run-off staining; this can be minimised by detailing to throw water away from the cladding surface by incorporating projecting surfaces such as sills and copings. In the long term, it should be accepted that some cleaning may be necessary. BS 8221-1 offers guidance on cleaning.
Stone is vulnerable to impact and chemical and frost damage. Edges chip, frost damage causes spalling, precast concrete may crack or craze superficially or structurally. Where damage is noted the causes should be determined and repairs put in place. BS 8221-2 provides guidance.
Building Performance Group specialises in whole-life performance using software tools to determine best value options based on lifecycle costs, payback and cost benefits analysis.
BLP Construction Durability Database at www.componentlife.com has durability information for building components — and access is free for registered social landlords.
For further information contact Peter Mayer by email at p.mayer@bpg–uk.comor telephone 020-7583 9502.