A Guide to repointing Stone and Brickwork

A Guide to RepointingStone and BrickworkConservation of BuildingsRepointing is the process of taking out andreplacing the mortar (‘pointing’) from the faceof a masonry joint. Done properly, this helpsexclude the weather and slow the deteriorationof the wall. Regrettably, repointing is oftenundertaken unnecessarily or unsatisfactorily.Unnecessary repointing, however carefullydone, risks damage to the edges of bricks orstones, as well as the loss of valuable clues to abuilding’s construction and history.Seriously decayed pointing will let moistureinto the wall. Saturated masonry willdeteriorate more rapidly and is likely to causedamage to interior plaster and woodwork.Is Repointing necessary?This needs to be carefully assessed beforestarting. Generally speaking, if the mortarjoint has weathered back to at least theequivalent of its width or is soft and crumbling,then repointing in those areas is advisable.Areas which are firm, and any that would needstrong chiselling to take out the mortar, shouldgenerally be left. Complete repointing of anelevation is rarely needed or advisable.Repointing Good PracticeThe wrong type of repointing will also haveserious consequences, causing further damageto stone and brickwork, trapping excessmoisture and increasing the rate of decay.Cement based mortar should never be used torepoint historic walls, it is too strong, inflexibleand impermeable.Historic buildings were built in lime mortarwhich is flexible and permeable, and allows thebuilding to ’breath’. Lime mortar must alwaysbe used and repointing carried out carefully.Good repointing of brickwork.1. Joints should be carefully raked out to adepth at least equal to the width of thejoint and, generally to a depth of not lessthan 40mm (just under 2 inches) in rubblestonework & 20mm (just under 1 inch) inbrickwork (and fine ashlar stonework).The repointing of very fine ashlar joints isgenerally not desirable due to the risks ofdisfiguring the appearance of thestonework. Where joints are wider ingeneral ashlar work, finely crushed stoneis often preferable to coarse sand as anaggregate. (Ashlar is stones cut to evenfaces and square edges laid in regularcourses).Old pointing which is sound and doesn’t needrepointing.2. Rake out with a bent screwdriver or spikeshaped tool, leaving the back of the jointsquare. Where hard patches of old mortar

LimeThis is available in 3 main types:1. Matured Lime PuttyPutty is is sold wet and in tubs, it is generallya high quality product, but expensive. It isusually sold ‘matured’ referring to the length oftime it has been since it was ‘slaked’. Thelonger matured the better, 6 months is theusual standard for a good quality lime. Limeputty should always be used for internalplasterwork and can be used for exteriormortars and renders. If used in a mortar it isadvisable to use a ‘pozzolan’ additive whichacts as a catalyst to help the mortar set andimprove its durability. Pozzolans includematerials like Pulverised Fuel Ash (PFA),metakaolin Metastar 501, Trass fromGermany, and HTI (ceramic ‘high temperatureinsulation’). They should be added at a ratio of10% to the mortar just before use, as it willimmediately start off the setting process.2. Hydrated limeThis is the dry powder you are likely to be soldat the DIY store or local builder’s merchant. Itis commonly used in cement mortars as a‘plasticiser’ (WARNING: this does notconstitute a ‘lime mortar’). Hydrated lime canbe used for proper lime mortars (withoutcement) but we recommend that lime putty isused instead (see below). If Hydrated lime ischosen then for the best result the lime shouldbe converted to lime putty by the addition ofwater. Add the lime powder to water in asuitable clean bin and mix to achieve a thickcream. Keep for at least 24 hours before use asa putty – the longer it is left to ‘mature’ thebetter the lime will become (ensure the lime iscompletely covered by c.2cm of water). Use of apozzolan as indicated above is recommended.Good Practice for mixing lime puttyFor best and consistent results with either theLime putty (1) or Hydrated Lime (2) the sandand lime putty should be thoroughly andevenly mixed beforehand with just sufficientwater to achieve a plastic fatty consistency.Putty is best mixed in a paddle mixer, by handon a board or in a bucket, but not a standardcement mixer. The mixture is called ‘coarsestuff’ and should be stored in bins with airtightlids, or wet sacking cover, for a week beforeuse. It can be “knocked up” and gauged withthe pozzolan when required for the work.3. Naturally Hydraulic lime (NHL)Sometimes this is sold simply as ‘Hydrauliclime’ which makes it easily confused with‘Hydrated lime’ (see above). Hydraulic limewill always display NHL and a number on thebag. This refers to the clay content of the limeand is an indicator of the level of chemical set.The chemical set is similar to the settingcharacteristics of cement but not as rapid. Itcan be used as a substitute for cement (bearingin mind the use of coarse sand referred toabove), but it requires greater aftercare andattention to ensure it does not dry out before ithas fully set (see point 10 under ‘Good Practice’above). NHL 3.5 is a ‘moderately’ hydrauliclime appropriate for very exposed positions, formost re-pointing nothing stronger than a‘feebly’ hydraulic should be used (NHL1).Good practice for mixing Hydrauliclime mortarHydraulic lime of NHL 3.5 or less can be mixedand stored for a short period before use. Thelower hydraulic limes (NHL 1-2) can be left forup to 4 hours and ‘knocked-up’ prior to use.This will aid the setting process. Moderatelyhydraulic mortars (NHL 3.5) should not be leftmore than an hour or two before use.Table highlighting the main points with respect to each type of lime.ProsConsLime Putty Hydrated Lime Hydraulic lime (NHL)• Good quality• Highly ‘breathable’• Flexible• Sacrificial• Historically authentic• Expensive• Better mixed by hand orpaddle mixer• Cheap• Widely available• Can be turned into putty• Poor quality• Should be converted toputty• Better mixed by hand orpaddle mixer• Quality acceptable• Easy to mix• Cheaper than putty• Inflexible• Less breathable• Less authentic

Mortar MixesAppropriate mixes will depend very much onthe type of stone or brick to be repointed.Suitable general guide mix is:1 : 3 lime to sandWater : must be clean.Gauging : measuring proportions of thematerials should be done accurately. Separatemeasuring containers should be used for limeputty and the sand.MixingLime putty: This is best mixed in a paddlemixer, by hand on a board or in a bucket, butnot a standard cement mixer.Hydraulic lime: can be mixed in a cementmixer.Water: the mix should not be made too wet,enough water should be added to allow themortar to stick to an upturned wooden hawk.It should not be sloppy.Problems & remediesProblem Cause How to avoidMortar cracks Mortar too wet Less water in mixMortar dried butnot hardened/setMortar was not compressed sufficientlyMortar dried out too quicklyInsufficient pre-wetting of masonryWall not protected from wind and/or sunMortar damaged by frost before settingPress mortar tightly into jointsPre-wet joints & protect afterwardsSpray joints thoroughly with waterone hour before, and 10 minutesbefore pointingAlways cover with Hessian anddampen if hot/windyNever point when frost is likelyTools for the jobBrick Pointers (left) or ‘pointing irons’ arenot widely available at DIY stores but somebuilder’s merchants and a number of on-linesites sell them. They come in a variety ofwidths from 3/8 inch; ½ inch; 5/8 inch.Wooden hawks are useful but not easily found, they can be made byattaching a short piece of broom handle to a 6 inch square board.Alternatives include a plastic plaster’s trowel or just a piece of wood.Churn brushes get their name from their use for cleaning out milkchurns. They are just right for ‘stippling’ the mortarjoints but are not widely available on the high street. Astiff bristled dust pan or scrubbing brush will suffice ifthe bristles are hard enough.long handle churn brushMore detailed information on repointing can be obtained from:The Society for the Protection of the Ancient Buildings, 37 Spital Square, London E1 6DY.Ask for: SPAB Technical Leaflet No. 5 or the following by John Ashurst:‘Pointing Stone and Brick Walling’ and ‘Mortars Plasters and Renders in Conservation’There are also a number of web sites with repointing advice:www.periodpropertyshop.co.uk/acatalog/Repointing.htmlwww.mikewye.co.uk/mikepointng.htmwww.womersleys.co.uk/pdfs/pointingwithfatlime.pdf