Using a render pump to achieve a roughcast finish

There are many different makes and models of render pumping machinery that may be used for monocouche render application of both weberpral M and webertherm M1 when creating a textured ‘rough cast’ finish.

Weber would recommend the use of a machine with automatic variable speed control. Invariably batch mixing diesel pumps have this capability; however some electric machines do come with this function inbuilt. The expectation of quality standards is to achieve consistency in texture and colour, the most common defect in roughcasting is referred to as merging. 

The effect of merging is a concentration of material forming too quickly in an isolated area. As the material cures the issue is highlighted with a lightening of colour as cement rich laitance dries on the surface of the work. The indicator for merging is a combination of a change of texture and colour. Principally, merging is a workmanship defect resulting from an inability to maintain a consistency in the spray. However, this problem is much more likely to occur if the speed of the pump cannot be controlled accordingly. If the rate of spray is too quick the ability to manoeuvre around scaffolding will be more challenging.

Full access should be available between scaffold and the wall to enable a continuous wet edge of material to be maintained across the complete elevation. Particularly when texturing standards and ledgers should be spaced sufficiently away from the wall to allow applicators to maintain the gun nozzle in a 90 degree angle to the wall. In some cases it is feasible to undertake an adaption of machines that do not have a variable speed control setting. Most render pump suppliers/distributors of these machines will provide relevant training on these specific requirements.

Machine application

For the use of a continuous mixer without variable speed control the output should be adjusted by utilising an appropriate rotor and stator. As an example on standard applications (non rough casting texturing) a D6-3 (20L/Min.) rotor and stator with approximately 1 meter square per minute at 18mm is in order. This is too fast to use for texturing. For texturing a rotor and stator with less output is a D 4-3 (12-14L/Min.) with approximately 0.3 of a meter square per minute at 18mm. This will provide greater control. You may also use a D3-4 (8L/Min.), this would be ideal for texturing but too slow to use for the initial render pass.


It is imperative that consideration is given to the water settings on continuous mixers. This can be difficult to adjust and read if the rotor and stator with a low output is used. If this is the case you would need to change the water float (red) to a lightweight float (green). Many machines have a water reading chamber with both markings red and green. Usually the green markings for the lightweight float are to the back, simply loosen the barrel and twist to show the green markings at the front. If this is not the case you would have to change the barrel as well as the float.

It is essential that the correct mixing pump is employed for successful texturing. The initial basecoat should be applied by machine at a thickness relevant to satisfy the exposure rating. The base coat should be ruled level and flat and then allowed to pick up for a period of between 1 and 2 hours before application of the second pass.

The second pass gives the textured finish to the desired effect. Various degrees of textures can be achieved from a Tyrolean fine finish through to a heavy roughcast effect. Regardless of texture chosen it is imperative that the combined final thickness of the render, at its lowest point, relates to the exposure rating.

Due to extensive choice of different textures it is advisable to prepare an on-site sample for client approval prior to commencement of works. The process of achieving these textures is by lowering the pressure of the render pump for heavier textures and speeding up for finer finishes. Additionally the nozzle size of the spray gun can be varied to assist in these textures. A consistent texture is normally produced by moving the spray gun with a circular action, maintaining a regular distance and orientation between the nozzle end and the render panel. During the application of this pass a flowing edge must be maintained and thought should be given regarding potential obstructions when applying texture (design of scaffolding), the less number of interruptions to this operation will allow for the best finish.

It is worth noting for all machines during texturing when the texture gun is shut off by the valve, the pressure within the pipes will force the render to ‘ooze’ out at the end releasing any pressure. When the machine is started back up, you must allow the pipes to build back up to the same pressure as before it was turned off (spray into a bucket). By not doing so this may give a different texture to the finish. 

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