Rendering Exterior Walls - How to Render an Outside Wall Including Getting the Right Mix and Correctly Applying and Finishing the Render

Summary: How to render a wall, particularly how to render exterior walls follows a very similar process to that of plastering internal walls. It’s pretty much the same job but done outside or externally. In this DIY guide you will learn about the different types of render such as sand and cement and lime, how to get the correct mix for your render, how to apply the first coat and then subsequent second coat and get them flat and level and also why you should then sponge the render over. We will also tell you how long you should leave between coats and what you need to do to ensure a strong and lasting bond to the wall beneath. In some situations, your wall may need to comply with Building Regulations in terms of the thermal qualities of the finished job so this is an important point to be aware of.

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Like plastering, rendering is a job that requires some skill and plenty of practice. We don't advise you to start on rendering house walls without some experience, but there's no reason why you shouldn't have a go at a garden or garage wall to start with.

Preparing Surfaces for Rendering

Unless the wall has been recently built, you will need to clean it well – remove any loose paint, and dust or loose debris, plants growing up it, scrub off any mould with a bleach, or fungicide solution, and hose down the wall to remove any dust and dirt that could stop the render from adhering.

After washing down It will be Okay to render the wall if it is still damp but do not try and render while it is still running with water.

Try to make sure you have a couple of fine days to do the job in – if it rains too soon after you apply the render, it will be washed off the wall. A little forethought here could be handy so think about fixing a tarpaulin or strong polythene sheeting to the top of the wall you are working on so if the rain does come down you can simply drop the protection to keep it dry.

Getting the Correct Mix for Your Render

For detailed information on how to correctly mix your mortar see our project on Mortar Mixes. Your equipment, including mixer, water, soft sand, sharp sand and cement should be as close to the work as possible to save on time running backwards and forwards. Also remember to clean your tools thoroughly after and between usage.

Generally speaking, the render you are applying is there to form a solid, strong, waterproof surface to cover the layer below, be that brick, block, stone or even old render. As such the mix needs to be strong and wherever possible, this strength is increased by adding some sharp sand (or grit) to the mix.

The sharp or coarse sand is a lot grittier than soft sand or builders sand. Soft sand contains more clay than sharp sand and is a lot more flexible. The sharp sand is added to reduce the flexibility of the render and to give it greater strength against both cracking, shrinkage and erosion.

The video at teh base of this page shows the correct proportions of soft to sharp sand and the mix used on the wall is a top quality strength mix which will protect most external walls very well.

When rendering a chimney stack the mix should be a little stronger as the weather erosion is greater so follow the mix guidelines on the link above to our mortar mixes project.

If it is likely to be very cold in the evening with even a hint of frost or ice, then the work must be covered (preferably by hessian sacking or even a old blanket) to stop the water in the render freezing.

If it is allowed to freeze it will expand and on thawing it will leave a void which will again fill with water when it rains. This will also freeze and so the process goes on. See our project on Freeze-Thaw action for more information.

The best way to mix the render is with a cement mixer as this ensures that the mix is even and that the correct amount of water is added. Mixing in a wheelbarrow is fine but almost always there is a little unmixed sand and cement at the bottom which does not become apparent until emptying out the barrow.

Mixing sand and cement render in a mixer

A mixer is the best way to mix sand and cement render

How to Apply Render to Your ExteriorWalls

Applying the First Coat of Render

If you have a wide wall to render, setting up screed battens can be very helpful. These are 15mm thick wooden battens, which you fix to the wall. See our project on fixing to masonry to accomplish this.

Screed battens fixed to wall to help level render

Screed battens fixed to wall to help level render

Use a straight edge to level up render

Use a straight edge to level up render

Place them about 900mm apart and make sure the screws are flush with the face of the timber because you will be using these battens to help you level the render as you can see in the image above.

These battens serve to divide the wall up into more manageable portions, and also gives you a starting point so that you know how thickly to apply the render. Dampen the wall before you start if it is not already damp after cleaning down.

Applying the first or scratch coat of render

The first coat of render is also called the scratch coat

Apply the render with a steel plastering trowel using a firm hand to press and ensure it sticks to the damp wall. The first coat of render should be a very thin coat of about 5mm thick. This coat is pushed well into the wall.

The thickness is important because we are asking the wall to start “sucking” the render in for it to adhere properly to the wall. It can only do this effectively if there is little weight being applied as, if we try and get too much on the wall in one go, gravity will start to pull it off.

5mm of render is about the thickness that the wall can start to absorb and which will “stick” almost immediately. Once the 5mm is on all over the wall, we can return to the second coat after “scratching” the first coat.

Use a scratching comb to key render for next coat

Scratching comb for keying first coat for better bond to second coat

Scratch the surface of the first coat to enable the second coat to bond. Professional plasterers use an over-sized comb type tool for this (senn in teh image above). You could make your own by hammering four or five nails into a piece of wood or simply running over the surface with a screwdriver.

There is no need to scratch right down to the original wall surface, just deep enough to provide a key for the second coat.

Applying the Second Coat of Render

The second coat can be applied within half and hour or so of the first, just long enough for the first 5mm coat to become well adhered to the wall.

For the second coat, which should be about 10mm thick, build up the coat to the thickness of the batten and even a little beyond as it can then be levelled it by using a straight-edge as shown above. Work up the wall from the bottom, moving the straight-edge from side to side as you move upwards.

As you drag the straight edge up the wall it is inevitable that you will leave some holes as the render clings together. Fill in any holes with the trowel and repeat the straight edge levelling process using the battens as guides.

When the render has been on the wall for an hour or so, remove the battens carefully and fill in the holes left by them. This is often best done by using a pointing or a gauging trowel. Scrape the surplus render from the straight edge back onto your spot-board.

See our project on base coat plastering as the principle here is exactly the same.

Polyurethane rendering float

Polyurethane rendering float - Get one here in our online store

Floating up the Render

Once the battens have been removed and the voids filled to be level with the rest of the wall, the wall should be left for another hour or so to start the drying out process. Once it is hard enough to push a rendering float onto the surface without render bulging out around the float, you can begin to float the surface up.

Floating has the effect of closing up the render. All the little air holes are filled and the render becomes smoother and more waterproof.

Floating up render with a polyurethane float

Use a wooden or polyurethane float to get a good render finish

The float should be pushed fairly firmly (only practice will show you the pressure required) onto the surface then moved in a circular motion as you can see in the below video. Cover the whole wall and then go back and start again. You will end up with a lovely smooth, closed surface.

It sounds simple but you'll find that the technique takes a bit of practice to get used to it and it may take you a while before you can manage a nice smooth finish.

The battens fixed to the wall will make your first couple of attempts a lot easier as the wall (providing it was fairly flat in the first place) should be flat. If you try to apply the render free-hand and you do not have the experience of getting a uniform thickness on the wall, then the difference in thickness will not only lead to a difference in surface level (leaving it almost impossible to trowel smooth) but could leave it prone to cracking where thick render meets render of a different depth.

Sponging Down the Rendered Wall

Many plasterers (Renderers) will use a large ordinary car sponge which is very slightly damp, to give a final pass to the wall. By rubbing very gently, as you can see from the video, the wall surface becomes completely closed, with no holes making it very smooth, very waterproof and easy to paint.

Sponging wall after trowelling render

A sponge finish on render makes it really smooth and waterproof

Delay Time Between Coats of Render

The guidelines given in this render tutorial are exactly that - guidelines! It does not matter if you leave the wall a week or even a month, between cleaning it down and rendering it. Just make sure that the wall is slightly damp when you start and that you have brushed it over to remove any new dust that may have accumulated.

Similarly, it does not matter if there is a week or two between the first coat and the second coat as long as the same dampness is applied to the wall before the second coat is applied.

Spraying wall with water to damp down for render

Damping the render down gives better adhesion

The reason for the surfaces needing to be damp before applying any render so the wall does not suck out the moisture from the render mix too quickly. If this happens, the wal simply absorbs the water from the render leaving effectively a dry mix on the surface which will soon crumble and fall off.

If the render is allowed to dry out slowly (achieved by a slightly damp wall underneath) the render dries while soaking slowly into the wall forming a  much stronger layer which is firmly fixed to the wall.

Using Lime Mortar for Rendering

If you live in a listed building or your walls were originally built in lime mortar, you will be expected to complete any rendering in lime.

Lime does the same job as cement in that it holds the particles of sand together while setting (albeit much more slowly) but its real value is that it allows any moisture trapped in the surface of the building to evaporate out.

This evaporation is called “breathing” so when you her the phrase “The walls must be allowed to breathe” this is what it means.

Lime mortar is also much more flexible than cement mortar and given that older walls move more than their newer counterparts, the lime allows the walls to move more without cracking.

Lime is normally mixed with the sand at a 3 to 1 ratio. Some builders merchants sell bags of lime which is especially designed to work with cement so a mix could be used which incorporates both. This gives some breathability while setting harder more quickly but advice should be sought when considering using lime mixes of any kind at home.

Notes about Rendering and the Building Regulations

If you do plan to render a house wall you should be aware that changes to the Building Regulations dictate that you must check with your local Building Control Department of your council as to whether your changes (i.e. the render) will mean that your wall must be upgraded to comply with the current “Thermal Element” regulations.

A Thermal Element is a wall, roof or floor which separates any heated part of a building from and space which is unheated, e.g. outside.

If your proposed rendering will cover more than 25% of the wall you may have to upgrade the wall to comply with current insulation requirements if this is possible. For a cavity wall for example, this may mean you need to insert cavity insulation.

More information can be found in Part L1B (Appendix A) of the Building Regulations Document on The Conservation of Heat and Power in Existing Dwellings. For more information see our project on Building Regulations Approved Document L.

Additionally, if you are considering rendering a wall or walls then check out our project on the different options for rendering found here.

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