Sources of stone and amounts required

In general, the best stone for use in walling is the local stone. Even if other stone can be obtained more cheaply, local stone preserves the continuity between the natural landscape, the older walls in the vicinity and the new work. Occasionally local stone is not to be preferred, if it is extremely fissile and weathers poorly, or where the only local source are limestone pavements or other sites of special geological or botanical interest.

The majority of walling work involves rebuilding existing derelict, unsafe or non-stockproof walls, of which there is a huge mileage in Britain. The existing supply of stone may be sufficient, although normally there has been some loss of stone. The stone supply tends to diminish as stones get broken, become buried in the ground or dispersed. Stones are also deliberately removed for reuse elsewhere, both legitimately and illegally. Hearting tends to get buried and dispersed. In some cases new stone is not available, particularly on inaccessible sites, in which case the challenge to the waller is to make the best of what is there. In other cases, new stone must be brought in to augment the existing supply.

There are four sources of stone to consider:

1. Surface field stones, other than limestone pavements. Early farm walls were often built in part to clear the land. Often in areas of rough grazing there are still ample supplies of stone near to hand, requiring only picks and crowbars to remove. Even where bedrock does not form convenient outcrops, glacial till may supply stones large enough for walling, although these tend to be less easy to work with than quarried stone.
2. Stone from derelict walls and buildings. This stone will be weathered, and will blend in with existing stone. It will also be sorted into suitable types, which will ease the work of reusing it. Sometimes old stone is not suitable, such as some Cotswold stone and fissile shales and slates which become unsuitably soft and crumbly through weathering. Highly dressed stone is no good for dry stone walling since the faces are too smooth to bond securely. Always check with the landowner before using stone from old walls or buildings, no matter how decayed they seem. Removing an old wall will inevitably destroy the habitat, although the new wall will form a replacement habitat, but somewhat different in character.
3. Stone from abandoned quarries or self-quarried stone. ‘Vest pocket’ quarries often dot the line of long enclosure walls, usually as near as possible to the uphill side. Sometimes these can be re-opened. Limestone faces usually produce good-sized though irregular chunks with a little pick and crowbar work, but outcrops of weathered sandstone often yield unusably big boulders. Weathered granite is very difficult to extract, but schist is less troublesome. Whinstone and slate or slatey shales can be taken from rock faces without too much difficulty. Disused quarries may have heaps of waste stone which is usable after sorting. Always check with the landowner and the local authority before removing any stone. Some sites are protected as Sites of Special Scientific Interest (SSSIs) or Regionally Important Geological/geomorphological Sites (RIGS), and stone removal may not be permitted.
4. Professionally quarried stone. This is now probably the main source of supply where additional stone is needed for walling. It is expensive, but where costs of employing a skilled waller are also high, the cost of stone becomes less significant. Inquire locally as to the best source to match the existing stone. The Register of Certificated Professional Wallers/Dykers and Sources of Stone, published by the Dry Stone Walling Association, may also be of guidance.

Amounts required

The amount of stone required for building a wall depends on:

1. The wall’s dimensions.
2. The regularity of shape of the stone, and the skill of the waller.
3. The density of the stone.

Dimensions and amounts are given here in metric, as quarries deal in tonnes rather than tons.

The wall’s dimensions

The higher or wider a wall, the more stone it will require for any given length. In order to calculate the wall’s volume, calculate the following:

Length x height x average width

The height is measured from the bottom of the foundations to the top of the coping. The average width is calculated by adding the base width (below the foundations) to the top width (at the top of the topstones) and dividing by two.

For example, taking a wall 0.8m wide at the base and 0.4m wide at its top, 1.6m high and 100m long:

Average width = (0.8m + 0.4m)/2 = 0.6m

Volume = 0.6m x 1.6m x 100m = 96 cubic metres

Regularity of stone and skill of waller

Not all of a wall’s volume is stone, as it also contains air between the stones. As a very rough guide, allow the following:

Regular, level bedded stone will create a tight, well packed face, limiting the amount of air to as little as 10%. Sawn slate off cuts, with slate chipping hearting, is possibly the most regular stone available, and air content may be reduced to as little as 5%, but this is exceptional. Irregular and rounded stone tends to create a slacker face, and with larger stone the gaps can be quite marked. The air is likely to comprise about 20%.

The skill of the waller is also significant, with a skilled waller building a wall with perhaps 10% less air than a novice. The regularity of the stone and the skill of the waller also has implications on wastage. Less regular stone can be dressed to a certain extent to produce a tighter wall, but this becomes more difficult with larger and harder stone. There is likely to be more wastage with a novice than with a skilled waller. Estimates are difficult, but even for irregular stone used by novices, wastage should be less than 5%. With regular stone there should be no wastage.

Density of stone

Different types of stone vary in density and hence weight. In order to know how much stone is required you need to know its ‘bulk density’, which is measured by how much a solid cubic metre of it would weigh, and is normally expressed as tonnes per cubic metre (t/m3). It can also be expressed as millions of grams per cubic metre (Mg/m3).

Basalt, for example from Skye, is one of the heaviest walling materials, with a bulk density of around 2.95t/m3. At the other extreme, Oolitic limestone from the Cotswolds, one of the lightest stones, can have a bulk density of around 2t/m3.

If an allowance of 15% air is made, the 96m3 volume (from the example above) would require 82m3 of stone.

For a basalt at around 2.95t/m3 this would require 242 tonnes.

For an Oolitic limestone at around 2t/m3 this would require 164 tonnes.

Generalisations within a stone type are also problematic. Oolitic limestone provides a good example of this. Oolitic limestone from the Cotswolds is likely to fall in the range of 1.9-2.1t/m3, whereas Oolitic limestone from Portland is likely to fall in the range of 2.1-2.3t/m3. These variations occur because of the slight differences in the stone between quarries, and even between two faces in the same quarry.

These differences can have serious implications for cost and wastage. When combined with the difficulty of assessing air and making any allowances for wastage it can be very easy to overestimate, and to pay for stone which you do not need. This extra stone will also have to be cleared up and transported away, involving more time and expense. Equally, you can easily run out of stone. On a large job this might not be a problem, beyond the extra cost, as you can order another 16-20 tonne load. For smaller lengths a shortfall of 5 tonnes will seriously affect the price as the transport costs are likely to increase out of proportion to the amount of stone required.

There is also a problem in obtaining information about the exact bulk density from the quarry concerned, as they may not know it, or give rather inaccurate estimates. The table below gives a rough guide to the various main types of stone, but note that there will be variation between different areas and different quarries. A variation of +/- 5% should be allowed.

It is a rather inexact science, and accurate estimates can only be made with experience. The problems of estimating can make it difficult for dry stone walling contractors involved in competitive tendering for work, and it is well worth being aware of the pitfalls before a tender is made.

Other considerations

Unless you know the quarry company well, it is always advisable to inspect the stone before it is delivered, as the interpretation of ‘walling stone’ can vary. With regular, level bedded stone there shouldn’t be a problem with suitability, although sometimes a lot of the stone is too big and slabby and has to be reduced with a sledgehammer. The less regular the stone the greater the problem, and walling stone may tend towards being the material which the quarry can’t otherwise get rid of. This will make walling work difficult, and may result in a lot of wastage.

Often it works out more cost effective to buy hearting separately, as smaller stone is usually cheaper to buy. However this will depend on the regularity of the stone, and on how individual quarries operate. Generally, the more regular the stone the less hearting occurs ‘naturally’ within a load. A certain amount of hearting will be generated through dressing stone, but it can be wasteful to smash up good building stone to create hearting, if the hearting can be purchased more cheaply. With larger, rounded and irregular stone it is going to be harder to ‘create’ sufficient hearting if it does not occur naturally in a load. Washed stone of 5″-1″ (125-25mm) makes good hearting, and some of the larger pieces will make suitable face stones.

Purchasing hearting separately does not affect the total tonnage you require, but you will have to make some calculation of how much you need as a percentage of the overall mass of the wall. This will be affected by the dimensions of the wall, and the relative size of the face stones. For example, many walls built out of smaller stone can contain 30-50% of their bulk in hearting, whereas walls built of larger stones and slabs can contain as little as 10%.

Generally it is safest to slightly underestimate the percentage of hearting required, as you can always ‘waste’ some of the building stone by reducing it to hearting. Conversely, if you have to substitute many of the face stones with larger hearting the stability of the wall may be compromised.

If you require throughstones you may need to purchase these separately, as with many stone types they do not occur ‘naturally’ within a load in sufficient quantity. Some good quarries sell suitable throughstones separately, whilst with others you will need to visit the quarry and hand select the stone.

The number required can be calculated from the pattern of throughs to be used in the wall. It’s advisable to order about 5% extra, to allow for any damage in transit, and to give a wider choice in use. Try to arrange a separate delivery of the throughs so they are less likely to be damaged, and can easily be kept separate on the work site for ease of use. Either deduct the weight of the throughs, as supplied, from the total walling stone required, or for smaller amounts, ignore the weight and regard the additional stone as part of wastage.

Most of the considerations for throughstones also apply to coping, but the quantities involved are much greater. If you do require a separate order you will need to calculate the volume of coping required, with allowance for air and wastage. As the coping is a significant proportion of the total volume, this needs to be deducted from the total walling stone required in order to avoid unacceptable wastage.