design considerations

When selecting your render, it is important to ensure that the substrate is suitable for receiving the selected render.  All too often we see render that has cracked, failed or suffered due to poor substrate, building design and preparation.  Here we have outlined some points for consideration and further information can be supplied for specific applications and sites as required. 

We recommend the use of factory blended renders from JUB, K-Rend, Scotrend and Parex.  By utilising these pre-blended products, the render will be manufactured to a consistent quality and colour in factory conditions with technical back-up and in some cases a full manufacturer’s warranty. 

Render manufacturers spend a lot of time and money developing products that work together to allow substrates to breath and for renders to perform in an anticipated manner. By using a base coat from one manufacturer and a top coat from another and creating a hybrid system, problems may arise in the future, and there will certainly be questions asked regarding warranties.  

We offer K-Rend, Scotrend, JUB and Parex products, and we work very closely with these manufacturers. They have considerable expertise in developing, testing and supporting the best products available. They support these products over the lifespan of the completed render and work with us to develop new products and to produce site specific specifications. Our policy is to offer products in manufacturer’s branded packaging to ensure there is no question over the source and history of the material.  This will offer the specifier, applicator and contractor peace of mind and absolute confidence in the selected product.

Before applying a render, here are some points for consideration;

  All renders are inert and have no structural capability. They are designed  as decorative products with certain weatherproofing properties.

  Renders are suitable for most substrates, however certain precautions and preparations may be required.

  The substrate should be designed in accordance with good building practice and the structure should be able to accommodate movement through drying out and normal flexural movement.

  Render should not be applied to sloping or horizontal surfaces.

  The substrate should be checked for line and level before applying a render.

  All substrates should be stable and mature. They should be a minimum of 7 days old, or possibly as much as 28 days old, depending on the weather at the time of construction.  Seven days of 25 degrees with no rain is very different to 7 days of 5 degrees in the rain.  Advice should be sought if there is any question about the maturity of the substrate.

  All surfaces must be clean, suitably dry and free from any friable material that may compromise the adhesion of the render to the substrate. It may be necessary to dampen the surface after extended periods of dry weather.

  Blockwork joints should be mature and not deeply racked back to prevent ghosting through the finished render.

  Substrate cracking (and subsequent render cracking) can be significantly reduced by incorporating bed joint reinforcement in the mortar throughout the building.

  The brick reinforcement manufacturer should be consulted regarding the placement of the reinforcement.

  Renders can crack at high stress locations. In the main these are around doors, windows and weep vents.  Although it will not stop a substrate from moving and cracking, the incorporation of a 160gm alkali resistant mesh cloth at these points will aid the performance of the render.

  The inclusion of 160gm alkali resistant mesh cloth in all base coats can increase the long-term performance of the render and should be considered if budget allows.

  Temporary rainwater goods should be considered and designed for use during application and then all gutters and downpipes should be installed and completed as quickly as possible after the render has been finished.

  Renders should not bridge a DPC.

  The use of different materials on a single elevation should be avoided as it can lead to differential movement and subsequent cracking. If it is not possible to avoid multiple substrates on a single elevation, advice should be taken regarding the base coat selection and the inclusion of reinforcement.

  Sills, copings, flashings, overhangs and soffits should be designed to ensure water does not wash or splash down the render. This can cause unsightly staining over a period of time. Joints in copes and flashing should be designed, installed and positioned to prevent water dripping onto the render below.

  All structural movement joints should be mirrored through the render system

   A continuous bead of mastic or silicone should be applied where the render meets other materials. For example, around utility boxes, door and window frames, vents and facias etc to prevent any water ingress.

   Beads should be aluminium, stainless steel or PVC to avoid rusting.

  Different manufacturers produce blocks to different specifications. Always request the water absorption figure from the brick or block manufacturer as this can have a significant bearing on the preparation required for a successful render.

Contact Us

If you need any further installation advice or would like to discuss your project with us, call one of our advisors who will be happy to help you.

Tel:  0141 548 6010
Email:  rowebb@rowebb.com