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 guidelines have been produced to help you and your organisation understand the requirements when using Limestone in your construction project.
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)
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.
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.
Where an air-entraining agent is used or plasticiser too much additive should be avoided as excessive air in the mortar can result in water penetration. The air content within mortar should be no greater than 18 to 20%.
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.
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’.
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 65 mm, 90 mm, 115 mm, 140 mm.
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.
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 BS EN 1996-1-2 1995 (Eurocode 6 for design and masonry structures). When choosing a tie, the following should be taken into consideration.
It would be a good idea to consult a structural engineer with the answers to the above for further advice.
Guidance in the BS EN 1996-1-2 2005 (Eurocode 6 Part 2) gives the following information.
The maximum horizontal distance between vertical movement joints is recommended as below.
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!
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.
If building with limestone below DPC, water saturation is likely. Guidance BS EN 1996-1-2 1995 (Eurocode 6 design of masonry structures ) states that the masonry above the first 150 mm and below 150 mm of ground level are most at risk from frost damage. The builder should be aware to keep the cavity open to 150 mm below ground level in order for the stone to breath.
The options available are:
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.
Northamptonshire Sand Stone (Brown stone)
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.
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.
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.
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.
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. The use of water saturated stone should be avoided during winter months, particularly when freezing conditions are expected.
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.
Any steel dowels placed into the stonework should meet with BS EN 1996-1-2 (Eurocode 6) or any subsequent British equivalent. Metals should be corrosion proof. Drill holes should not allow the ingress of damp or water, and should allow for thermal movement and expansion. Other steel restraints should be corrosion proof, screws or fixings should be sealed against ingress of damp or water. All should allow for thermal movement in extremes of weather.
The following is a guide to membrane requirements within build structures. Membranes should be adopted at low level in order to prevent raising damp saturated the stonework.
Membranes should be adopted to separate and protect the stone from any hard landscaping, including block pavers, slabs, tarmac paths etc.
Hard landscaping such as slabs, concrete or tarmac should not be allowed to make contact with natural stone due to the need for movement. Any landscape surface should be a minimum of 150mm lower than the damp proof course. Any natural stonework should be separated from soil, bark, grass or other foliage by using a suitable protective membrane.