Rendering using the traditional method using ordinary Portland cement and the correct type of sand can last a lifetime if done correctly. My property is over 80 years old. No render has come blown-off the brickwork and looks like it never will. It is usually normal for a building to have a few cracks in render as it is a part of the settling-in process of the structure. A property that is not built on a solid concrete foundation, that has many openings and long walls will have slightly more cracks than normal.
These days expansion joints have been introduced to avoid cracking, more flexible and breathable renders have become available. The latest type of render is monocouche (one coat), a pre-coloured render. It is available in several colours and requires no painting once dry. It costs much more than the traditional method and is floated when dry using a spiked trowel in a circular motion to remove some of the render to leave a rough flat finish. It looks great however for my extension I have to match the existing render so is has to be the traditional 2 coat sand and cement method.
For the first coat or base coat I used a new cement call Multi-cem that has additives to make it more workable. The mix I used was 5:1. That’s 5 plastering sand and 1 cement. I did not add any waterproofer or plasticiser. I bedded a reinforcement mesh at the top corners of the openings as there were visible cracks in some areas the concrete blockwork. I assume this was due to the drying out and expansion/contraction of the building. Hopefully, the mesh and render will help.
Beforehand, i fixed mesh beading around openings using galvanised nails or screws. I left approx. 15mm spacing for the render. I used my laser level late in the evening so i could see the laser better to check the levels. I few dots and dabs of 5:1 render made sure they will not move when floating.
The process… I soaked the walls using a hosepipe an hour before I started and again with a slight mist before I applied the render.
I used small buckets to measure the proportions correctly into a large plastic tub. I usually would have used a cement mixer however i was only rendering a small area my first time. I found by pouring the water with the bucket tilted allowed the water to flow to the bottom of the dry mix and made the whole mixing process easier. Also, um-mixed hard lumps would be left at the bottom of the tub
Once it seemed mixed I spun the paddle mixer to introduce some air into the mix and to make sure it was fully mixed.
Note to bed the reinforcement mesh the mix has to be quite runny to allow you to push it in and trowel it over. Once it goes into place more render needs to be applied in some places to build up a thickness of approx. 8-10mm. I used an aluminium straight edge to flatten the surface by scraping of any high spots. Any low spots were filled and levelled off again. At this stage, it does not need to be perfect. Just before it goes off the surface need to be scratched so it provides a key for the top coat. I made a tool myself using some timber by hammering in 6 long nails. I left it to dry for 1 day before top-coating
For the top coat, I used a slightly weaker 6:1 mix however this time I added 1 hydrated lime. A 6:1:1 mix. Hydrated lime is used instead of a plasticiser that is said to provide a more flexible render. It is also fully breathable that will not trap any moisture in the walls. Hopefully, this is the best choice as only time will tell.
Before I mixed any render I scratched off any loose bits. In my first coat, I found the sand has too many large stones. I used a sieve and washed the sand into buckets. It was very time-consuming. When the bucket was almost full I left the particles to settle and tilted the bucket to pour of most of the clear water out. I then placed the sand into a woven bag. It allowed any excess water to drain off.
I made up a mix and applied it almost to the final level. I used a large plastic float to level it off. I then used the smaller float in firm circular motions to work the mix into a level surface. I some areas i sprayed a little water using a hand sprayer to bring out the finer material. You really have to get the timing right. Too soon the material will loosen up, too late you will have to constantly wet the wall to work it.
A sponge float or a simple car washing sponge is usually used to close the render to it’s final appearance. I found just by using the plastic float it provided a more flat closed finish. By the end of the day i had pretty much mastered it. I could notice that where i began the rendering was not as a good finish to what i did last. I did not apply enough pressure when running the float or the mix was too stiff.
The following day it had almost dried out. A couple of very fine cracks appeared at the bottom of the render where i assume the water had run down. Too much water in these small areas shrank the render as it dried out. A week later no more cracks have appeared where I was expecting them around the openings.
I was looking forward to painting the render to make the whole extension look more completed. I had Sandtex smooth magnolia or Dulux weather shield paint in mind. Dulux paint is not suitable for lime render, also Sandtex even though they are very breathable. I found that lime render requires special paint. 5L can cost over £90.