Technical Q&A 11 : Repointing

 
The SPAB has recently brought out a revised edition of one of its best-selling publications, Technical Pamphlet 5 on repointing. Ideas on what constitutes good practice have advanced over recent years. Douglas Kent, the Society’s Technical Secretary, focuses the central issues.
 
 
Q. What is repointing and why does it demand care?
A. Repointing is the process of taking out and replacing the mortar (‘pointing’) from the face of a masonry joint. Done properly, this helps exclude the weather and retard deterioration of the wall. Regrettably, repointing is commonly undertaken unnecessarily or unsatisfactorily. Unnecessary repointing, however carefully done, risks damage to the edges of bricks or stones, as well as the loss of valuble clues to a building’s construction and history. Unsatisfactory repointing can not only be visually disturbing, but harmful to the actual fabric it is intended to protect.
 
Q. When is repointing necessary?
A. Repointing is premature until mortar has weathered back to a depth equivalent to the joint width or is very loose. The lime mortar used for old buildings is more permeable than the brick or stone, concentrating frost and salt action in the joints. Unlike hard cementitious pointing, this protects the masonry units because the mortar erodes in preference (in other words, is sacrificial). Irregular weathering means only localised repointing is usually required. Repointing is not justified simply because mortar is soft.
 
Q. Should I remove inappropriate hard cementitious pointing?
A. The routine use of weather-struck cement pointing in old buildings is a common mistake. Not only is it disfiguring, but it leads, more seriously, to faster deterioration of bricks and stones as they become the most permeable part of the wall and suffer the greatest frost and salt action. Removal should only be attempted if a trial indicates this can be achieved without further damage. Otherwise, the pointing is best left to work loose.
 
Q. Should I normally match the existing pointing?
A. Almost always if possible, the principal exception being where inappropriate repointing has taken place. Old pointing is often best observed in sheltered areas, such as under eaves, and should be matched in terms of material and finish. Where it is necessary to design a new mortar mix instead, this must be weaker than the bricks or blocks and take into account site exposure. If in doubt about the joint profile, a flush or nearly flush finish is frequently appropriate. The production of mortar samples and trial panels is strongly encouraged.
 
Q. What pointing mortar might be suitable?
A. The binder (which should usually be lime, not cement) and aggregate (generally sand) are selected to ensure the existing mortar is matched in colour, texture and strength. In most cases, existing mortar can be adequately assessed from visual inspection coupled with local knowledge.
A weak hydraulic lime binder was preferred historically for pointing, but, if unavailable, purer local, non-hydraulic lime was used (sometimes with brick dust or another ‘pozzolan’ to compensate). Recently, cement has been added to modify lime mortars, but this is discouraged now a wider range of limes is obtainable.
The local aggregates used historically varied widely. Frequently, however, a well-graded sharp sand would be suitable. The largest particle size should not exceed one-third of the joint width.
 
Q. What joint finish might be suitable?
A. Unless a specialised form of pointing exists, a flush or nearly flush finish will often be sympathetic. Many now favour finishing joints with a fuller profile than in recent years, following historical precedent and providing better protection to the wall. Consequently, recessed pointing, particularly with stonework, may soon be considered a late-20th-Century fashion. Sometimes, though, there may be a case aesthetically for pressing the mortar back, as when repointing very localised areas or where bricks and stones have heavily eroded edges and joints would otherwise appear excessively wide.
 
Q. How do I minimise the risk of pointing failure?
A. A high proportion of failures occur because adverse weather is ignored. Where possible, avoid repointing in winter. Whatever the time of year, ensure new work is adequately protected from frost, rapid drying (by the wind or sun) and rain. Poor preparation also causes failure. Existing mortar should normally be raked out to a depth equivalent to twice the joint width (and deeper with wide joints).

 

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Further Reading
 
Repointing Stone and Brick Walling, SPAB Technical Pamphlet 5