External Rendering Outlined
The information on the following pages is only to be used as a general guide; further information is available in I.S. EN 13914: 2005 Parts 1 & 2 Design, preparation and application of external rendering and internal plastering. In addition, the guidelines outlined by the designer should always be followed.
External rendering should be durable, water resistant, weather uniformly and be attractive in appearance. The recommended order of building up render is:
1) Scud coat
2) Scratch coat
3) Final coat
4) Finish coat
External rendering durability
A number of factors affect the durability of rendering. Detailing, degree of exposure of the building, proportions of the mix, bond between the rendering and the wall, and the general standard of workmanship are all factors that affect its durability.
Resistance To Moisture
Since moisture is more likely to penetrate through some absorbent material than through cracks in strong dense rendering; a detailed design and the design of the mix being used are of huge importance for avoiding water penetration.
Uniform weathering depends on many factors. The finished surface, the nature of the mix as well as structural details such as sills, eaves and so on all affect the ability of rendering to weather uniformly.
Specification For Plain Rendering
Blockwork should first be scudded with a mix: 1: 1.5/2 – cement:sharp sand
Should then be finished with 2 coats of render mixed in the proportions: 1: 0.5 : 4/4.5 – cement:lime:sand
Manufacturer’s recommendations for placticisers should be strictly followed where used. In order to prevent excessive air in the mix resulting in a loss of strength and durability, exercise caution when mixing. Avoid overdosing and overmixing the mix.
Volume Batching & Scud Coats
Properly constructed gauge boxes should be used when carrying out volume batching.
Rendering should never be applied to blockwork unless an adequate key exists to hold the rendering both during and after its application. Stress between the rendering and the background occurs when the rendering shrinks. The bond between the rendering and background must be
strong enough to resist this stress. The scud coat is recommended as it provides a key.
The scud coat mix should be quite dry, with just enough water to give the mix the consistence of thick slurry. It is usually applied using a hand scoop to throw the mix at the wall to a thickness between 3 and 5mm. While the scud coat is hardening, the surface should be wetted periodically.
Scratch Coat (First Undercoat)
The scratch coat should be between 8 and 12 mm except for in localized areas when the maximum should be 16 mm. Once applied and left long enough to be firm, the scratch coat should then be scratched or combed so as to provide a key for the next coat.
Final Coat (Second Undercoat)
Once the scratch coat has dried and hardened enough to provide adequate suction, begin applying the second undercoat. If necessary, the suction of the scratch coat can be lessened by applying water. The final coat should be less than the scratch coat in normal finishes, the thickness of the final coat should be between 6 and 10 mm. The final coat should also be applied using a suitable float (wood or plastic). A steel trowel is not suitable and should not be used. Over-working should be avoided also.
Ensure that the nominal thickness of the two undercoat applications is greater than 20 mm.
It should be specified that additional render coats should be less strong than the previous coat or background.
Float-ready plaster is premixed mortar produced in mixing plants and delivered to site. It is created especially for external rendering and internal rendering and should be used in strict accordance with the manufacturer’s instructions.
Types Of Finish
Finishes are not considered undercoats as they are decorative coatings/textures that are applied on top of the undercoats. In general, textured finishes are easier to make look uniform: they are less prone to cracking or crazing, although they are more susceptible to dirt lodgement. A rough surface tends to even out discoloration which makes dirt less obvious. Also, the distribution of flow of rainwater lowers the risk of rainwater penetrating through the rendering.
There are 2 general categories for external renderings: trowel finishes and thrown finishes.
This is a finish that requires a high standard of workmanship in order to avoid crazing and/or discoloration. For this type of finish, a wooden trowel is used. A steel trowel should never be used for this type of finish.
Scraped Or Textured Finishes
There are a couple of ways of achieving a textured finish; it can be achieved using a trowel or using rendering machine. A scraped finish is achieved by letting the final coat harden for several hours, then scraping it with some appropriate tool.
Where exposure is high, thrown finishes are generally preferred to trowel finishes as they provide greater weather resistance, they are less likely to crack, and they are more durable. Roughcast and Drydash finishes are usually used on strong backgrounds if backgrounds are weaker; the mix properties will need to be changed to account for it.
For a roughcast finish, the final coat of rendering is thrown on as a wet mix. This final coat is then left un-trowelled to set. The shape and size of the coarse aggregate in the mix determines the “roughness” of the finish.
For a dry dash finish, a layer of mortar known as a buttercoat is applied. While the mortar is still wet, crushed rock chippings or pebbles are thrown at the wall.
Ways to minimize the risk of crazing:
Use of properly graded sand. Avoid using very fine material, such as silty dirty sand.
By using a mix with a relatively low cement ratio.
Avoid over working; over working draws up an excess of laitance to
Avoid allowing the final coat to dry too quickly.