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Considerations to Reduce Staining on Natural |
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New Build |
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Introduction
Natural stone cladding is selected for its aesthetic appeal and good weathering characteristics, so it is a matter for concern when staining and other blemishes disfigure it. Such problems can usually be avoided by the correct selection and detailing of the stone at the design stage.
This note discusses some of the commoner causes of disfigurement of cladding and gives general guidance. Reference should also be made to BS 8298:1989 "Code of practice for design and installation of natural stone cladding and lining", which deals with the wider questions involved in cladding design. Many of the comments made are applicable to stone masonry generally. Guidance on cleaning stonework is to be found in BS 6270: Part 1: 1982.
Stone is a naturally occurring material and so is subject to unavoidable colour variations which should not be confused with staining.
Minor colour variations are usually far more readily seen in homogeneous light coloured stones than in heavily patterned ones, and are accentuated by some surface treatments.
Close consultation with a Stone Federation member at the design stage is strongly recommended if it is required to minimise colour variations between adjacent panels.
Weathering
Likely changes in colour and texture of the stone, in response to weathering, should be considered. For example, many sedimentary stones retain their colour where washed by rain, but darken where sheltered.
If the intended architectural effect was one of uniformity, then these changes with time would be disappointing, although entirely predictable.
Water Run Off
Conspicuous disfigurement and/or staining can be caused by the uneven washing of cladding by streams of water run-off from glazed areas, sills and other projections. Sills should be provided with adequate stooling, or other efficient end stops.
Textured surfacing to the stone will help to distribute water run-off by breaking up localised streams, but this is only of limited effect.
Considerations To Reduce staining on natural stoe facades
Water Absoration
A number of light coloured stones, including several imported granites, darken noticeably when wet due to water absorption. Uneven we from whatever cause, gives rise to a patchy appearance which can take a long time to dry out, depending upon the weather.
Stones prone to this problem are frequently known by name, but can usual be recognised as being susceptible to darkening because they have a relatively high water absorption and high absorption rate.
Rising Damp And Splash back
Cladding panels close to ground level can exhibit a ‘tide-mark' either due to rising damp or splash back. Rising damp can be drawn from almost anything placed against the cladding, .including apparently impervious things such as steps, if they channel water back to the cladding.
Disfigurement of this sort can be avoided by using plinths of impervious and preferably dark coloured stones.
Metal Staining
Adjacent metals, for example lead and bronze, and lightweight metal fixtures attached to cladding can cause staining, due ether to the corrosion of the metal or to run-off. It is absolutely essential to fix letters etc, to the cladding, then only stainless steel should be used.
Poor quality scaffolding erected close to stonework can give rise to rust staining as can metal filings from adjacent on site cutting operations.
Timber Staining
The run off from virtually all unsealed hardwood window frames and sills will stain most stone claddings.
Loose timber, especially when treated, should not be stacked on or against stone.
Spacer DAB ‘Shadows’
Spacer dabs behind panels can lead to unsightly damp patches on Some light coloured marbles and granites. These patches usually come and go with wetting and drying, and frequently disappear completely with time, although isolated persistent problems are known.
The precise mechanism is unclear, but a waterproofing coating on the back of the panel at erection normally eliminates the problem.
Sealant Staining
Oil based mastics must be avoided, but even some silicone sealants can give rise to darkening of the adjacent panels in the immediate vicinity of joints,due to absorption of the silicone fluid .A test for stability is given in BS 3712 : Part 2: 1973.
Alge and Lichen
Damp north facing walls and sloping sandstone surface can develop algae and lichen growth .Coarse textured finishes should be avoided in these locations |
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