Why use lime mortars for re-pointing old walls in Cornwall?
Traditionally, all old stone walls had some form of pointing application to the outside elevation, chiefly as a weather protection. Walling from the 1700’s (up to as late as the 1890’s) would have had an earth mortar ‘pointing’ for the jointing, but predominantly it was the 1800’s that appreciated and extended the popularity of using lime mortars as binders and applied materials. Mortared joints are not only a significant part of the appearance and character of a stone or brick built wall they also help to keep the building dry. There are two main reasons why; by preventing rainwater finding its way into the inner part of the wall and by allowing inherent moisture already in the wall to dissipate during drier conditions. In essence, breathability should be achieved through the mortar joints and not the principal fabric. If a hard, cementitious mortar is used, then all moisture movement is concentrated in the stone or brickwork, which could cause irreversible frost-related damage and/or impairment from the crystallisation of soluble salts. Lime pointing in Cornwall is unquestionably best practise for historic stone built walling and old brick walls.
Lime in Building
The use of lime and lime mortars have been traditional construction materials since Roman times. A substantial part of the UK’s historic building stock was constructed using simple lime mortars and testimony to its benefits as a building medium are reflected by the continued existence of such buildings. Today, traditional lime mortars resume their role as a staple for repairs, rebuilding and restoration of the historic built environment. Having an established and significant presence within the construction industry, lime mortars are now also being used extensively on contemporary construction projects, notably for their favourable characteristics in relation to a healthier living environment, in particular lime plasters. This commentary does not expand on the varying types of lime mortars that were and/or are being used as a construction material, however, credence has been included where deserved. Further reading on Natural and Non-Hydraulic Limes can be found at the Building Limes Forum or the Cornish Lime Company.
Why use lime mortars on old buildings?
Principally, natural lime mortars allow old walls to breathe. The properties of natural limes and the mechanisms that realise permeability are fundamental in allowing moisture movement and ultimately moisture dissipation. Their ability to absorb and release moisture enables old, solid walling to breathe and to an extent, allow for structural movement. If moisture content within the wall becomes excessive and has no means of release, accumulated wetness will almost certainly cause problems (skip to Damp Walls). Undoubtedly, the largest contributor to damp in historic walls has been the widespread use of cement-based materials. Whilst cementitious mortars hold (and deserve) an important position in the market-place, their usage on historic walling such as cob, stone and/or old brick has been proven to be detrimental to the very fabric of its restorative intention. Cement-based mortars have many attributes and prolific usage on contemporary builds is unquestionably a tried and tested practice, however when these hard, impervious products are applied to old, solid walls complications will arise.