Guidesheets:  Rebuilding a stone wall in lime

• Do I need planning permission and listed building consent?
• Do I need Building Regulations approval?
• What size foundations are necessary?

The requirements for repairs to an existing wall will differ from a new wall separating your patio from the neighbour's! The type of stone or brick used will affect the strength of mortar in which to bed them. Prior to 1919 most walls were of traditional solid construction and masonry bedded in lime putty mortars, often with earth (subsoil) mixed in as well.

Rebuilding and repairing with lime mortars offers a number of advantages:

Before starting any work, always check that the building is not listed, if it is contact the local authority conservation officer to discuss the project.

Preparation:
Try to select stone from a local quarry to match the existing. Take a sample along to the quarry if you're not sure. You can usually get two size ranges; 4"-6" and 6" - 9". For the bedding mortar use a volume mix of 7 parts coarse sharp well graded sand ( from fine up to 8mm size) and 2 parts mature lime putty, mixed in advance for at least a week. Lime mortars gain strength from carbonation with carbon dioxide from the air. In damp, frost prone or very exposed situations it may be appropriate to add an extra ingredient to a lime putty mortar to increase its compressive strength and frost resistance. Traditionally volcanic ash or brick dust were added, these are forms of burnt clay called pozzolans after the Italian town of Pozzuoli where volcanic ash was used by the Romans. We use a calcined clay Argical M-1000 at a volume gauge of 10:1 up to 6:1 by volume depending on the degree of exposure and the job required of it. It won't give an overnight set but will slowly begin to add extra compressive strength to the mortar after a few days.

Building:
If you're a novice, position a stone dry first to make sure it looks right and you have got the best face showing. Stagger the vertical joints so there isn't a vertical joint running continuously up the wall

Premixing:
All lime putty mortars benefit from being pre-mixed for perhaps a couple of weeks and then "knocked up" again prior to use to plasticise them - this reduces shrinkage in the mortar. Natural hydraulic limes (NHL) benefit from premixing by an hour then mixing again just prior to use.

Application:
Use a mortar bed just thick enough to spread the load evenly, finishing just beyond the front face and then trimming flush with the edge of the gauging trowel. Use a through stone that can tie together the entire thickness of the wall or thereabouts, one every square metre of wall face, pinning the wall together from both faces. The mortar shouldn't dry out too quickly - protect from sun, wind and rain with damp hessian cloth. Protect from rain if necessary. Build up to a maximum of 1 metre high at a time and then let the lime mortar cure for 2 to 3 days. When dry, the joints can be brushed with a stiff brush to expose the aggregate.

Time of year:
Please note that great care should be taken not to be applied too late in the year or too soon in spring or else frost damage may occur. It is important to prevent frost crystals forming within the mortar soon after application. The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it may take 20 months before mortar has carbonated to a depth of 20mm.

Please contact us full more guidance and information.

Safety:
Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.

Our advice and information are given in good faith. It's important that users satisfy themselves that they've chosen an appropriate product and have a suitably skilled workforce.

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