Good building practice with natural stone

The following guidlines have been produced to help you and your organisation understand the requirements when using Limestone in your construction project.

Storage of limestone goods during winter periods
Part built structures
Patina
Mortar for natural stone
Building with Limestone ‘Cavity walls’
Building with Limestone below DPC
Cavity wall ties
Expansion joints
Structural Steel

 

If built correctly limestone will last for a thousand years or more, this is evident by the hundreds of killometers of dry stone walling which stretches through the cotswolds, rutland and lincolnshire amongst other counties. Also as seen in the many churches and cathedrals which have stood the test of time since medieval times. However, if not built correctly most limestone would be lucky to survive one or two extreme winters. Some beds of limestone are more resilient to frost than others and rely on experienced masons to identify the most suitable stone for application.

Long gone are the days when masons and quarry owners could leave stone out to the elements to ‘prove’ over several winters, but with today’s building technology and a wide range of damp proof materials and membranes in addition to a raft of metal products such as fixings, wall ties, etc. incidents or failure can be minimised by good building practice.

As a rule of thumb, most limestone is very happy with one or two faces to the weather, so long as there are no other forces at work such as rising damp or water soaking from the rear or above.

A common reason for the failure of some limestone is the tarmacing or other hard landscaping of the adjacent ground surface which is allowed to abut the natural stone. This should be avoided. A gap should be provided for rainwater to drain prior to soaking the limestone wall.

For applications such as, in the external walls of a house where the roof keeps it dry from above, and the damp proof course prevents moisture from the ground, no issues should be encountered, providing the stone is allowed to breath with the adoption of a suitable mortar mix (See our recommended mortar mix here). Where natural limestone is required below D.P.C it is advisable to seek advice from our sales team. Also see our technical paper (building with limestone below D.P.C).

Other matters that effect natural stone is quarry sap. This is a natural inclusion in most newly quarried stone and usually takes between 12 months and three years to dry out. When the quarry sap has dried out of the limestone, it’s considered at its hardest, and most weather resilient. On rare occasions, slight spalling from the faces may occur following freezing conditions, this however usually stops after the first one or two years following construction.

Storage of limestone goods during winter periods

During winter periods when freezing conditions are expected, all stored limestone should be protected by a minimum of two layers of dried insulating material. Dry straw or any other suitable material. A polythene outer cover or tarpaulin should be provided in order to prevent water ingress.

The packaging provided upon delivery is not on its own considered suitable for winter protection.

Part built structures

Part built structures and walls are extremely vulnerable to frost damage during the winter periods, and should be suitably covered with sacking or hessian type material (minimum of two layers in extreme frost). Polythene or tarpaulins should be provided in order to prevent water ingress.

Patina

The patina is the act of the newly exposed stone reacting to air which starts to mature the coloration, for example from white to a honey color.

Mortar for natural stone

The besetting sin of much modern pointing is to mix too much Portland cement into the mortar. All stone absorbs a certain amount of moisture in wet weather, and in order to dry out again it must be able to breath freely through the mass in every direction. unless this can happen, some types of stone will fail, and can be adversely affected. Very hard non-porous joints impede this aeration and are therefore wrong.

The aesthetic effect of too much cement in the mixture is invariably bad. A good working mixture for most purposes is ten parts of clean sand to two or three parts lime, beaten up with water, with the addition of no more than one part ordinary Portland cement. The object of adding more cement is to help the mortar to harden, and no more should be added than is sufficient to make sure of that, nor should any be added until just before the mixture is going to be used. On exposed sites and with harder stones, a slightly harder proportion of cement is sometimes acceptable. For historic buildings on the other hand, Portland cement is never right and is no longer used by the department of the environment.

Source: ‘English stone building’ (Alec Clifton Taylor & A.S Ireson)

Hydraulic Lime

Many buildings were built before the use of cement. When re-pointing, alterations or repair of a historic building is required, it may be necessary to use hydraulic lime. One part hydraulic lime should be mixed with two parts sand.

White Cement

The use of white cement can be more aesthetically pleasing than grey cement. White cement however is around 33% stronger than grey which should be allowed for within the mortar mix.

Take a look at Pete Fords recommended mortar mix video

Building with Limestone ‘Cavity walls’

When building with natural limestone, it is essential that air flow is able to circulate behind the stonework. The provision of weep holes at ground level and above all lintels is usually sufficient ventilation. Where insulation batts are adopted, an air gap of 25mm minimum should be maintained for the stone to breath.

Where this is difficult to maintain, a product such as Surecav is designed for this purpose. The cavity should in no circumstances be fully filled with paper based products, polystyrene pellets or any other type of injected insulation. See section ‘Cavity wall ties’.

Building with Limestone below DPC

If building with limestone below DPC, water saturation is likely. Guidance Eurocode 6 design of masonry structures BS EN 1996-1-2 1995 states that the masonry above the first 150mm and below 150mm of ground level are most at risk from frost damage. The builder should be aware to keep the cavity open to 150mm below ground level in order for the stone to breath.
The options available are:
1. Incorporate engineering bricks below the damp proof course.
2. Request highly frost resistant limestone such as a weather bed quality stone from our sales team.
3. Provide underground drainage so that ground water can discharge before rising into the stonework. Remember that the mortar mix should be compatible. See technical paper on mortar mixes. All limestone exposed to freeze thaw requires air flow to the rear of the stone. The cavity should therefore be clear of groundwater, mortar, debris and insulation.

Cavity wall ties

There are hundreds of different wall ties on the market, made in different materials and different strengths. Some are rigid, others flexible. The specification of cavity wall ties is extremely specialist. The prevailing standard is now Eurocode 6  for design and masonry structures. When choosing a tie, the following should be taken into consideration.

• Location of the project within the country and height above sea level
• The likely wind speeds that the ties will have to cope with on a regular basis.
• The thickness of the inner and outer leaves.
• The compatibility of stiffness between the two leaves.
• The width of the cavity and materials being used in the outer and inner walls.
• The type of insulation proposed if any.
• Any backing products such as Surecav.
• The proposed mortar mix.
• The number and position of the proposed ties.
• The type and number of wall ties are extremely important to the integrity of a project.

It would be a good idea to furnish a structural engineer with the answers to the above for further advice.

Expansion joints

Guidance in the Eurocode 6 Part 2 BS EN 1996-1-2 2005 gives the following information.
The maximum horizontal distance between vertical movement joints is recommended as below.

Aggregate concrete blocks and manufactured stone masonry 9m
Autoclaved aerated concrete masonry 9m
Natural stone 20m
The above applies to where the build ratio length V height is 3 to 1 or less. The distances should be reduced where the building design lies outside of this ratio. The use of movement tolerant ties are recommended!

Structural Steel

Sometimes structural steel is required in a stone building. Steel may be required to maintain the structural integrity of stone work. Our staff will not seek the services of a structural engineer unless requested prior to the production of stone products by the client.Costs associated with structural calculations will be passed to the customer. Any request for calculations should be in writing or email.
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