Natural Stone laying guide
Natural Stone Laying Guide
Health and Safety
Safe working practices should be employed and followed at all times during the construction process and all necessary Personal Protective Equipment (PPE) should be worn.
Excavation
To allow the new paving to be installed correctly, careful excavation will be required. The depth of this excavation will depend upon the thickness of the required sub- base plus the sand and/or mortar, and the paving flag thickness. An extremely important factor to consider when working out the depth of excavation is that the finished surface level of the paving, when being laid up to an existing structure, must be a minimum of 150mm below the DPC to prevent problems with rising damp in the structure.
Edge Restraints
Edge restraints should be sufficiently robust to resist the lateral displacement from imposed loadings placed upon the pavement and are installed prior to the installation of the sub- base. The restraint must provide a consistent vertical face to a level below the laying course material.
Sub-base
Material Selection
Granular sub-base material should be well graded (40mm to dust) Type 1 quality material. Inferior quality material may be liable to failure under loading and be susceptible to frost or moisture movement. Recycled materials such as crushed masonry or concrete can be considered, provided it is well graded and compacts to give a close textured finish. Materials containing organic matter should not be used.
Creating the base
Base requirements depend of the kind of traffic the paving will be exposed to. As such, a garden path will have different base requirements to a driveway, where the base will fulfil a load-bearing capacity. In the case of a driveway, the concrete base and sand & cement screed base should transfer and spread the traffic load to deeper layers. The integrity of the finished surface is directly dependent on the quality of the base construction.
Sub-base construction
Sub-base material should be placed in layers not exceeding 75mm in thickness or twice the nominal maximum aggregate size. Each layer should be fully compacted before the next layer is placed. Sub-base tolerance to be +5 -10mm from specified levels. The surface should be clean and suitably close textured to prevent migration of finer material through the construction. A minimum longitudinal fall of 1.25% (1 in 80) and crossfall of 2.5% (1 in 40) should be incorporated in the sub-layer construction to provide adequate surface water runoff from the wearing course.
Traffic Category 1: Pedestrian Traffic ONLY
This guide is suitable for footpaths, terraces, and other areas with virtually no vehicle load-bearing requirements.
Material requirements
· Paving or setts: Natural stone with a thickness of 30mm or more.
· Structural membrane: Non-woven geotextile membrane for placement directly on top of the soil substrate.
· Sub-base: A minimum 100mm layer of MOT Type 1 aggregate.
· Laying course (mortar bed): A sand and cement screed, typically a 1:4 ratio of Portland cement to aggregate by volume, with a compacted thickness of 50mm or more.
· Bonding primer: A slurry primer designed for natural stone to be applied to the back of each stone unit.
For 20mm thick paving however, the base preparation needs to be different as the thinner paving material possesses lower compressive and flexural strength. It transfers loads more directly to the layer below. Without a strong, rigid base, a concentrated force could cause the slab to flex and crack. The action of beating or tapping 20mm thick paving on a wet or semi-dry laying course will result in high rise of damage and therefore is not suitable.
Material requirements
· Paving or tiles: 20mm-thick natural stone.
· Structural membrane: Non-woven geotextile membrane for placement directly on top of the soil substrate.
· Sub-base: A minimum 100mm layer of compacted MOT Type 1 aggregate.
· Concrete base: A 100mm base of suitable concrete (C20 grade).
· Bonding primer: A slurry primer for bonding the paving units to the concrete base.
· Flexible adhesive: A high-quality, flexible (S1-rated), cement-based tile adhesive suitable for outdoor use and natural stone, such as Ardex X32.
Traffic Category 2: Pedestrian, Cycle and Light Vehicle Area
This guide is for paving areas that will be subject to vehicles up to 3500kg, such as domestic driveways, cycle paths, and small car parks. The base must be significantly more robust and rigid than for pedestrian-only areas to withstand repeated vehicle loads.
Material requirements
· Paving: Suitable paving units, which may be thicker (30mm+) and more robust than those used for footpaths.
· Structural membrane: Non-woven geotextile membrane for placement directly on top of the soil substrate.
· Sub-base: A layer of MOT Type 1 aggregate, a minimum of 150mm thick, or thicker depending on the underlying soil's strength.
· Concrete base: A 150mm thick concrete base layer, such as a C20 grade mix.
· Laying course (wet screed): A sand and cement wet screed, at least 40mm thick.
· Bonding primer: A slurry primer to ensure a strong bond between the paving and the mortar bed.
Traffic category 4 - Light commercial use, such as a service yard, and residential streets with regular van and maintenance vehicle access
In applications where vehicular use is regularly vehicles over 3500kg, advice should be sought from a structural engineer for the most suitable construction build up.
Paving Construction
Paving units should be cleaned by washing the units with a sponge and clean water.
This is to remove any dust, loose material, packaging, or production aids. To avoid damaging the units, stack the units on timber battens with spacers between them. The backs of the units should be primed using a proprietary priming product or fine mortar slurry prior to placement upon the bedding mortar.
Flags should be supported on a full 'wet' workable mix mortar bed of 1 part cement to 3 or 4 parts building sand. The mortar bedding should be laid according to the recommendations above; however, some adjustment may be necessary to ensure that the units are fully supported and do not rock or move. A PVA, SBR or equivalent bonding agent can be added to the mortar to assist bonding. An appropriate plasticiser can also be considered to be added to the mix.
Keep checking levels and gradient across the units. String lines can be helpful to define levels and lines within the laying pattern. Maintain the correct joint width by use of plastic spacers.
Jointing and pointing guide
Jointing for smooth paving units (non-riven)
· For joints less than 8mm, the use of a wide-joint, exterior-grade grout is recommended.
· Product recommendation: Ardex-Flex FL is a suitable option for achieving a durable and consistent finish in these applications.
· Important note: Butt-jointing or joints under 3mm are not recommended, as the natural variance in stone cutting tolerances can make achieving a uniform joint difficult and may compromise the integrity of the pointing.
Jointing for riven paving and cobbles
· For wider joints (8–15mm), the following specialized options are recommended:
o Traffic Category 1 (Pedestrian): For areas with foot traffic only, consider using a high-quality jointing compound like Nexus ProJoint Fusion
o Traffic Category 2 (Vehicular): For areas that must withstand vehicle loads, an ultra-high-strength epoxy grout such as Nexus ProJoint Titan is required. Nexus ProJoint Titan is suitable for vehicular traffic up to 10 tonnes.
· Traditional method: A robust alternative is a damp mortar mix comprising 1 part cement to 4 parts building sand.
· Cleaning procedure: Any mortar residue on the paving surface must be cleaned off immediately with a damp sponge rinsed frequently in clean water to prevent staining.
· Prohibited method: The practice of brushing in dry or semi-dry sand/cement mixes is strictly prohibited. This method does not result in a rigid, durable joint and often leads to irreparable surface staining on the paving units.
Cutting
Cutting may be carried out using a diamond blacked power saw, mobile bridge sawn or split with hammer and bolster for riven and cobbles. It must however be noted that the aesthetic finish achieved will depend greatly upon the choice of cutting mechanism and level of skill. Specific equipment or blade types should be used when cutting natural stone units as those designed for cutting concrete only may blunt easily. If more than 25% of a flag or slab requires cutting then the remaining piece should be cut from the internal corner of the cut to the external corner of the flag or slab, at an angle of preferably 45°.
Inclement Weather
Laying operations should be discontinued (and any open work face covered) if weather conditions are such that the performance of the paving may be jeopardised. Laying operations should not be undertaken when the temperature is below 3°c on a falling thermometer and below 1°c on a rising thermometer. All unfinished areas and stockpiles of materials should be covered in the advent of inclement weather to prevent saturation.