Rendering tips for patch and repair work

Ernie Cook, the master bricklayer, who I had the pleasure of working for in my teens used to say. “The worst thing that has happened to the building industry is cement.” It was a bit of an exaggeration but we spent a lot of time travelling around London and the South East putting right the work of bricklayers who were born into the cement age and knew nothing of working on buildings built with sand and lime.

The results were cracks appearing where they had never been before and spalling brick faces caused by moisture trying to find an escape route. All this because the brickie had the wrong mix for the job.

“If only they would use a bit of lime” said Ernie.

“If only they would use a bit of lime we would be out of work” I replied.

Lime is a magical material that has the ability to move and reconstitute itself in mortar, render and plaster. It is said that during the Blitz many of the buildings built with lime would jump minutely to absorb the impact of a nearby explosion whereas the sand and cement buildings would crack apart and stay cracked.

It is a golden rule of bricklaying that the mortar should be no stronger than the material it is joining together. That in a nutshell is the reason that Ernie Cook had such a poor regard for cement mortar. It wasn’t the fault of the cement so much as the lack of knowledge of those using it. Nowhere is this more evident than in modern building with lightweight aircrete blocks. The mortar used to build block walls should be around 1 part cement to 8 parts of soft sand but it is rare to see bricklayers changing the mix (gauge) between the blocks and brickwork which is typically laid 1 part cement and 4 or 5 parts sand.

limey

The result is that the block work moves at the weakest point which is the block itself and not the mortar. This produces cracks at the place where the blocks are most free to move, which is usually directly under windows. The cure, according to some builders, is to use an even stronger sand and cement render mix as a base coat for plaster. I have seen this done on so many occasions that I have lost count. Of course now the use of dot and dab plasterboard hides the cracks from view.

Another common problem is a strong mix of sand and cement render on exterior walls. Some plasterers think this stops cracking but sand and cement will always shrink and it pulls the brick or block with it producing hairline cracks. Until you’ve seen it you won’t believe how much water can run down a wall and track in along horizontal hairline cracks. The capillary action sucks it in to soak internal faces. I have seen walls covered with the white web of dry rot simply because water has tracked in through cracks that you would hardly notice with the naked eye. Even worse is when well meaning builders rake those cracks out and fill them with even stronger sand and cement or resin thinking that it will stick the building together. What then happens is the seasonal movement in the building causes the cracks to push upon the hard repair and lever the cracks open even more.

So faced with all this grief, why are brickies and plasterers still so anti-lime and pro-cement?

The reason that many won’t kick the habit or at least cut down is because putting in less cement makes the mix difficult to work with. It loses moisture into the bricks and won’t stay on the trowel. The simple answer is to use an equal part of hydrated lime and cement but this means ordering two different materials and mixing them together. The modern answer is to use plasticiser which mimics lime by putting lots of tiny balls of air into the mix to help it flow. While plasticiser introduces air to increase workability and help frost resistance it doesn’t have the same stickability as lime. It does however allow the brickie to use less water which helps reduce shrinkage so it is a good second best.

The same effects can now be achieved by using enhanced cement which has plasticisers in a powder form but, in my experience, it still isn’t the same as putting hydrated lime in. The ratio of hydrated lime to cement is important. The lime should not exceed the cement. If you use more lime than mortar it is likely to fail if the frost gets to it. A typical mix is 1cement 1 lime 6 soft sand for bricklaying or sharp sand for rendering. If you want an even weaker mix for internal block work then a 1.1.9 can be used.

Not only does the hydrated lime help the workability and adhesion of the mortar it also retains the moisture during the initial set to assist hydration. The set takes place before the mortar has fully shrunk which means it is less likely to crack. Even if it does crack, say with building movement, the lime helps the mortar to self repair by leaching lime into cracks. The lime then reacts with the atmosphere to form limestone in much the same way that stalactites are formed. This makes it the ideal material for repointing brickwork.

The most important thing you have to get to grips with is that hydrated lime is different to hydraulic lime.

Many people confuse hydraulic and hydrated lime. The use of hydraulic lime in bricklaying, rendering and re-pointing is old school but is now largely confined to historic buildings. Hydraulic lime sets in days, sometimes weeks and is much too slow for most builders looking for fast track solutions.

Hydrated lime mixed with cement is far quicker to set. It is the reaction between the hydrated lime and the cement which makes it set, if you leave out the cement it simply won’t set. Although hydrated isn’t a direct substitute for hydraulic lime, it does give some of the benefits. If you are extending an old building for example and want a good colour match but don’t want to go to the trouble of using hydraulic lime then hydrated lime is the next best thing.

If all this has started your head spinning just retain one thing. Next time you are doing a bit of above ground bricklaying or rendering buy a bag of hydrated lime from the merchants and add it to the mix in equal quantities to the cement. The instructions should be on the back of the bag. If you do this you will never go back to straight sand and cement.

We would very much welcome your comments below.

About Roger Bisby

Roger Bisby
Roger Bisby is an English television presenter and journalist, known for his expertise in the British building industry.

41 comments

  1. Hi Roger, I think all your tips are brilliant. I am self building but my plasterers are making an awful job of the external render, so I ‘ve decided to have a go. I have multicem cement from Hanson mixed at 6:1:1. What is the situation with adding water proofer and fibres ? and also is this enhanced cement okay to use or do I need ordinary cement.
    Your advice will be gratefully received,
    Thank you.

    • Roger Bisby

      That is fine. That cement has lime in it so it will work well. Fibres are only good in the base coat. Better still is fibreglass mesh in the base coat. You can buy it and push it into the scratch coat. It is available from Teco Products in Shoreham and it will stop cracking

      • Thank you very much. There are too many cowboys out there and it is appreciated that you spend time helping others with good solid sound advice. Thanks again.

  2. Hi im a bricky by trade and am looking to render dormer window cheeks,which are already plyed and felted with the beads on and coverd in eml what would be the best sand and cement mix for the scratch coat and the top coat and also what coat do i put the waterproofer in.

    Thanks dave

    • Roger Bisby

      Hi Dave

      I am assuming that you are using a breather membrane under the eml to stop any damp getting into the plywood. If so then the water proofer isn’t so critical. If you use a 5-1-1 sand cement and hydrated lime that will be good. I would put on the scratch coat and then put a little SBR in the finish coat and that will act as a waterproofer.
      You can use the same mix on both coats but the top coat will be thinner.

      • Hi thanks for the reply,
        yes its breathable felt and do you recommend sbr instead of a liquid water proofer additive for example opti-mix ?

        Thanks dave

  3. Hello Roger

    Thanks for your great Rendering Tips. This explains why I have been having so much trouble trying to get a decent finish. I do have a couple of questions if you would be kind enough to help.

    You recommend a mix of
    1 cement, 1 Lime to 6 sand for block work. Does this apply to all block work regardless of the density. I am using Dense 7 nm blocks for a retaining wall.

    Where can I buy the Round Bonnet Sponge you recommend in the video.

    • Roger Bisby

      You can use a stronger mix on dense blocks. The mix should never be stronger than the block so you could even go for 3 to 1 lime, 1 cement but really that is a below ground mix so maybe go 4-1-1.
      The sponge is called a drywall sponge. They come from Gyproc tools and Refina. Good builders merchants should be able to get them next day through Toolbank or try William Way Godstone Surrey. Ask the boys and tell them I sent you. They will post you one out.
      Address: 38-42 High St, Godstone RH9 8LW
      Phone:01883 742757

  4. Hello Roger

    Thanks for your great Rendering Tips. This explains why I have been having so much trouble trying to get a decent finish. I do have a couple of questions if you would be kind enough to help.

    You recommend a mix of
    1 cement, 1 Lime to 6 sand for block work. Does this apply to all block work regardless of the density. I am using Dense 7 nm blocks for a retaining wall.

    Where can I buy the Round Bonnet Sponge you recommend in the video.

  5. Hi Roger. Recently put a scratch coat on consisting of 4-1 sand and cement with waterproofer. I SBR’D the red brick wall prior to scratch coat. Then decided on using lime on final coat. Consisting of 4-1-1 with SBR in mix. Went on a dream and then when drying it has developed “Hairline cracks” they just look like fine lines. Any idea what may have caused this. I used plasterers sand in the final skim.

    • Roger Bisby

      Hi James
      It might have helped if you had used the lime in both coats so you had uniform movement. The cracks are shrinkage of the top coat, sometimes the sand is a bit sharp and a little building sand mixed in gives it a bit of body but they are nothing to worry about. If you are masonry painting them the paint will fill them. If they are left bare the lime will leach into them. If you tap the wall there shouldn’t be a hollow sound and that is what matters.

  6. Thanks Roger, did the job for a family member and he said they seem to dissappear when rubbing the wall.

  7. Hi Roger,
    I have a 1.5m brick wall between my property and next door. It retains about 1m of earth, on their side.

    It is fairly old and showing signs of spalling (crumbling bricks and some vertical cracks). A BC officer who was inspecting some other works we were having done suggested that I could use a mesh and rnder (or resin?) over the wall to give it a few more years.

    Could you recommend a mesh product (I’m assuming it’s a fibreglass mesh?) and would you recommend resin or render – like your video above?

    Do you need to attach the mesh to the wall first (somehow?) or do you render, push mesh in, then skim over?

    I was going to drill some weep holes and insert a (probably copper water pipe) through at variuos points along length to assist water release, as I think this is part of reason for damage. Sound sensible?

    thanks
    James

    • Roger Bisby

      I would start with the drain pipes to get rid of the water behind it. Then I would stitch the cracks with helical ties. That will put the strength back in it. Then put on a slurry coat of sand cement and SBR. Then apply a thicker coat of render and press glass fibre mesh into it. You can get this from Teco in Shoreham Sussex. Press the mesh into the base coat and scratch the surface for you top coat which can go on thinner. You can use SBR in this coat to aid adhesion but it will probably be fine as it is.

  8. Hi Roger,
    Reluctamt DIY learner OAP here. This is short notice I know but I’m half way through a 7′ high (approx) party wall wall render repair, up against a conservatory. Am having to re-render behind glazing also. Have done an “inadequately deep” scratch coat & have to “try” a top coat tmrw. Have used 4:1 building sand & cement with SBR prepared surface & in mix but found huge problems in keeping required depth of render to stay in place. Trowelled it in for hours & eventually had to remove an area & apply again.
    It is an uneven level to finish. Deeper one side than the other. 3/4″ & 3/8″ (very approximately) about 4′ wide, narrowing to half that for half the height.
    Because of above problem & having to hand mix, I found it impossible to make scratch coat deep enough on deep side but ok on thinner side.
    Is it feasible to effectively do two scratch coats please, as I don’t think it will take all that depth in one go, on that deeper side?
    Sorry this is a bit confusing.
    Many thanks
    Lee

    • Roger Bisby

      Hi Lee
      It sounds like you are having a nightmare there. A bit of lime in the mix will help it hang and hold together and give you something nice and fatty to scratch up. You can put the scratch coat on in as many coats and patches as you like. If a bit falls out of the middle leave it until the rest is set and then fill in the patch it is way eaiser. Sometimes we put up wooden battens and rule off to them and then when the rest is set take the battens out and fill in the grooves, then apply a thin top coat all over. Whatever works.

      • Many thanks for those insightful tips Roger.
        It certainly merited being called a nightmare yesterday but today was easier. The mistake I made yesterday, was making the mix too wet. My theory was that it would help it access the fine surface fluctuations on the base but .. it was more problem than help.
        Contrary to my expectations, I’ve actually done a very good job & sponged off to a very acceptable finish in one go today …but … I didn’t have any Lime handy & no time or vehicle to get any. As a result, even before it has properly set, there are already signs of fine cracks appearing in a few places.
        Funnily enough, the larger, deeper area at the top was easier than the thinner narrower area below. I thought, with a narrower area at the bottom, the moisture from the larger area above maybe seeped down & made it too wet. Anyway, I removed the section, let it dry off for a while & re-applied & all is OK, so far.
        If you have any miracle tips about how to overcome fine cracks as mentioned, I’d be very grateful but I guess a good dollop of masonry paint is perhaps all one can do.
        Yourl website was very helpful in my achieving success as far as it has been.
        Thank you very much indeed for your kind help.
        Regards
        Lee

  9. Hi Roger

    Firstly, thanks for all the advice you give freely it is much appreciated.

    I might need to render over some “Heritage” Cotswold style cast concrete blocks on our “new” 60s house. There are 4 panels of “stonework” directly below some downstairs windows. The vertical sides are bordered by normal brickwork piers standing about 70mm proud of the blocks. Do I go to the trouble of cutting the dressed faces of the blocks to give a more uniform surface level (very labour intensive as the windows are 2.4m wide) or bring the level flush by putting an initial coat of render on to fill in the imperfections? Will the latter method need mesh or metal lath to prevent cracking due to non-uniformity of depth? Can the render finish tight against the brick work piers? Presumably as this is going on fairly dense concrete I need a 4:1:1 or stronger mix.

    Many thanks again

    Steve

    • Roger Bisby

      Hi Steve

      You can render directly onto the blocks. Use some SBR in the 4.1 cement mix and make up a slurry coat to gain a key then prick that up as it dries to make it spikey.

      You can then make up a mix of 1.1.4 again with a small amount of SBR in the water. Press some fibreglass mesh (from builders merchants or TECO Building Products into the mortar and let that go off. Apply a scratch key on the render as it dries.
      You can use fine chopped fibres for render (Selco) in the next scratch coat to bind it and again a little SBR. You should then be able to apply your final coat of render.
      The important thing is that each coat is slightly thinner than the last. Ignore the thin slurry coat that is for adhesion.

  10. Many thanks for the reply Roger that’s saved me quite a bit of labour and mess over my initial thoughts.

    You’re a gent.

    Cheers

    Steve

  11. The black arts of true masonry are always difficult to discover! A google search of ‘aggregate void ratio’ will open many people’s minds to these lost secrets. My personal favourite mix for laying brick is 1:2:9 (OPC:Hydrated lime:Aggregate) also 1:3:9. Ensure the powders are carefully measured (by volume) & thoroughly mixed before addition of water. Water should contain an admix of Sika or Feb plasticiser at 5ml per litre. Most everyday builders don’t use hydrated lime; yes partly due to poor education but mainly because of cost (arguably a function of poor education). A bag of hydrated lime is 5x & sometimes 7x more expensive than the cheaper Aluminosilicate ‘clinker’ based cement. Good quality sand or sharp sand of known water content with an acceptable void ratio is necessary for these mix ratios to work effectively in the associated application. OPC is still a valuable component to the overall mix because it increases durability & surface hardness. Rendered surfaces should avoid being painted as this can affect the breathability of the substrate; cladding is a significantly preferable option for unsightly exposed weathered brickwork.

    Bricklayers & Masons in the good old days were highly respected craftsmen: https://www.youtube.com/watch?v=5xuUqJH3ehM&feature=share

    • Roger Bisby

      I was told that hydrated lime will not set without cement so you should always have equal parts of lime and cement. A 1.1.6 mix is common and gives weather resistance and flexibility.

  12. Addition of lime extends usability of OPC.
    Give it a try using a trial batch measured accurately using a small volume vessel.

    https://youtu.be/duB8idWVHoE

    Thanks for all the excellent work you do for the community.

  13. Hi, great stuff here.

    I’m going to be rendering my old French farm house on the INSIDE with sand and cement render – 1 scratch coat and 1 top coat. I’m going for a ‘not perfect’ finish to reflect the ambiance of the house.

    The house is quite damp in places as it’s stone built and i read that sand and cement render was good at holding back damp.

    I’m going to use a plasticiser with my mix and was planning on around about 5:1 sand and cement.

    Have you got any advice for me?

    Thanks, Phil

    • Roger Bisby

      If you are going to hold back the damp then don’t use a plasticiser because that provides lots of tiny holes.
      The best approach is to use Sika 1 and follow the manufacturers guide lines.

      alternatively you can let the wall dry out by using a lime renovation plaster which allows the moisture to evaporate into the building. The advantage of the renovation plaster is that you don’t get condensation forming on it because it absorbs and then dries out. With waterproof render any airborne moisture will form condensation.

  14. liked your YouTube rendering vid, it looked like a dogs dinner at one stage but you brought it back to a good finish.

    Have you ever rendered over a timber framed extension ? I can’t find any info. The osb board is covered is breather paper. Battens are then nailed over this and another layer of breather paper and then eml. (expanded metal lath) The second layer of breather paper to stop render filling up the cavity I have ascertained this much but cannot find how the created cavity is closed at the bottom to stop rodent and insect ingress but also retaining air circulation. I notice that the Americans put mesh direct onto the breather paper omitting the battens, second layer of breather paper and not creating the cavity.

    I have tried plastering sand from various retailers and one stood out as being much better than the others presumably because it had silicon added. I was told that they used to sell internal and external rendering sand the difference being the external had the silicon added but for logistical reasons they added silicon to all their rendering sand to simplify things. Sadly now their sand seems no different to the others presumably because the silicon is now absent. Do you know of any silicon additives.

    A reply would be very much appreciated. Regards Rick

    • Roger Bisby

      Hi Rick

      I am not sure what you have done and what you are planning to do. If you were starting with a blank canvas I would not bother with the eml. The quicker easier method is to screw cement board onto the timber frame and then use a renders primer from Weber with a fibreglass mesh embedded in it. Then before it sets prick this up. I use a wood float to suck it up slightly. You only have to dab it and lift it and it makes a fantastic keyed surface. Then you can go over it with a bagged product such as monocouche or a sand and cement mix with 6 sand 1 cement 1 lime. If you put a small amount of SBR in the mix it will be there for life.

      If you have already started with the mesh then by all means continue.

      Another great thing to do is to use the boards and some mesh in primer then instead of pricking it up trowel it smooth and then use an acrylic product. You can get this in tubs again from Weber. It is often used on loft conversion dormer cheeks to create a render finish rather than tile hanging.

  15. liked your YouTube rendering vid, it looked like a dogs dinner at one stage but you brought it back to a good finish.

    I have tried plastering sand from various retailers and one stood out as being much better than the others presumably because it had silicon added. I was told that they used to sell internal and external rendering sand the difference being the external had the silicon added but for logistical reasons they added silicon to all their rendering sand to simplify things. Sadly now their sand seems no different to the others presumably because the silicon is now absent. Do you know of any silicon additives.

    A reply would be very much appreciated. Regards Rick

    p.s. comment broken into two parts as it kept saying duplicate comment

    • Roger Bisby

      Ha! thanks Rick. I like that dog’s dinner comment. It highlights a problem that many novices have and that is that they try and get a finish before they have the sand and cement on the wall.

      I can see there is more below. I am not sure about the silicon. There is silica sand which is sometimes used in high end work to match in with Portland stone but I don’t know about silicon. I don’t always trust merchants, they sell stuff that the often know little about.

  16. Hello Roger – I’ve watched many of your videos and think they’re excellent, so thanks for sharing your knowledge. I need to re-render part of the facade of our Regency townhouse in Brighton but am unsure about whether to use waterproofer in the mix. I planned to mix the scratch coat 5.1.1 sand.lime.cement and the float 6.1.1 but having read far too many forums all I see is conflicting advice. Could you steer me in the right direction please?

    • Roger Bisby

      Sorry, Simon, I missed this one. I tend to use SBR in the scratchcoat for adhesion and provided you have a really good key the top thinner coat will stay on with no problem. Most people advise against waterproofer as such because it traps moisture. However, SBR is a waterproofer as well as an adehesive hence the reason I use it only in the scratchcoat. That way the top coat can absorb water and evaporate. That said if you are painting the render you are applying a water resistant coating but it doesn’t seem to trap moisture. I think that water vapour gets through but water droplets are repelled.

  17. Hi Roger, Great advise and tips in your rendering video.I use lime in my mix all the time and mesh the scratch coat before top coat is applied.
    I have a north facing gable end to render which is rather exposed and at times catches the wind.
    Scratch coat dryed to quick for my liking but seems nice and solid.
    I will cover scaffolding to protect from weather but how can i slow the set time down to give me more time on the wall ?.
    Dave south coast

    • Roger Bisby

      There are commercial retardants that are used in silo mixes etc Mostly these are used for bricklaying mortar so you have to be careful that they don’t cause an unwanted effect in rendering. Mostly they have one of the following ingredients.
      • Unrefined lignosulfonates containing sugars
      • Hydroxycarboxylic acids and their salts (i.e. sugars)
      • Carbohydrates including sugars
      • Gelatin (Sodium Heptonates – animal or fish fats)
      • Hydroxylated polymers
      An important sub group of retarders are those used in the production of ready to use ready
      mortar, they are covered by BS EN 934 -3: (Admixtures for concrete mortar and grout: Part
      3 -Admixtures for masonry mortar). Their use enables mortar delivered to site to be used
      for up to seventy- two hours after manufacture. Conventional retarders are used at a high
      dosage rate in combination with air entraining/plasticizing admixtures. The air entrainment
      assists in the retention of workability and the minimisation of bleeding.

      You might get better information from Cemex technical department but my approach is as follows.
      If you protect the wall from sun and the wind and you hose down the scratch coat the day before and again on the day and make sure your mix water is cold then you will increase the open time. Even storing the cement in a cool environment can help because the setting is aided by heat.

      • Thanks roger,dont want to start adding chemicals to mix ,will do as you say at the end of your reply.
        Would it help to brush on a cementone waterproof pva ?
        Around a 3.1 mix ratio.or a diluted sbr ?
        Dave

  18. Hi Roger, thanks for clearing up a load of questions with you’re videos, I’ve been asked to render my next-door neighbours outbuilding which has been built in two parts. The first section is brick built which is no problem but the second section is timber frame and I’m not sure what product to use as a base for me to render to, it is literally timber frame with kingspan in-between. Any suggestions greatly received thanks Jim..

    • Roger Bisby

      Hi Jim

      The modern material to render onto is cement board. You have to prime it with a key coat of Rendaid from Weber and then you can apply the sand and cement. The joints need to be covered with fibreglass mesh.
      In days gone by people used stainless steel expanded metal but you waste a lot of material and it is prone to cracking.

  19. Hi there,

    Thank you very much for doing the article on me!
    http://skill-builder.uk/day-life-richie-maxwell-screwfix-top-tradesperson-2016

    Please follow me bk,

    I’m extremely interested in doing product testing video in the future, is there any advice you could give me and point me in the right direction to get involved?

    Any help would be very much appreciated

    Thanks again guys,

    Richie

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