Lime in Construction – Information and Guidance

Click on the links below for guidance and information on the use of construction limes, including natural hydraulic lime and traditional lime putty.

NATURAL HYDRAULIC LIME (NHL): Powdered lime binder.
LIME PUTTY: Traditional form of air drying lime.
QUICKLIME: The raw material used for making lime putty.
HYDRAULIC LIME: A little different than natural hydraulic lime.
HYDRATED LIME: Builders bagged lime additive.
SETTING/CURING: How long will lime take to set and cure?
MIXING: Relatively straight forward but needs to be correct.
ADDING HAIR: Important addition for some work.
POINTING: Select the correct lime mortar and employ correct techniques.
RENDERING: Examples specifications for rendering with NHL.
BUILDING: Techniques and mixes to use.
DECORATING: Use compatible paints with lime products, here are some options.
LIME PLASTER: Definition and which mix to use?
LIMEWASH: Used for thousands of years – we take the mystery out of limewash.
STORAGE: Hydraulic limes have limited life, if stored well, this limit is extended.
TRAINING: Practical one day courses from Mike Wye.
LIMECRETE: A functional floor can be important for traditional buildings.
INSULATING LIME: Consider reducing your heating bills when rendering or plastering.
WHY USE LIME?: Some compelling reasons to use lime.
WHAT’S WRONG WITH CEMENT? Some compelling reasons not to use cement.
LIME PUTTY OR NHL?: Confused? Read this section.
SLAKING: An important process explained.
POZZOLAN: Making fat limes hydraulic – the Roman way.
HARL/SCAT COAT: Essential for most applications.
COST: Find out the best value brands and how to purchase it.

Natural Hydraulic Lime (NHL)  

Secil Natural Hydraulic LimeNatural hydraulic lime (or NHL) comes from limestone that has natural impurities of clay and other minerals, the amount of impurities within it determines how hard it will set.

It sets with water (rather than with air as with lime putty) hence the term ‘hydraulic’.

Natural hydraulic lime powders come in 3 European grades:

NHL Grade Conpressive Strength (MPa) @ 28 days Examples of Use
2 >2 to <7 Pointing internally, plastering
3.5 >3.5 to <10 Bedding, pointing
5 >5 to <15 Flooring, below DPC or chimney re-build

Natural hydraulic lime can be used when a faster strength gain is necessary or for exposed work.
You should be aware that as you increase the strength of a mix the vapour permeability (breathability) will be reduced.

The ratio of sand to binder varies depending on what job you are doing. A 3:1 mix may be used for pointing of top coat renders, but for floor screeds a stronger 2:1 mix would be used.

Hydraulic Lime

Hydraulic lime (HL) consists of lime and other materials such as cement, blast furnace slag, limestone filler or other materials that change the properties of the binder. The additives do not have to be declared which is legally different to natural hydraulic lime.

Formulated Lime

Formulated lime (FL) consists of hydrated lime and/or natural hydraulic lime with added hydraulic or pozzolanic material. Inclusion of any cement or cement clinker must be declared and at a limited percentage. This is legally different to a natural hydraulic lime and formulated lime.

Lime Putty

Lime puttyLime putty as a binder has been used for thousands of years. It has many names, such as non-hydraulic lime, fat lime or air lime, which often causes confusion.

It is made when quicklime is mixed (slaked) with water and left to mature for a minimum 3 months until in solidifies.

Lime putty cures by carbonation with the air (at a rate of approximately 1 mm per month) and can therefore be kept for many years in an air tight environment (actually improving with age).

Hydrated Lime

Hydrated lime is often confused with natural hydraulic lime due to them having similar names and supplied as a powder but is not to be used for the same applications. It is produced when a small, controlled amount of water is added to quicklime whilst still retaining it’s solid state.

It is usually used as an additive in cement as a plasticiser and it should not be used as a binder in its own right.


Quicklime (calcium oxide) is the raw material that is used to make lime putty. It is made by burning limestone or chalk in a kiln resulting in a highly reactive material. Quickilime is available in a range of sizes from lumps down to very fine powder.

Hydraulic Lime Mortar (HLM) and Applications

Please use the following tables to assist with strength and mix ratios relevant to the job at hand:

Building element
Hydraulic Lime Mortar Designation
Internal walls 0.5
External walls 0.5 – 2.5
Facing to solid construction 1.0 – 2.5
Walls close to/below ground 2.5 – 3.5
Sills, lintels and cornices 2.5 – 3.5
Copings and capping 2.5 – 5.0
Chimneys 3.5 – 5.0
Earth retaining walls 3.5 – 5.0
Masonry below water level 5.0
HLM Designation
NHL2 lime:sand by volume NHL3.5 lime:sand by volume NHL5 lime:sand by volume
Mean compressive strength (MPa@91 days)


It is essential that lime is uniformly dispersed and that any fine agglomerations are broken down. The time of mixing will be controlled by the efficiency of the mixer. Roller-pan mixers and screed mixers have the most efficient action but simple tilting-drum cement mortar mixers can be used if a longer mixing time is allowed. If the job is sufficiently large use a mixer with a capacity for a full bag of lime.

The following sequence will be suitable for a tilting-drum mixer.

  • When mixing wear protective goggles and waterproof gloves.
  • Introduce half of the sand and add all of the lime, mix well for 2 to 5 minutes until a uniform colour is achieved.
  • Stop the mixer and isolate the drive. Scrape down any material adhering to the back. Add the remaining sand and mix again for 2 to 5 minutes to get uniform dispersion.
  • Continue mixing adding water slowly over at least 10 minutes and giving plenty of time for water to be fully incorporated. The mortar should be more like a dough than a slurry and the less water added to achieve this, the better the mortar performance will be.
  • The longer the final mixing time the more workable (fatter) the mortar will be. Workability will be improved by allowing mixed mortar to stand for 15 minutes or longer before re-mixing for a further 5 minutes (in hot weather do not over-mix as water will be lost through evaporation).


Gauging hydraulic limes is not normally required although addition of pozzolanic materials can improve the hydraulic activity and performance in some applications. Materials such as used crushed brick, fly ash, ground granulated blast furnace slag (GGBS) or metakaolin (Argical) may be used to increase the mortar strength designation.

The addition of hydrated lime or lime putty can improve the plasticity but may reduce the mortar strength designation.

It is recommended that trial mixes be produced to establish the optimum properties for a particular application.

Pointing  Click here for a lime pointing pdf guide

Carefully choose which lime is most appropriate for the situation, selecting a weaker mix for softer masonry.

If not purchasing a premixed mortar then ensure you select the correct sands for texture and colour as required.

Generally, NHL 2 or lime putty mortar for internal use, and either NHL 3.5 or lime putty mortar plus pozzolan is recommended externally.


Any existing defective pointing must be raked out to a depth usually equal to twice the width of the joint, but generally not less than 20 mm. The back of the joint should be roughly square in profile. Plugging chisels ensure that the stone or bricks aren’t forced apart. Never use an angle grinder or similar type of equipment which may damage the masonry.


The joints must be dampened with enough time left for the stone or brick faces to dry to prevent smearing. The mortar should be as dry as it is practicable to point with. This allows maximum compaction in the joint, reduces shrinkage cracking and reduces the tendency to smear on the stone faces.


Lime mortars benefit from being premixed before use. Allow 30 minutes for a natural hydraulic lime mortar  compared to up to a week for lime putty mortar. Pre-mixing is obviously unnecessary when purchased as a ready to use mortar.

When you’re ready to carry out the work, the mortar is “knocked up” (agitated/mixed) immediately prior to use to plasticise the mix  and help to reduce shrinkage. Any pozzolan additive should be mixed in at the final knocking-up stage and not before.

Mortar Selection

It is very important to select the correct grade of natural hydraulic lime and then mix the correct ratio of sand to lime in order that the strength and vapour permeability is appropriate for the masonry.


Start at the top of a wall to allow for cleaning up and spraying to continue. Use a pointing spatula/small tool and push the mortar in from a hawk.

Joints deeper than 20 mm may need an initial dubbing out as shrinkage can occur otherwise.

Finish flush or rebate a little if the joints have widened with age or for personal preference as rebating highlights the stone more (weather striking).


When the mortar is “green hard” (firm enough to brush without smearing but still malleable enough to work) brush or tamp the joints with a churn brush to enhance the aggregate and give a coarser texture to the pointing. During this process, it is possible to throw a dry indigenous soil at the soft lime mortar to pre-weather and allow it to blend in to the wall as a fresh, bright lime mortar is not always desirable.


External pointing should be mist sprayed to control drying and protected from direct sun and wind. In winter it should be protected from rain and frost. Hessian cloth is recommended.


20 kg of Lime Mortar will point 2-3 m² of average sized stonework or 1- 1.5 m² of brickwork based on a 10 mm joint and 20 mm depth.

Time of Year

Great care should be taken not to apply lime mortars too late in the year or too soon in spring to avoid damage from frost.  The ultimate hardening process takes up to a month for each millimetre of thickness. Therefore it could take 20 months before mortar has carbonated to a depth of 20 mm.

It is recommended that trial mixes be produced to establish the optimum properties for a particular application.


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

Please contact us full more guidance and information.


The following is an example of a rendering specification that could be suitable for many situations but please ensure that it is suitable for your own particular project:

  1. Ensure that appropriate scaffolding is in place and the work site safe for workers and public.
  2. Take off the existing render, except any existing sound lime mortars, taking care not to damage the structure. Look out for very thick patches of render that are effectively load bearing. It may be preferable to render on top rather than risk rebuilding an area.
  3. Dub out any deep holes in the wall with a haired lime mortar, rebuilding defects with cob blocks, bricks or stone as appropriate.
  4. Treat wooden lintels with preservative and counter batten with oak lath if rendering over them.
  5. Apply a 3 mm harled coat of Secil Consolidation Mortar to provide a key to the wall.
  6. Next, a 12-15 mm scratch coat with hair, fibres or mesh included to smooth the contours of the wall. The fibres reduce any slumping whilst applying and shrinkage cracking whilst curing.
    Each backing is keyed with a scratch comb. The mortar should be NHL3.5 mixed to a ratio of around 2.5:1 of lime/sand.
    Any subsequent coats should be a weaker mix and/or thinner than the previous coat to encourage moisture to be drawn out of the wall.
  7. Apply a top coat of floated or hand harled coat as desired. The mix should be NHL3.5 mixed at 2.5:1 or 3:1 with sand and applied at 6-8 mm for a float coat or 3 mm for a harled coat.

Damping & Curing

It is very important to control suction from the wall by light spraying with water half an hour before applying each coat (especially cob and porous brick) and in warm weather it will be necessary to spray each coat afterwards. Whatever the season, protect each render coat during the curing process from all the elements such as hot drying wind, strong sun, rain and don’t apply in temperatures below 5 degrees centigrade or if there’s a risk of frost. A heavy cloth such as hessian sheeting will provide a suitable physical barrier and should be left in place as long as required.


All coats need to be given at least a few days to harden before subsequent coats are applied. To test whether a coat is ‘green hard’ the surface should be resistant to a fingernail.

Many factors will influence the timing such as the season, exposure of wall and the thickness of the coat but it’s normal to expect a couple of days for the harled coat to harden and perhaps 4- 7 days for each of the thicker coats.


For the example specification per m²:

  • 3 mm scat coat of NHL3.5 unhaired mortar at 2:1 = 6 kg/m²
  • 15 mm scratch coat of NHL3.5 haired mortar at 2.5:1 = 30 kg/m²
  • 7 mm floated top coat of NHL3.5 unhaired mortar at 2.5:1 = 14 kg/m² or repeat the first scat coat for a rough cast finish.

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.
We estimate that 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 20 mm.


We offer a one day Practical Lime Course in using lime in renovation and sell a training DVD based on the course.


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

Please contact us full more guidance and information.


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

  • They match the existing structure for porosity and density, allowing moisture to move in a similar way.
  • They can accommodate general movement better than a hard mortar.
  • Soluble salts will be less likely to crystallise in the stone or brick faces.
  • They match existing walls aesthetically.

Before starting any work, always check if building is listed. If it is listed, contact your local Conservation Officer to discuss the project.


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 select an appropriate strength for your masonry, a regular mix would be NHL3.5 mixed at 1 part lime to 2.5 or 3 parts sharp wash pit sand. The sand should be a good blend of coarse and fine particles. The largest size particle is ideally about 1/3rd the thickness of the bed to be laid, so for a 10 mm bed, 3-4 mm is ideal.


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.


See mixing hydraulic lime section on this page. Natural hydraulic lime (NHL) benefits from premixing by an hour then mixing again just prior to use.


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 m² 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 ‘green hard’, 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.

We estimate that 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 20 mm.


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


Lime putty has to dry through evaporation before it can carbonate. Carbonation may take many years before fully completed and fully hardened. Any lime with hydraulic qualities, be it formulated, natural or ‘hydraulic’, will set in the presence of water and doesn’t need to dry before hardening.

All lime mortars benefit from protection from rapid drying and frost before fully cured. We recommend using hessian.

For curing to complete effectively, it is important that moisture is present for at least 72 hours before being allowed to dry out slowly. Premature drying can result in a feeble mortar or plaster. Saturated substrates and/or low temperatures will result in extended curing times.

Re-pointing an old wall may result in rapid hardening from high suction from old dry masonry and mortar (remember to dampen down first!). Pointing however, is not structural and will rarely be a major issue if it is weak. 

Bedding mortars may take a week or more to gain significant strength.

Rendering and plastering layers are best applied on to ‘green hard’ recently applied coats. This controls the suction and helps create a good bond. If previous applications are left too long it can make application of new layers difficult to control and work.
Our lightweight Secil ecoCORK insulating render can be applied on to previous layers more quickly than normal lime renders or lime plasters.

Example Build-ups and Timings

Internal Lime Plastering
1st Day Scat/harled coat (if required)
2nd-3rd Day Scratch coat
10th Day Float coat
14th Day Skim coats
18th-21st Day Limewash coats x 4 (one coat per day)
28th+ Day Other breathable paint finish
External Lime Rendering
1st Day Scat/harled coat – if required
2nd-3rd Day Scratch coat
10th Day Float coat
17th-20th Day Limewash coats x 4 (one coat per day)
24th Day Silicate Primer
25th-26th Day Silicate Paint coats (one coat per day)
External Lime Render with Secil ecoCORK
1st Day Scat/harled coat
2nd-3rd Day ecoCORK coat with 4 mm fibre glass mesh
4th-5th Day Additional ecoCORK coat (if required)
6th-7th Day Finishing render coat
21st Day Silicate Primer
22nd-23rd Day Silicate Paint coats (one coat per day)

Why Use Lime?

Before this century building techniques and materials were very different from those employed today. Traditional properties need to “breathe” to allow moisture inherent in a solid wall construction without a damp proof course to evaporate from the external stonework or render.

Many old buildings are constructed from materials such as brick, cob and stone which are relatively porous and often of lower strength. Lime mortars were normally used for bedding and plastering. It is a relatively softer mortar and therefore it is able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes in ground conditions.

Lime mortars and plasters are vapour diffusible and hygroscopic. They can therefore manage humidity and allow moisture to evaporate, helping to keep a building free of damp and create a healthier internal environment.

What’s Wrong with Cement?

Cement_pointing_longwell-green-bristol2-160Apart from the adverse effect it has on the environment in general, cement mortar is usually hard, brittle and less porous than lime mortar. It often contains additives to make it sometimes completely waterproof and is damaging to traditional buildings for several reasons.

Cement mortar is often harder than old bricks, cob or some types of stone, therefore when movement occurs it may damage these softer traditional construction materials. Hard cement mortar can trap moisture behind it causing damage to the structure and encourages ground water to rise up a solid wall by capillary action. Trapped water in the wall can cause poor insulation, decay and crumbling. In severe cases a cob wall can fail. The brick, cob or stone can also be subject to frost damage if moisture levels are to high.

Lime Putty or NHL?

Natural hydraulic limes set even when wet and come in a range of strengths. They are useful for building with stone or brick where the earlier set may speed up construction. But you can also add a pozzolan to a lime putty mortar.

We suggest that for external bedding and pointing late in the year it would be better to use natural hydraulic lime. It is capable of a faster initial set in cold weather.

For most external rendering and internal plastering jobs, the fattiness of lime putty makes a superior mortar that allows coarser sands and thicker coats to be applied without shrinkage.

If the budget is very tight and there are no compelling practical reasons to go for lime putty then a natural hydraulic lime binder mixed with a carefully selected locally sourced sand invariably works out less expensive .

If you are a total novice to lime work, it is usually best to use a premixed product to minimise the potential issues that could arise. Premixed lime putty mortars/plasters have a long shelf life and need to be kept free from drying out until used.

Premixed hydraulic lime mortars (HLM) have a shelf life limited in part to how it is stored and needs to be kept dry until used.


Slaking is the process for making lime putty. Quicklime is added to water and a chemical reaction occurs resulting in the release of a large amount of heat from the quicklime creating a boiling liquid.

Chemically the calcium oxide is converted to calcium dihydroxide.

The longer the slaked lime is matured in the tank the better the lime putty. We leave ours for approximately 3 months before mixing with sands to create lime putty plasters and mortars.


For lime putty mortars, the sand used for building, pointing and backing coats of render and plaster should be a washed sharp coarse sand. Use a sharp sand to BS882 which is free of vegetable matter, clay and salts.

While a NHL based mortar also requires a sharp, washed, well graded sand, the sand is often slightly less coarse than that used for a lime putty based mortar. You always need to be mindful when using less coarse sands that they may need to be applied in slightly thinner coats to avoid the risks of shrinkage cracks. For finishing coats of internal plaster use a very fine sand with the lime putty or NHL2.


Premixed lime putty mortars can be stored for many months if sealed from the atmosphere and safe from frost in dumpy bags or sealed tubs.

Mortars with hair teased in will gradually lose the hair as it dissolves in an alkaline wet mortar and you will end up with an unhaired mortar.

Hydraulic lime mortars have a shelf life and should be stored dry and sealed from the atmosphere to prolong its life. If stored correctly, the bags of dry powder could last anything from 4-12 months from manufacture.

When Do I Add Hair? 

Lath and plasterTraditionally, some form of fibre was added to a lime mortar when applying it to the first application on to a lath wall or ceiling. For all other applications, the addition of fibres are optional and depend on the build-up and make-up of the plasters.

With NHL mortars we would suggest that 8-12 mm layers will require fibres and anything above 10 mm with lime putty. On to solid walls, it is acceptable to apply thinner coats without fibres.

Traditionally, animal hair such as horse, cow or goat, is added to the backing coats of lime render and plaster. In some instances, white goat hair can be found in the final skim coat.

Adding hair gives extra strength and minimises shrinking and cracking. It allows thicker coats to be applied to uneven walls and holds the plaster keys in place when plastering onto lath.


Natural Hydraulic Lime Window DetailDon’t paint a lime render or plaster with a modern paint laden with acrylic additives.

Externally, it is preferable to use a Silicate Masonry Paint.

Internally, the choice is far greater, for example: Distemper, earthborn Emulsion, Clay paint, Limewash

Lime Plaster 

Lime plaster is the finishing plaster used as a final skim internally.
Mike Wye supply two grades of lime plaster – Heritage Plaster and Regency Plaster.

Heritage Plaster is our standard and most popular lime skim.
Regency Plaster is appropriate for matching the finest historic finishes and can also be used a filler for minor cracks.

If making your own lime plaster it is vital to source a suitable washed, kiln dried sand of 1 mm or less grain size. It is recommended to mix the lime putty and sand together at least two weeks before application to lessen shrinkage cracks. Sometimes a plaster is created by mixing NHL2 with sand. This needs to be used within a few hours of mixing with sand and water.


Limewash at it’s most basic is a traditional, breathable paint made from lime putty which is thinned with water.
Mike Wye limewash is approximately 1 part mature lime putty to 1 part water for optimum performance and minimal number of coats required. Typically, four coats are needed on new external render and three coats on new lime plaster internally.
It can be coloured with pigments and can be used internally or externally on lime plaster, lime render, stone or brick. It works best on porous surfaces and hardens as it absorbs carbon dioxide from the atmosphere to form calcite crystals, giving it a unique appearance.

Limewash can be used inside or outside. Before making a decision on which  paint to use, it is important to understand that limewash can be a little powdery if not applied well or on to a surface that isn’t suitable. It can also have a non uniform colour due to differences in suction on the surface, this is usually due to differing moisture levels. The most obvious cause of this is rain hitting an external wall. Some people love this authentic appearance, some people loathe it.
Choose an suitable alternative if you don’t like the effects, in association with your Conservation Officer if the building is listed.

Click here for a full guide to applying limewash.


A pozzolan is an additive that reacts with the lime in a mortar to create harder chemical set and so is very useful for damp or frost-prone environments. We use a metakaolin from burnt clay and advise it is used for most external work in strictly regulated amounts.


Harling is a technique for applying a coat of render or plaster. The lime mortar has extra water added to bring it to a runny consistency similar to that of porridge. It is then cast on the wall using a special tool called a harling trowel which has a curved blade.

The harling coat provides a rough texture when cured that gives extra key for the next trowelled coat and also controls suction with very thirsty materials such as cob and brick.

Insulating Limes

A number of manufacturers now produce insulating renders and plasters. Some are based on lime putty and usually mixed with hemp and pozzolans. Others are based on hydrated lime and cement with various forms of insulation such as hemp or perlite.

Our main offering is a lightweight, dry mixture of natural hydraulic lime and cork. Being free from cement, it has good eco credentials as well as offering improved thermal performance.

ecoCORK is a product formulated exclusively with natural hydraulic lime binder and free from all cementitious content. Mixed with a largely cork aggregate, this offers a lightweight, vapour permeable and environmentally friendly render or plaster.
This render is ideally decorated with SecilTEK Silicate Primer and SecilTEK Silicate Paint or a similar silicate  masonry paint.


Limecrete is a modern interpretation of a traditional solid lime or gypsum floor that can still be seen in many historic or well preserved older homes.

It is vital to have an alternative option for a traditionally built house as a relatively impervious, non flexible cement floor may create problems with dampness being forced into the walls at ground level. This can result in poor insulation, mould growth with associated health problems and even structural damage. Many conservation officers with responsibility for listed buildings would encourage the use of the limecrete option rather than modern concrete floor buildups required by Building Regulations. Always use the correct materials and techniques on a building irrespective of whether it is listed or not.

Limeash were generally used on suspended timber floors onto a raft of reed and it is thought used primarily as a fire break. These floors were originally made with lime putty and lots of a pozzolanic additives to provide an hydraulic and hard set. Although very effective, this method needs a very long time to harden and carbonate.

A Limecrete floor is made using a natural hydraulic lime powder that sets much quicker and potentially harder than the limeash floor. A mixture of NHL5 and well graded aggregates can be used to create the limecrete slab but it is also possible to introduce breathable insulation using an insulating aggregate and even underfloor heating (UFH).

For more details about limecrete, please visit our dedicated website

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