Good building practice with natural stone

If built correctly a Limestone building will last for a thousand years, as evident by the many parish churches and cathedrals throughout the UK. If inappropriate mortar is used or other defective building practices are adopted, this may be reduced to 10 years.

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

1. Mortar mixes
1A. Pointing
2 Cavity walls
3. Choosing the course height
4. Using quoins
5. Cavity wall ties
6. Expansion joints
7. Structural steel
8. Building under DPC
9. Types of building stone
10. Part built structures
11. Packaging
12. Deliveries
13. Laying stone safely
14 Storing stone

1. 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 higher 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.

Mortar must be strong enough to physically support all that is built above it, however it must be soft enough to expand and facilitate movement. A good mortar mix should be weaker than the weakest stone laid. Lime only mixes can be adopted! The downside of lime only mixes is that it can be slow to set and may not meet the time expectations of the builder.

Take a look at Pete Fords Recommended Mortar Mix Video, available from the Goldholme website video gallery section.

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1A. Pointing natural stonework

[ps2id id='1' target=''/] The method in which building stone should be pointed is extremely important. The vast majority of limestone pointing is called ‘flush pointing’ or ‘wrag pointing’. This is considered the most appropriate for new limestone. Too much smearing of mortar over the limestone faces should be avoided; sometimes builders choose to highlight pointing giving it a convex ‘V’ shape profile , otherwise described as a beak or double truck joint. This method should be avoided unless matching up with existing. Another style is ribbon cement mortar where the joint protrudes – this should also be avoided. Recessed pointing can create good effect but is not appropriate to Stamford vernacular.

Setting mortar joints should be brushed off in a timely fashion using a churn (stiff) brush. If the mortar becomes too hard it may be necessary to resort to a wire brush which will leave marks spoiling the effect. Colour pigments should be avoided when pointing stonework. New mortar is extremely vulnerable to rain and frost and should be protected immediately. Hessian sacking is the favoured frost protection by builders which is available from builder’s merchants.

Dry stone walls are not laid in mortar nor pointed. The exception may be the coping stones in order to prevent unauthorised removal; these are usually recess jointed.

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2. 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’.

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3. Choosing the course heights of natural building stone

Most manufacturers offer a selection of course heights which can be varied throughout the build. The standard building stone course heights are designed to work to the quoin sizes. Traditionally builders would use the larger heavier stone blocks at the bottom of the build, and diminish in size as rising up the walls. In parts of Yorkshire the standard construction method is to adopt a single course height often quite large and to use only that size of building stone throughout the build. This method of construction would look completely out of character if used in many other parts of the UK. The usual course heights of walling stones are 65mm, 90mm, 115mm, 140mm.

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4. Using quoins in your building

Quoins or no quoins? This is one of the builder’s biggest dilemmas! Quoins are the large blocks of building stone that are frequently used to provide formal corners to a new dwelling. Quoins can be cut to any size. The standard versions are calculated to work with walling stone at a range of standard course heights. Quoins undoubtedly give a touch of class to any new build. They are traditional looking and give great assistance to the builder in terms of plumbing up the walls and speed of construction. Projects built without the use of quoins always appear unfinished and of budget construction.

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5. 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.

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6. 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!

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7. 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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8. 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.

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9. Types of Building Stone Quarried by Goldholme

Goldholme’s group of uk quarries provide a wide variety of building stone, and variations within these two quarries mean that we can match almost any UK regional variation you require. Examples of building stone we can provide are:

Stamford Stone
Clipsham Stone
Lincolnshire Limestone
Ironstone
Oxford Stone
Cambridge Stone
Yorkshire Stone
Cotswold Stone
Grantham Stone
Northamptonshire Sand Stone (Brown stone)
Oundle stone

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10. 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.

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11. Ways in which building stone can be packaged and delivered

Long gone are the days when stone was drawn by teams of Oxen or relied on river barges to reach its destination. Modern building stone is delivered carefully sized and stacked on shrink wrapped timber pallets. These can be off loaded by Hiab crane or Moffit Mounty type forklifts which arrive with the lorry. Dry stone walling or naturally bedded stone may arrive in single use 1 tonne builder bags as favoured by builder’s merchants. Smaller quantities of stone can be collected from your local building stone supplier. Merchant or directly from our works. Building stone is an expensive commodity and should be protected from the elements and avoid unauthorised removal. Stock takes should be undertaken on a regular basis.

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12. Building stone deliveries

When the building stone arrives it is important for the buyer to be satisfied that the correct amount has been received. Any shortfalls or comments should be immediately written on the merchant’s copy of the delivery ticket. Whilst not usual, unauthorised removal by workers does occur creating unnecessary disputes between the customer and the supplier.

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13. Laying natural building stone safely

Do not rise the stone work faster than the mortar can withstand. An experienced builder can advise. It is not unheard of for a newly build wall to come tumbling down as the newly laid building stone buckles under its own weight. A good tip is to ask yourself ‘how high you could stack the stone dry without it collapsing’! Sometimes it’s safer to call it a day rather than risk a collapse. The following day the mortar should be set enough to continue.

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14. Storing your stone

Building stone is an expensive commodity and should be protected from un-authorised removal. It should be protected from rain and frost until such time as required for use. Stone should be stored in a dry condition.

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15. Sample building stone panels

Sometimes it is of great benefit to build a test panel prior to your main building project. These are usually in the order of 1m square. A good merchant should provide such stone without charge. Many local authorities insist on test panels prior to discharge of conditions.

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