WHY USE LIME PLASTERS ON OLD BUILDINGS
For hundreds of years we have applied internal plasters to the walls of our buildings for decoration, trend or consolidation of substrate purposes. Early Cornish plasters consisted of sieved, clayey earth that would have been applied by hand (literally) or with a wooden float. Typically, the first coat of plaster would contain a fairly coarse sand or aggregate with animal hair, chopped straw or teased hay added to bind the material together. The top-coat plaster would comprise of finer clays and sands without a binder. Commonly, and particularly during the 18th and the first half of the 19th century, quicklime was added for strength, workability and to achieve a faster set. Surprisingly [to a few] some of these historic plasters remain effective today, both on walls and ceilings. From the middle of the 19th century until as late as the 1920’s, earth plasters were being surpassed by a more affordable and available lime mortar substitute. The types of lime plasters and lime mortars used during this period is a discipline in itself and therefore far too extensive to be included within this narrative, however there remains a healthy proportion of surviving examples. So much so, many historic walls with modern-day plasters conceal these traditional backgrounds providing testimony to their effectiveness and longevity. These days we are using very similar or in some situations, the very same lime mortars that our forebears used with only concession to manufacture and transportation.
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 plastering on old buildings?
Principally, natural lime mortars used for lime plastering allow walls to breath. 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.
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