Information about coloured renders, silicone render, acrylic render finishes & Cement render Pre-bagged render such as K Rend Weber Sto and Parex
We originally intended to take it easy on the text
But we felt that the knowledge gained from over 40 years’ experience surveying and applying exterior wall renovation products may be appreciated by those that may be embarking on a rendering project.
We often hear the same remarks when chatting with customers one thing we often hear is builders have different opinions especially about the very important surface preparation and material specification
All we intend to do with this section is to hopefully explain the ins & outs of rendering and general wall care.
Rendering a house with scaffold costs, whether its a new build render or refurbishment of old render is not a cheap exercise.
Not only is the financial outlay a big decision often running into thousands of pounds a badly designed or installed render or coating can seriously ruin the fabric of the building, the last thing you want to do it to have to take it off.
So Please Do Read on:
Over the last decade or so pre-bagged factory render systems have become very popular.
Before these pre-made proprietary systems became readily available rendering contractors, builders, plasterers etc mixed up their own renders on site.
This is nothing new, of course, renders have been around for thousands of years. In Rome, you can still see some fine original examples of lime-based Stucco scagliola, etc. that still look good today.
Some Renders fail and delaminate soon after being applied
The failure of most renders can be attributed to a few main flaws.
Listed below but in no main order are some common defects and mistakes that will cause cracks and the premature failure of render
The mortar mix.
Using a made on-site mix of unknown ingredients what we call “bucket chemistry render mixes”
Wrong sand (more on this below)
Incorrect use or overuse of admixtures such as latex SBR, PVA, waterproofer, accelerant/ frost proofers, plasticisers (some builder even recommend adding washing up liquid to render mixes)
Incorrect ratios of ingredients
Too much or too little water
Inadequate preparation of the substrate or background onto which the render is going onto.
The surface could just need a simple wash down to remove friable material or moss, this minimal task is often ignored
Just a little more on the subject of rendering over unsound or painted surfaces.
We see many failed jobs when sand and cement renders have been stuck over painted brickwork or other painted unsound renders.
Care should be taken especially with lime-washed surfaces as most modern renders will not adhere to this finish
Don’t let any builder persuade you that the PVA glue he’s is going to apply will provide the key and do the job.
Chances are the glue will emulsify and the render will crack and fall off!.
We normally water jet and steam paint, blast or chemically remove it. If it’s a painted render surface we normally take the render off and the paint with it.
If this is not possible, cut deep chases into the render or if its brick or block grind the horizontal mortar beds back 10-15mm, wash down the surface and then apply the render.
The new rebates you have made provide a good key for the render.
The best renders for the base coats over unsound or unusual surfaces are normally a render with a high polymer content and microfibres.
K Rend make such a product called HP12
If it’s a damp solid wall, cement may not be the best finish, a lime product such as Parex Parlumiere may be a better alternative
Polymer renders can stick to glass and will provide a good base, Ardex X7G can also be used to provide a high adhesion base coat on difficult substrates. (on the right wall build)
Back to other mistakes
Not controlling suction of the material, (the surface sucking the moisture too quickly from the mix causing renders to crack and delaminate, very commonly seen on aerated blocks )
Insufficient mixing of the combined ingredients to form a homogeneous mix
Poor quality workmanship, such as applying in bad weather, applying too thinly, overworking the render with plastic or wooden floats
Use of hard cement renders on old stone brick walls (more on this later)
A basic render most commonly consists of sand and cement
The binder or Cement…..AKA ordinary Portland cement (OPC)
Sometimes Lime (NHL, lime putty with a pozzolan hydrated lime and cement)
Although not that common nowadays, but making a return amongst the more learned, lime was all that was used before the last century or so
Sand is mixed with water and the binder at around four parts sand to one-part binder (cement or lime)
One very important part of the factor when mixing this lot together is the sand.
The sand used is critical in determining the quality of a render mix and the longevity of it.
Many builders use incorrect soft yellow sand or fine silica sand (like play pit/ kiln sand)
Remember the sand has to be glued together by the cement or lime to form a cohesive layer when it cures. If the sand particles are too fine they don’t lock together properly.
Also, the shape of the sand particles plays an important role in a good mix. If they are angular in shape and irregular, they will lock together better.
Imagine trying to glue lots of marbles together, you can imagine it would be a difficult task, prone to failure but if you smashed them up with a hammer the sharp fragments would bind and lock together with much better adhesion. A well-made render will not crack or break down and become powdery.
So sharp sand particles lock together better than round ones
But that’s not all. The sand should be well graded, clean and free from impurities such as mud clay silt etc.
More information on sands if you google search this:
BS EN. 13139:2002
A lack of skill and knowledge with an over-reliance on pre-made renders and plastic coatings has ruined many a fine building.
Best Way to make a good render
A good general guide if you want to make your own modern render mix for a cavity wall build is to use sharp sand for the base coat with a finer graded sharp plastering sand and cement with a little latex SBR added to the gauging water at 4/1 or 5/1 The SBR makes your mix a polymer render, which is another buzzword in the rendering industry.
You can also add chopped glass fibre strands to the base coat
(or hair of any type if you’ve got some kicking about)
Render for the base coat/s should be greater in strength than the one going over it so you build up in weaker mixes as you go.
The benefits of a factory-made pre-bagged render
Having a render that you can buy in the bag mixed with the correct type and ratios of sand & cement and other ingredients (dyes and polymers) makes sense.
This pre-bagged approach has been made even more attractive especially when pigmented so the final finish can be through-coloured saving the need to paint the render. Self-coloured or through-coloured renders are not new. Products such as Blue Circle Tyrolean and Decromace have been around for decades.
Scagliola plasters and renders dating back to the Romans and beyond used pigments to create wonderful marble and stone effects
Sand & cement renders made on-site normally would not normally be pigmented, so you would have to paint the render, rather like plaster.
Having the render self-coloured (pigmented with mineral dye) can provide you with a good window of time (usually 5-10 years) before the render needs painting.
Dismiss claims by installers and manufacturers that they are self-cleaning, these claims should be taken with a large “pinch of salt”
Mother nature and the test of time will dictate otherwise.
To summarise with a pre-made proprietary render the contractor and the customer has some of the problems and risks associated with render failure reduced.
Just like having a cake mix product providing, they are mixed properly and in this case applied correctly.
Using a bag of render with all the ingredients inside the bag can minimise defects in years to come (or sooner)
We get asked the same old questions from clients fresh from googling
What is the best render for solid walls or what’s Monocouche render
Is K Rend the best render
(Monocouche just means one coat, layer, it’s a French term)
Not many people ask how thick you should apply the render and how many coats should it be built up from?
Normally renders are 10mm thickness minimum usually 15mm for block or brickwork
It’s surprising how often contractors are given the go-ahead for rendering contracts without actually specifying the thickness of the render
If you apply render too thinly you may get to see the outline of the underlying block or brickwork, ghosting or pattern staining as its called in the trade.
Also, if the render is too thin it could fail to protect against the elements and soon breakdown
Normally for brick or blockwork, a render is applied in two stages.
First, a base or scratch coat is applied. Described as a scratch coat as its marked and grooved to form a key with a tool (or with a chestnut stick for true aficionados of lime renders)
The base or scratch coat is about 10mm thick mixed around four parts sand to one part cement (or lime is added in the form of Hydrated lime to help plasticize the mix and offer a little flexibility, more on lime renders later)
Then a second float coat of 5mm is applied about mixed at a ratio of 5/1 or 6/1
Finishing the render
With made on-site renders this is normally ruled flat and true with a straight edge (also referred to as a darby) it’s just like shuttering a concrete floor, only vertically!
So around 10mm-15mm is the overall normal thickness, more if using lime in very exposed locations, especially on rubble or single-skin walls.
The finish coat is often worked with a wooden or nowadays a plastic float and sponged up to provide a flat neat finish, usually referred to as “rubbing up” in the trade.
The self-coloured renders, the factory-made ones (in the bag) are applied sometimes in two passes by hand or machine pump and then ruled off.
Once they are semi-hardened or “thumbprint green”: they are scraped back, by literally scraping them with a pronged float.
Rather like a trowel-float with nails in it
About 2mm is shed when this finish is carried out, so masking is the order of the day!
But things go wrong even with pre-coloured factory-made renders
Some newcomers to the through-coloured render scene, try and sponge the finish, (as they do with a made on-site renders that will normally get painted)
This is a big mistake and one we often see, clients are most unhappy with the lumps, bumps, trowel marks, and discolouration in their new render finish
The water the contractor flicks and sprays on the surface as they sponge up and the overworking of the render dilutes the dye out of the mix.
This makes a real mess of the job.
Even though the contractor often informs the customer that the finish will dry and become even over time, it never does.
Often the unhappy customer ends up painting it.
Painting through-coloured render kind of defeats the object of having a self-coloured render in the first place!
Can you paint coloured renders?
Yes but even a painted badly finished render will just highlight the undulations and imperfections even more.
If you are considering painting over self-coloured renders, be careful with adhesion as some of the through-coloured renders have a waterproofing agent incorporated into them such as a silicone
This additive is designed to make them less resistant to dirt pick up (so-called self-cleaning properties)
After a few years, this silicone integral additive should have been drawn out of the render by the sun, rain and wind so with a gentle wash down and acticide wash you should get good adhesion for the paint and be safe.
An excellent product for over-coating or painting render including self-coloured render is Sika 550 (formally decadex made by LPL) or the silicone paint made by Jubosil called Revital colour
We have been specifying these for over twenty years on the likes of K render etc. with great results
Another drawback of a through-coloured render is that they do not patch without signs of the repair.
Another way of finishing a render is to spray the final finish on and leave it with a texture, similar to Tyrolean or roughcast.
With the proper skills and techniques, textures of varying degrees can be achieved. From fine to heavy
Nowadays most customers want a smooth flat finish with no texture.
It should be noted that a textured finish weathers better than a smooth surface. Wind-driven rain is pushed across a textured surface and slowed down.
Roughcast, pebbledash or harling as it’s called in Scotland is a good example of why a textured surface has been the prefered choice for years for extreme weather protection and durability
So to summarise and offer some do’s
Make sure the render is at least 10mm thick and made up with the correct sharp sand and accurately gauged or use a pre-bagged render.
Make sure the contractor is familiar with the products he is using especially if it’s a pre-bagged product
Ask to see some work examples
Ensure the render is reinforced with nylon mesh. The nylon scrim as its called is sandwiched between render coats, this dramatically reduces the chances of cracks, water ingress and ultimately the failure of the entire render.
Make sure the surface to which the render is going onto is sound and clean.
Make sure your contractor is not using galvanised metal beading, (beading is the trim fixed around window and corners to form a neat edge, PVC-u or stainless is the best material to use for beading)
When using concrete render (sand and cement) allow for expansion joints on big sections.
Ensure the weather is correct not too cold (5c above and rising) and not too hot for the finish coats.
Do not render down to the ground level or below the damp proof membrane ( DPM) unless some protection against rising damp has been designed into the detail.
Make sure proper scaffold is in place… You will not get a good job done off bits of tower scaffold.
Make sure that paths, roofs, windows etc with low tack protection and clean up after each day, polymer renders do not clean up easily.
A Silicone render refers to the final finish of render this is the last coat that is applied usually to provide the decorative finish and texture.
Silicone renders are often confused with other products such as K rend FT i.e render with silicone additives
Through coloured renders or self-coloured render are completely different products. They are pigmented cement renders which sometimes contain a colourless silicone water repellent, hence they are sometimes referred to as silicone render.
The new generation of silicone renders are made up of an emulsion of synthetic silicones and polymers. They have high PH content making them a little resistant to algae and lichen growth. But as I have said before in this site take any claims of self-cleaning with a “pinch of salt”
They do get dirty especially if trees are nearby
They exhibit excellent flexibility properties making them the no one choice for anti-crack renders. They are suitable for application over existing renders or over a new render and also as a final finish for external wall insulation systems.
The ultra-violet resistant pigments of a silicone or acrylic render allow for large areas to be rendered with perfect colour consistency.
What are the benefits of using silicone render rather than acrylic?
Both have excellent water repellency properties and are UV stable.
The main benefits of a silicone finish V acrylic are better dirt repellence, Some degree of vapour permeability.
Silicone renders are also able to accommodate pronounced movement in the underlying material, making them the number one choice for external wall insulation
Silicone renders have a longer lifespan.
When used over a quality render you should expect a 20 year + lifespan.
Once they do get soiled and tired they can be freshened up by painting with a silicone smooth paint which can be roller applied.
A good render base for the silicone would be a polymer render, reinforced with nylon mesh.
The main benefits of either finish over a through-coloured render are:
Corner beads do not show through the render. It is not a good detail to have a white corner plastic bead poking through a self-coloured render.
Although you can get colour matched beads they are still prominent and they can fade and change colour differently to the render over time.
Silicone renders completely encapsulate corner beads so you get a solid, neater finish.
Another big advantage of silicone or acrylic renders is the colour range. They can be made up in just about any colour and are available in various textures from very fine to course.
Silicone renders are less prone to cracking especially when applied over good quality render bases
As mentioned above they are more resistant to dirt and grime than cement finishes
These silicone render products originated mainly in European countries. Firms such as Baumit are huge European manufactures of all kinds of mortars, paints and EWI systems.
Over the last few years, many firms are making or marketing these products in the UK, including Jubosi, Sto, Parex, Weber, K Rend, & Baumit.