Achieving a Flawless Finish for the Outside Walls

Similar to plastering, rendering is a task that requires a certain level of skill and ample practice. We do not recommend beginning with rendering house walls without prior experience. However, there's no reason why you shouldn't attempt rendering a garden or garage wall as a starting point.

Preparing Surfaces for Rendering

If the wall has not been recently constructed, it is necessary to thoroughly clean it. This involves removing any loose paint, dust, or debris, as well as any plants growing on it. Additionally, scrub off any mold with a solution of bleach or fungicide, and rinse the wall with water to eliminate any dust and dirt that might hinder the adhesion of the render.

After washing down the wall, it is acceptable to proceed with rendering if it is still slightly damp. However, attempting to render while the wall is still dripping with water should be avoided.

It is preferable to choose a couple of consecutive sunny days to carry out the job. If it rains too soon after applying the render, it will simply wash off the wall. To prevent this, you might consider attaching a tarpaulin or a sturdy sheet of polythene to the top of the wall you are working on. This way, if it does rain, you can easily drop the protection to keep the wall dry.

Obtaining the Proper Mix for Your Render

For detailed instructions on correctly mixing your mortar, you can refer to our project on Mortar Mixes. Make sure to have all your equipment, including mixer, water, soft sand, sharp sand, and cement, close to your work area to save time running back and forth. It is also important to clean your tools thoroughly after each use and in between.

In general, the render you are applying serves to create a solid, sturdy, and waterproof surface to cover the underlying layer, whether it be brick, block, stone, or even old render. Therefore, the mix should be strong, and if possible, adding some sharp sand (or grit) will further enhance its strength.

Sharp or coarse sand has a grittier texture compared to soft sand or builders sand. Soft sand contains more clay and is more flexible, while sharp sand reduces flexibility and provides greater strength against cracking, shrinkage, and erosion.

The video found at the bottom of this page demonstrates the correct proportions of soft to sharp sand. The mix used on the wall is of high quality and offers excellent protection for most external walls.

When rendering a chimney stack, it is advisable to use a slightly stronger mix due to the increased exposure to weather erosion. You can follow the mix guidelines provided in the aforementioned link to our mortar mixes project.

If there is a likelihood of very cold temperatures in the evening, with the possibility of frost or ice, it is crucial to cover the work area. Hessian sacking or even an old blanket can be used to prevent the water in the render from freezing.

Allowing the render to freeze can cause expansion, and upon thawing, it will create voids that fill up with water when it rains. This cycle can perpetuate. For more information on this freeze-thaw phenomenon, please refer to our project on Freeze-Thaw action.

Using a cement mixer is the most effective way to achieve a uniform mix and ensure the correct amount of water is added. While mixing in a wheelbarrow is acceptable, there is often some unmixed sand and cement left at the bottom, which may only become apparent upon emptying the barrow.

How to Apply Render to Your Exterior Walls

Applying the First Coat of Render

For wide walls, it is beneficial to set up screed battens. These are 15mm thick wooden battens that you attach to the wall. Our project on fixing to masonry provides further guidance on accomplishing this.

Installation of screed battens to wall for the purpose of achieving a level render

Achieve a level render by utilizing a straight edge

Position the screed battens approximately 900mm apart and ensure that the screws are flush with the surface of the timber. These battens will serve as a guide for leveling the render, as depicted in the image above.

These battens have a dual function of dividing the wall into more manageable sections and providing a starting point for determining the thickness at which the render should be applied. Prior to commencing, dampen the wall if it is not already moist after cleaning.

The initial layer of render is commonly referred to as the scratch coat

Apply the render using a sturdy steel plastering trowel, exerting pressure to ensure proper adherence to the damp wall. The first layer of render should be quite thin, approximately 5mm in thickness. This layer needs to be firmly embedded into the wall.

The thickness of this layer is crucial as it allows the wall to effectively absorb the render, promoting proper adhesion. If too much render is applied at once, gravity will cause it to detach from the wall.

A thickness of 5mm is the point at which the wall can begin absorbing the render, resulting in immediate adhesion. Once this thickness is uniformly applied to the entire wall, proceed to the second coat after "scratching" the first coat.

Utilize a scratching comb to create furrows on the surface of the first layer, enhancing its bond with the second layer. Professional plasterers often use a specialized comb tool for this purpose (as seen in the above image). Alternatively, you can create your own tool by affixing four or five nails to a piece of wood or simply run a screwdriver over the surface.

There is no need to scratch all the way down to the original wall surface; the furrows must be sufficiently deep to provide a key for the second layer.

Applying the Second Layer of Render

The second coat can be applied approximately 30 minutes after the first coat, allowing sufficient time for the initial 5mm layer to firmly adhere to the wall.

For the second coat, which should be around 10mm thick, build up the render to match the thickness of the battens, or slightly beyond. This excess can then be leveled using a straight edge, as demonstrated in the above image. Begin working from the bottom of the wall and move the straight edge from side to side as you progress upwards.

As you move the straight edge vertically, gaps may appear in the render. Fill these gaps using the trowel, and repeat the leveling process with the straight edge, using the battens as guides.

After the render has been on the wall for approximately an hour, cautiously remove the battens and fill in the resulting gaps using a pointing or a gauging trowel. Scrape off any excess render from the straight edge and return it to your spot-board.

For detailed information and guidance on base coat plastering, please refer to our project: Base Coat Plastering Project. The principles discussed there are applicable in this context as well.

Find a polyurethane rendering float in our online store by clicking [here].

The Art of Floating Render

After removing the battens and filling in the voids to align with the rest of the wall, allow the wall to sit for approximately an hour to initiate the drying process. Once the render is solid enough for a rendering float to be applied without causing bulging, it's time to start floating the surface.

Floating serves the purpose of refining the render. It fills in all the tiny air pockets, resulting in a smoother and more waterproof finish.

To achieve a quality render finish, use a wooden or polyurethane float.

Apply moderate pressure (the required force will become clear with practice) to firmly press the float against the surface, then move it in a circular motion, as demonstrated in the accompanying video. Work your way across the entire wall and then repeat the process. This will leave you with a beautifully smooth and seamless surface.

While it may seem straightforward, mastering the floating technique requires practice, and it may take some time before you can achieve a flawless finish.

If the wall was initially fairly flat and you have the aid of fixed battens, your first few attempts will be easier since the wall should be flat. If you attempt to apply the render without the assistance of battens and lack the experience to ensure a uniform thickness, the variations in thickness will not only result in an uneven surface (making it nearly impossible to trowel smooth), but it can also lead to cracks where thick render meets areas with different depths.

Applying the Final Sponge Finish to the Rendered Wall

Many plasterers and renderers employ a slightly damp, large car sponge for the final pass on the wall. By gently rubbing the sponge, as shown in the video, the surface becomes completely smooth, free of any holes, and highly waterproof, making it ideal for painting.

A sponge finish provides an exceptionally smooth and waterproof render.

Waiting Between Render Coats

The instructions outlined in this render tutorial are just that - instructions! It doesn't matter if you leave the wall untouched for a week or even a month between cleaning and rendering. Just ensure the wall is slightly damp when you begin and brush off any accumulated dust.

Likewise, there's no issue in waiting a week or two between the first and second coats, as long as the wall's dampness remains consistent before the second coat is applied.

Dampening the render promotes better adhesion.

The reason for dampening the surface before applying render is to prevent the wall from absorbing moisture too quickly from the render mix. If this occurs, the wall will effectively drain the water from the render, leaving behind a dry mix that will eventually crumble and detach.

When the render dries gradually (due to a slightly damp wall underneath), it penetrates and bonds with the wall, creating a much more resilient and securely attached layer.

Using Lime Mortar for Rendering

If you reside in a listed building or if your walls were originally constructed using lime mortar, it is necessary to utilize lime for any rendering.

Lime performs the same task as cement by binding sand particles together during the setting process, albeit at a much slower pace. However, the true value of lime lies in its ability to facilitate the evaporation of any trapped moisture on the building's surface.

This evaporation process is commonly referred to as "breathing," so when you hear the phrase, "The walls must be allowed to breathe," it essentially means enabling this evaporation.

Compared to cement mortar, lime mortar possesses superior flexibility. This is particularly beneficial for older walls, which tend to experience more movement than their newer counterparts. Lime allows these walls to adapt to movement without the risk of cracking.

Typically, lime is mixed with sand at a 3 to 1 ratio. Some suppliers offer lime specifically designed for use with cement, allowing for a combination of the two. This mixture provides a certain level of breathability while setting faster and harder. However, it is advisable to seek guidance before using any lime mixes in a residential setting.

Important Reminders about Rendering and Building Regulations

If you intend to render a wall of your house, it is crucial to be aware of the Building Regulations' recent changes. You must consult your local Building Control Department to determine if your proposed changes, such as rendering, will require upgrading your wall to comply with the current "Thermal Element" regulations.

A Thermal Element refers to a wall, roof, or floor that separates any heated area of a building from an unheated space, such as the outdoors.

If the area you plan to render exceeds 25% of the wall's surface, you may need to upgrade your wall to meet the current insulation requirements, if feasible. For instance, in the case of a cavity wall, this might involve installing cavity insulation.

Further details can be found in Part L1B (Appendix A) of the Building Regulations Document on The Conservation of Heat and Power in Existing Dwellings. To learn more, refer to our project on Building Regulations Approved Document L.

Also, if you are contemplating rendering a wall or multiple walls, explore our informative project about the various rendering options available.