A practical guide to lime rendering for cob walls

For some, rendering onto cob can feel like a dark art, with often conflicting information available online and limited printed literature available. This guide introduces best practices and things to consider for rendering on cob, including substrate preparation, material selection and applying a bonding, scratch and float coat. It’s important to note that every project is unique; your material selection, mix and approach should be tailored to suit.

To begin, what is cob? Cob or cobb refers to shuttered or mass earth construction made from a stony clay subsoil with straw. It generally forms the main body of a wall but also can be found backing up a stone face. Cob has regional variations around the British Isles, as multiple types of subsoil, clay, and other materials and combinations may have been used. Generally, cob is always relatively weak in strength compared to other materials but maintains an excellent structural integrity. Work done by soil engineers at Plymouth University found that cob could withstand its own self-weight to an equivalent of thirteen storeys, making it an impressive and resilient material.

Preparing the cob substrate

Removal of old renders or coatings

Like any other material, cob walls require preparation before a lime render or plaster can be applied. Therefore, removing all old or inappropriate coatings on the cob is essential before beginning any new rendering or plastering work. Inappropriate materials include cement render, lime and cement render blends and synthetic non-breathable masonry paints. These materials restrict moisture movement and hold or trap water into the wall, saturating the cob material over time and turning it into mud. While not immediate, the result can be disastrous for the structure, with the wall collapsing like a landslide.

Assessment of cob stability and strength

Begin your substrate preparation with a thorough check of the cob and an assessment of its strength. The cob should be firm enough that you can not push a sharp chisel more than a few inches into the surface by hand. If the cob is softer to any significant depth, an assessment by an engineer will be beneficial to ensure that additional work is not required to stabilise the wall. When in doubt, it is always worth confirming that your background is stable before rendering or plastering work begins.

Removal of biological materials

Remove any biological growth on your cob substrate, including vegetation, lichen, algae or similar. Aside from growing back and creating issues, these biological materials produce damaging acids that can dissolve the lime in renders over time. Lastly, ensure care is taken during removal to prevent damage to the cob. For the removal of biological materials, D/2 Biological Solution is one option to consider.

Learn more about D/2 Biological Solution for removing staining for masonry

Filling holes, pockets or voids

If holes or pockets are present, these should be filled and brought roughly flat before rendering. If suitable materials are unavailable locally, a cob block can be mashed with water to create a very stiff render which can be packed into the voids. If using this method, wet the background before application, and fill at a thickness no greater than 25mm or 1″. To help reduce shrinkage and movement, we recommend including chopped straw or hemp in this mix. This earth repair may need re-tempering the following day to deal with plastic shrinkage.

After completing any required repairs, allow as long as possible for further shrinkage or movement to settle before beginning to render. In the case of extensive repairs, this could be up to one year. All necessary preparation, which will be unique to your project, should be undertaken to ensure the background is suitable to receive a protective lime render over the cob.

Render coats, thickness and exposure

The overall coats and thickness of the render required will be specific to your project. Generally, a three-coat render is advised externally for areas with higher exposure to the elements. For each layer of render, we recommend that a thickness of 10mm should not be exceeded.

There may be areas where a thicker application may be desired or required in isolation; for example, to level out areas or wobbly walls. While lime renders can be forgiving, be mindful of the overall thickness and take the appropriate measures. The thicker the render, the greater the risk of shrinkage issues.

Binders & pozzolans

Regarding binders, we typically advise a lime putty render, sometimes gauged with a pozzolan, depending on the project’s requirements. If using a pozzolan, keep doses low, especially if a more reactive material like Metakaolin, such as Argical M1000, is being used. We never advise using GGBS, as it is a pozzolan in name only and is as strong as Portland cement, which is incompatible with a softer background like cob.

Shop our range of lime and binders

In environments which are highly exposed, a high free lime NHL2 could be considered, such as Saint-Astier’s NHL2. However, caution should be taken, as the strength of natural hydraulic limes can vary, and some NHL2s can be considerably stronger than others. Further, an NHL3.5 should never be used to render cob, as it is far too inflexible.

Reinforcement and fibres

Fibres or hair are exceptionally useful additions to any render onto cob. Natural hair or high-quality polypropylene or alkali-resistant glass fibres are good options. Hair or fibres can be added to every coat; however, both the harl and finish coat do not require the addition of hair or fibres.

Mesh is also an additional, but not a mandatory, option for further reinforcement. Plasterers mesh helps reduce the chance of cracking around lintels or other more challenging areas. If the substrate is not in ideal condition, we advise using a heavier-grade fabric mesh, sometimes known as renovation mesh, for the affected elevation. View our range of plasterers mesh.

Sand

For the base and float coat, a good clean washed 4mm down sharp sand should be used. If a finer finish is required, this coat can be topped with a finer sand of 1mm to 2mm down. If the highest level of performance is needed, for example, in highly exposed areas, a harl coat as the last coat is an option.

Browse our range of fine sands

Three coat application

Harl Coat: Thrown or sprayed on bonding coat

Cob is often quite friable and porous, making suction notoriously challenging to control. As such, we recommend an initial bonding coat, thrown on as a harl coat, to assist with stabilising the cob and improving the bond and adhesion of subsequent coats.

An appropriate mix for this bonding coat could be as follows:

• 1-part Cornish Lime Mature Lime Putty (NHL2 if necessary)  
• 1-part Harling Grit 6mm down, Cornish Lime CLS57
• 1-part coarse sand, Cornish Lime CLS28

A harl coat is made by throwing or “harling” a very coarse render onto the wall, providing a very effective bond upon impact onto the surface of the cob. Of course, the exact texture and thickness of the harl coat will vary depending on the mix. Still, this will generally provide a well-bonded coat with an efficient mechanical key, texture or roughness, for subsequent coats of render.

While rendering onto cob, wetting the background down, controlling suction and avoiding rapid drying is critical. As such, this will demand the appropriate focus and consideration before, during and after all applications. Failure to manage this aspect is the leading cause of failure in achieving a successful lime render. 

Developing a sound bonding coat is best practice and of the utmost importance before subsequent coats of render are applied. As such, this coat should be allowed to cure for a minimum of two weeks in warmer months and longer during colder months, as the process of firming up will take longer. Properly curing and protecting a lime mortar is an essential part of the process. For more guidance on curing, see our helpful guide on curing lime renders and mortars.

Scratch Coat

The scratch coat is the first true layer of render and provides a suitable background for a float or finishing coat.

An appropriate mix for a scratch coat could be as follows:

• 2-part Cornish Lime Mature Putty (NHL2 if necessary)  
• 5-part coarse sand, Cornish Lime CLS28
• Belmix 18mm fibres

For those looking for a premixed coarse mortar, our Cornish Lime CLM28F Coarse Putty Mortar with fibres is a convenient and easy-to-use option made with our matured lime putty.

To begin, prepare the background by wetting it down. Depending on the amount of suction, this step may be required multiple times before applying the scratch coat.

Next, apply a 10mm coat of well-fibred lime render by hand using a trowel or by sprayer using a 4mm down sharp sand. Sablon type gun applicators which use compressed air are recognised as an effective method for spraying as the render is efficiently projected, and a sizable area can be covered relatively quickly.

Where you have different adjoining background materials, it’s worth further reinforcing these areas to mitigate movement cracking. To do so, embed a mesh into this coat by buttering up the wall, pressing the mesh in, and thinly over-topping the mesh in a 1 coat 2 pass type application. With mesh, you will want to overhang the relevant area by a minimum of 300mm on each side. For larger areas requiring the mesh to overlap, this overlap should be 100mm. 

Scratch this layer no deeper than a 1/3rd the thickness of the render coat. Damp cure this coat for a minimum of a week before moving on to the next coat. As a guide, it should be leather dry in consistency; you should be able to mark it with your nail but not with the flat of your thumb; if it’s still soft more time is required for the render to cure and firm up. Again, curing and protection are essential for lime mortars, so ensure best practice is followed.

Float Coat

The float coat is the final coat, and its finish is subjective depending on the final aesthetic you are trying to achieve. If looking for the utmost performance, a traditional harl coat or roughcast coat thrown on as a slurry or wet dash is an option, especially for exposed areas. The highly textured surface of a harl coat assists with water runoff away from the body of the wall and a higher surface area for any water that is absorbed to evaporate away faster.

With that said, a more contemporary smooth layer with finer sand is also a suitable option and popular for those painting or limewashing afterwards.

An appropriate mix for a smooth float coat could be as follows:

• 1-part Cornish Lime Mature Putty (NHL2 if necessary)  
• 3-part washed plastering sand, Cornish Lime CLS35

For those looking for a fine premixed mortar, our own Cornish Lime CLM35 Fine Putty Mortar is an option to consider, made with a fine flint sand.

Browse our range of lime putty mortars

To begin, wet the background enough to control suction adequately. Next, apply the render as evenly as possible to a thickness not greater than 7mm. Lastly, using a float or sponge after application, ensure to close the render surface tightly. As best practice, we often advise a “less is more” approach for top coats, as this can mitigate stressed induced cracking from the floating action.

It’s worth noting that achieving a relatively plumb or flat render finish with cob is challenging and often unlikely, so managing this will be subjective.

Further, it’s also important to note that due to the porous nature of cob, uneven layers can show up as ghosting. This type of ghosting won’t be the appearance of joints but rather result in darker areas that resemble damp patches.

Effectively mitigating this ghosting requires practice and experience but a good start is to ensure that suction is controlled adequately and that layers of render are applied as evenly as possible.

As with any lime render, we always recommend using a breathable paint or limewash instead of synthetic masonry paints. Find out more about breathable paint and its benefits for lime render.

Summing it up

Similar to lime, cob is a traditional building material that has been used for centuries. Before beginning any rendering work, always asses the stability and strength of the cob and what preparation will be required. It’s important to note that every project is unique, so your material selection, mix and approach should be tailored to suit your project. Once your rendering project is complete, as with any lime render, we always recommend using breathable paint or limewash to decorate.