What is Lime Render?
Lime render is a form of lime mortar that, instead of being used for pointing or bedding masonry, is spread on external walls to create a natural, breathable outer layer. And because of its natural properties, lime render has been used to externally protect buildings for thousands of years.
A combination of traditional lime, blended sands and aggregates, lime render is made by simply mixing the dry ingredients together with clean water to a usable consistency.
The render is 100% natural, breathable and sustainable, making it an eco-friendly building product. It does not contain anything artificial -- no additives or plasticisers and, importantly, no cement. Being a natural product, lime render is sympathetic to the underlying structure -- unlike cement which is non-breathable, and will damage the underlying structure. This ability of lime render to breathe helps to avoid potential issues from issues such as damp.
The Main Benefit of Lime Render is Breathability
Lime render's breathability means that moisture does not get trapped between it and the structure -- unlike modern sand and cement render, which is frequently used today. Another benefit is that the application of lime render disguises poorly built or uneven walls, improving the aesthetic of the wall or building as well as protecting it.
Breathability means that not only does lime render allow moisture to escape and evaporate from the structure, it also effectively draws moisture out from the structure. This is due to its exceptional capillary properties.
This process of drawing moisture out is important when considering the sympathetic conservation or renovation of old buildings with solid walls. The role of lime render is to act as an external protection by keeping water from getting in while still letting water out. Unfortunately, an old building's solid walls can absorb water like a large sponge. This happens in solid walls because there is no cavity to stop the transfer of moisture -- as you would find in a modern building. Fortunately, lime render will naturally assist with the evaporation of excess moisture absorption if this happens.
In a rendered property that was built before 1950, there is a good chance that it is finished with lime render. If so, this may have prevented long term issues with damp in the building. Damp issues tend to be short lived where masonry is fully breathable, as the moisture is eventually expelled. Subsequently, lime render allows the masonry to gradually dry once the root cause of the damp is located and fixed.
However if damp is present in an older building, it is likely due to being re-rendered with a modern cement render, or coated with a non-breathable coating -- such as standard paints and emulsions. Once a non-breathable material is added to a solid wall, large amounts of moisture can become locked in. Any moisture will generally find a way out in some isolated areas and manifest itself as a damp problem.
What is the Three Coat System?
Lime render is always applied and built up in thin layers called coats. Traditionally, a three coat system is the recommended method:
|2||Float Coat /
|3||Finish Coat /
† Non-hydraulic lime render assume warm, dry weather and may be significantly longer.
The scratch coat is the first coat applied to the substrate, this could be bare masonry or laths. It is made with a coarse aggregate to leave a rough surface, this is then heavily scratched (hence the name scratch coat). The rough scratched finish provides a good key for the following coat to adhere.
Float Coat / Brown Coat
The float coat (also known as brown coat) is applied on top of the scratch coat and is also made with a coarse aggregate. The surface is ruled to a relatively flat finish and is scratched lightly to leave a key for the finish coat.
The finish coat is the final, visible surface. It'a purpose is decorative and is made with a fine aggregate to produce a smoother, flatter finish.
Is Lime Render Always a Three Coat System?
The three coat system may not suit every scenario. However, the general principals can be adapted and the plaster can be applied using two or more coats of different thicknesses.
Build up gradually in thin coats
A coat should not exceed 15mm thick.
Lime render is heavy and cures very slowly compared to modern cement based, resin or polymer renders. Thick layers are less likely to adhere due to the excess weight and are prone to shrinkage and cracking with the increased amount of moisture.
Don’t proceed until it’s dry
Allow sufficient time for each coat to dry before applying the next.
The recommended drying time is 1 calendar day per millimetre; therefore a 10mm coat would be left to dry for 10 days before applying the next coat. This may vary slightly depending on site conditions but should not be less than 4-5 days in any scenario.
How Much Lime Render Will I Need?
If you are uncertain of how much you will need, we do have a lime render calculator that will calculate the exact quantity for you. It calculates by 'total area (m²)' or 'wall dimensions (m),' and also will take into account how thick (mm) each coat will be.
Preparation is Key Before Applying Lime Render
The lime render must be applied to a suitable background, such as bare masonry. Therefore, the preparation of the masonry is very important. Failure to prepare properly can mean impaired adhesion of the render. This procedure can require more effort than the rendering itself, but shortcuts here can be very expensive -- listed below are a few main contaminants to be aware of:
Existing non-breathable surfaces or barriers must be removed. This may include previous plasters, cladding, acrylic or other non-breathable paints. Lime render is highly breathable but that is of no benefit if it is applied over a non-breathable material.
Remove climbers, vegetation or other kind of organic growth from the masonry. Wherever biocides are used to remove vegetation ensure these will not react with the render and produce patches or staining.
The root cause of any damp problems should be identified and remedied before any render is applied.
Clean, solid background
The background for the render must form a solid base. Remove any loose masonry, mortar and plaster. Cut back masonry where in doubt, and ensure to clean thoroughly to remove dirt and dust.
Cracks and cavities
Cracks and cavities should be cleaned out, filled and sealed with NHL pointing mortar . Large cracks may be packed with low fire clay tiles, slate or natural rubble before the NHL mortar is applied. Such repairs should be allowed to cure for 1 – 2 days before rendering starts.
Heavily defaced surfaces or areas with a lot of damaged joints may require a "dubbing out coat" to even out the surface.
Where joints, cracks or holes are severe they may be backfilled with natural rubble.
It is most efficient to apply the "dubbing out coat" by harling. When set sufficiently it can be keyed (approx. 10 hours).
Allow sufficient time for the "dubbing out coat" to dry before applying any render. Site conditions will dictate, this could take 5 days or more.
Bordering different surfaces
Wherever the render meets different materials e.g. timber frames, stone lintels – it is necessary to insert a metal mesh at the joint (at least 100mm each side).
Hair or alkali resisting fibres can be added to the base coats or applied to metal rib lath to increase the bond and tensile strength.
Key for first coat
ime render will not bond well to smooth or dense surfaces. It may be necessary to provide a suitable key for the first coat - there are several techniques at your disposal.
Where masonry is reasonably porous, a sufficient key may be established by raking out joints. At least every other course of brickwork would be raked out. This may not be adequate for larger units of dense masonry.
A key coat (also known as a "stipple coat" or "slurry coat") is a product we sell separately on request.
Fibres are added to base coats only, they are never added to the visible finish coat.
Dampening the Masonry
Supporting masonry is often very porous, particularly old bricks. This must be severely saturated to reduce suction before applying the first coat.
The process of soaking the masonry would typically start the day before. This may require several iterations to saturate adequately, the last of which just before the plastering starts
Protecting the Render
The work is at risk until the render has adequate time to cure, protection is critical in the meantime, although often overlooked:
- Cold temperatures may cause the plaster to freeze.
- Heat may cause it to shrink and de-bond.
Sunlight can accelerate the drying process.
This leads to colour alterations (bleaching) and cracking (shrinkage).
The work should be protected with a breathable membrane such as hessian sheeting.
In temperatures above 10°C (with no risk of frost), the sheeting should be sprayed regularly with water to increase humidity and control moisture loss. This process is known in the trade as "cherishing" and is recommended for at least 5 days per coat.
Lime Render Products
Conserv® lime render is created with the traditional three coat system in mind. Lime renders are available in two formats:
Suitable for scratch coat and float coat, this material contains a coarser aggregate.
Suitable for finish coat only, this material contains a finer aggregate.
* Please note: Although lime render finish is described as fine, it may be coarser than some modern renders.
Mixing Lime Render
Our hydraulic lime render comes ready-to-mix. There's no need to worry about measuring before mixing the product. Mixing the render is simple -- all you need to do is stir the lime and aggregate together, then add clean water and mix to the correct consistency. The product is mixed in its entirety by using all the aggregate and natural hydraulic lime provided. This maintains consistent appearance and performance.
Non-hydraulic render comes premixed. Inside the bag you'll find a premixed lime mortar, that is ready to use. It is made with matured non-hydraulic lime (commonly known as “lime putty” or “fat lime putty”).
The reason our lime render comes ready-to-mix or premixed, is because getting it right is difficult. The science behind different mortar ratios is complex, as there are many variables to consider. One such consideration is how the type and gradation of aggregate effects the performance and working qualities of the render, mortar and plaster, for example. If you are interested in knowing more about the science behind using lime in building materials, we have collaborated with Dr Ash Ahmed, from Leeds Beckett university, for his academic work in this area.
Lime render can be painted with a breathable paint when it is fully cured. Standard paints and emulsions must be avoided as they will compromise the plaster's vapour permeability and lock in moisture.
There are some excellent breathable coatings available, in particular we recommend:
It is strongly recommended that the lime render is fully cured before applying a paint or coating. At the very least, consult the paint manufacturer if you plan to apply before the render is cured.
Selecting Lime Render for a Specific Project
Made with different grades of hydraulic lime, which is better suited to colder, wetter climates like the UK and Ireland. Setting happens in these conditions through the process of hydrolysis (_a reaction caused by water that results in carbonation_), where a non-hydraulic lime render would not set sufficiently. A benefit of this process is a relatively fast curing time -- typically about 90 days. After this period, it can then be painted with an exterior breathable paint or Conserv's lime wash.
Non-hydraulic lime render is made using lime putty -- which provides some benefits over hydraulic lime render. The addition of putty makes it quite "fatty" and therefore sticky, making it easier to apply to walls. This results in a render that is softer and more flexible with improved capillary properties, _which is perfect for the conservation of delicate masonry. However, it carbonates (re-absorbs carbon dioxide from the air) very slowly, which can make it impractical in damp or wet conditions, where it may not carbonate at all. As well as being slow to cure -- its lower strength can leave it more susceptible to frost damage than a hydraulic lime render.