How to mix cement
Mixing cement mortar is easy, however mixing a mortar that will stay stuck to what you’re doing is a whole new thing.
I can guarantee without any real experience or knowledge anyone can mix cement mortar that on first sight will look the business. Given a chance it will bond to trowels, plastic, rubber, your clothes, your car and a million and one other things, including your expensive patio slabs, and it be stuck there for 40 years, even when deposited by a passing glance, or accidentally dropped. If however you climb ladders or scaffolding to point up let’s say some ridge tiles, and then lovingly take your time, pressing the mortar firmly home whilst concentrating on a nice aesthetic finish, it will probably crack and fall off within 6 months to a year.
There is however quite a few basic steps you can take to seriously improve nearly any standard cement mortar mix, without having to get too fancy, start adding lime, or getting too technical. In this article I’m going to tell you how to mix a simple cement based mortar with standard off the shelf building sands, make it strong, resistant to cracking, and get it to stick.
Let’s take a look at some important factors that will directly affect the quality, bond, and longevity of your mortar or roofing mortar. These are all covered in this article in roughly this order…
- Cement ratio for mortar mix – The amount of cement you put in will change the workability, curing time and strength, different jobs favour different ratios.
- Common jobs and the correct mix – A list of common roofing, building and DIY jobs and some general sand cement ratios.
- Top mistakes to avoid – Having created a mix from the gods, it’s all too easy to error at a late stage and ruin a nice job.
- Weather – Our friend the weather. Often cold temperatures or freezing conditions are all you will hear about, but good weather can be your enemy too.
- Quality of sand – So often overlooked, but much more important than it ever gets credit for. Depending on the task, we can alter the type of sand for a better job.
- Mortar mass – A larger mass of mortar (a big splodge) may favour a change in the type of sand used to prevent shrinkage and cracking. Smaller masses may also dry too fast in the wrong circumstances.
- A good key – Often when people point a chimney, ridge tiles, or paving slabs, the depth of any rebate or surface roughness is overlooked. Checking this is in a good condition or has sufficient rebate depth to receive the mortar is called ‘the key’.
- The substrate – What the mortar will adhere to. It may be too dry, too sandy, or glazed, to give just a few examples.
- Cement additives – There are a lot of additives and agents you can add to a mix, the most common being a plasticiser to make the mortar more ‘workable’ or to add frost resistance.
- Quick drying cement – Just like the cartoons? not quite. It will still accelerate the drying time of mortar greatly, Ideal for time or weather pressure situations.
How to mix sand and cement mortar by hand
Here are two videos of how to mix a cement mortar. You can change the type of aggregate used with the cement to form other common mixes, apart from a building sand mortar mix if you wish, such as sharp sand or ballast to form a screed or concrete.
Spot mix – A simple mix on a plastic re-usable mixing spot. In this video I will take you through a few basic steps and quickly talk about different grades of sand.
Bucket Mix – Everyone should know how to do one of these, it’s simple, cheap and useful.
This is a beginners guide to pointing up. Great for brickwork, chimneys or roof details like flashings.
How much cement in a roof mortar mix
If we’re talking about common roofing jobs like pointing a chimney, laying ridge tiles or flaunching the top of a chimney, I would recommend a 3 : 1 mix (3 parts sand to one cement). This ratio of sand to cement is ideal for exposed areas such as the roof, without being stronger than the bricks or ridge tiles you are working with. If the mix is too strong as for instance with a 2 : 1 mix, not only may it dry too fast especially in sunny conditions, but any natural movement of the roof as seen with expansion and contraction won’t be catered for. A crack that would normally appear in the mortar with age, may instead damage the brickwork or roof tiles, creating structural damage and a possible leak. Mortar should not be stronger than the material that you are working with as a rule of thumb.
Cement ratios – If you look closely you will see that the same sand cement ratio has probably been used for the chimney that was used for the rest of the house, which is only 9 years old. Roofs and chimneys are exposed to the elements, so a stronger mix is required to stop wear like this.
Common cement ratios for household jobs…
- Roofing – As mentioned above this is nearly always 3 to 1 no matter what sand or combination of sands you combine (see further down). Not only will it be strong and resist the elements, but the high cement ratio makes it stickier and easier to work with. The substrate may need dampening with water to slow curing in dry conditions.
- Bricklaying – The cement ratio for a standard building sand mix used in brick laying is generally considered to be 4 to 1 or maybe 5 to 1. If you mix at 4 to 1 it will be easier to work with and is also ideal for exposed areas like laying bricks on a chimney stack, footings, or garden walls where strength and resistance to damp or the elements is required. A good 5 to 1 brick laying mix though is also favoured by some builders as it can give greater flexibility when required, and of course is more economical on the pocket when building a whole house for example.
- Pointing brickwork – I tend to use a 3 to 1 again here above the roof line to give strength, weather resistance and ease of application, often wetting the chase first with water and a hand held bottle sprayer to slow the drying. For normal masonry like walls, a 4 to 1 is favoured because it matches the colour of the existing mortar better. Sometimes a 5 sand – 1 cement and 1 Lime is used, like NHL3.5 (Natural Hydraulic Lime) to repair older traditional brickwork. To read more on lime mortar click here
- Pointing between slabs – Typically 4 to 1, but sometimes 3 to 1. I favour a sharp sand to reduce cracking, shrinkage and increase strength. Remember to respect the fresh mortar for as long as possible afterwards, walking all over partially cured mortar can and will ruin it. To get more in-depth on this read here.
- Screeding a floor – This normally hovers around the 4 to 1 range, often you may see 3 -1 or 4.5 – 1 quoted, usually with a good sharp sand and a minimum thickness of 50mm onto a good substrate. Basic instruction can be found here.
- Flaunching a chimney – Quite similar to screening a floor as seen above but with a 3 – 1 mix instead. If you are re-flaunching over the top of any existing flaunching, try to clean, and wet the previous mix as much as possible to avoid moisture loss from the new mix via the dry substrate beneath.
- Bedding a Verge – When bedding tiles or slates near a gable end or roof verge I find a nice crack resistant mix like wash sand works best, but can be hard to finish aesthetically. I bed my tiles or slates first on a wash sand type mix, followed by pointing with a building sand after the mix has hardened off just slightly, ( about an hour ) but before it drys.
- Mixing concrete – Concrete is most easily mixed with bags of ballast and cement off the shelf. Ballast is a combination of sand, grit and gravel, and is perfect for concrete. You can find it sold at nearly every large DIY chain in small and large bags. Sometimes it’s called ½ inch to dust and commonly a 6 – 1 ratio is considered to be a good general purpose mix like the ones used on paths, driveways, floors, or for general landscaping. A stronger 5 – 1 mix is used for structural purposes like forming ponds and concrete panels, and aids with making concrete waterproof. Large volumes and foundation filling often sees an 8 – 1 mix.
Common mistakes with fresh mortar
Sometimes I get asked by home owners why my mortar doesn’t crack or crumble, and how it stays stuck. Typically they may have done small repair works around the home like pointing gaps in crazy paving, only to find it has cracked or crumbled on them at a later stage. More often than not they wrongly assume the mix was too weak, or too strong.
- Dancing on your mix – Not literally obviously. But as an example, walking on or near your slabs after pointing between them will crack your mortar before it reaches a decent level of cure. A cement mix never stops curing, but will achieve most of its strength within 7 days (about 75%), and reach virtually all of its strength within 28 days. Walking backwards and forwards all over it whilst working, then inviting granny and the neighbours round for a look shortly afterwards is bad news. This is also true for working with any mortar on a roof like ridge tiles. Softly softly catchee monkey, give it time and space to cure.
- Flooding the mix – A good example of this can actually be found in my how to mix mortar in a bucket video. With my brain switched off because I’m making a video, I put way too much water in the dry mix in one go, ‘flooding the mix’. Whilst I was lucky and able to rescue the mix with dry stuff at the bottom of the bucket, this rookie error ruins the final strength of the mortar and would have to be disposed of in any real work situation.
Other important factors for good quality mortar are listed below…
How the weather will affect mortar
As you can imagine the weather conditions on the day you are working with any sort of sand and cement mix will have an great influence on not only what you can do, but the end result. If you can afford to wait for perfect, or near perfect weather conditions it’s a good idea to try. Often with me this will involve juggling jobs around the weather, or dropping off a job and going back to it on a more favourable day.
Rain and showers – There’s a lot of this type of weather in the UK. Working in the rain with a mortar mix will not only be miserable but is totally impractical, especially on a roof. But on showery days it can be possible to work with sand and cement for small repair work, such as re-setting a ridge tile or two, or small patch pointing jobs. If you were to rule out all mortar related jobs on all showery days, most professional roofers would have a huge backlog of work, and angry customers. The key to working on a showery day for me is correct task, spotting a window of opportunity, and the use of additives such as fast drying cement, which you can see below in the information about mortar additives.
Sunny hot weather – This can be as bad as constant rain for some jobs. Yes, you now have the perfect conditions for a nice tan, and with the radio on and a cup of tea in hand life has never been better. But hold on, there’s a lot to go wrong here. If you are doing any sort of pointing up, ridge tile bedding, or any sort of mortar work it can dry out too fast and be damaged, crack, or crack away from what you’re trying to bond the mortar to. Firstly and most obviously the sun will heat up the mortar you are working with, both in the bucket or on a spot. It will also heat up, or already have made hot the very thing you are sticking the mortar too, i.e. the brickwork or roof tiles. Forced drying like this is never good for sand and cement mortar mixes. Secondly, it will also have had an extremely drying effect on the substrate you are working with, more so if it has been dry for a few days running. Any sort of dampness in the bricks or tiles that would normally help to slow down the cure of the mortar is now long gone, and you have the opposite effect, a super dry moisture sucking substrate that will rob your mortar of its water content prematurely and suck dry it. Wetting the bricks or tiles you are working with repeatedly with a hand held water sprayer can be an option on certain jobs, but on very hot days the surface temperatures can be too great for this to be effective.
Normal warm dry weather – Slightly overcast or weak sun, no showers, not too hot, and not freezing. Yes, finally we’re getting somewhere. The conditions are now good for working with sand and cement mortar mixes. Make sure that your sand is suitable for the task in hand, keep your mortar fresh and in good condition, and don’t allow your substrate to be too dry if possible. Consider the use of a hand held water sprayer on any surfaces you are attempting to bond the mortar to on roofing jobs.
Damp misty days – Often seen around autumn, These conditions are the gold standard for me. Yes it’s not nice to work in, but the chances are that any substrate you are working with will be inherently damp. Lets take re-laying ridge tiles as a prime example, the ridge tiles and the roof tiles will have a fine surface water on them, and if the tiles are old and slightly surface porous the concrete will be damp internally too. What this means is the tiles will not want to suck moisture from any mortar it contacts with. The damp air also ensures that the mortar will cure much slower, and this is great news for the adhesion and strength of the cure as a whole. Slow and steady really does win the race.
Cold weather – Late autumn and winter in the UK. Watching the weather forecast on the television the evening before doing any cement or mortar work will pay dividends here. As a rule of thumb anything below 5 degrees centigrade and you are asking for trouble with mortar or concrete. Sometimes on a nice winters day it can be deceivingly warm, and well above 5º C, but remember winter days are much shorter, and when the sun drops, so do the temperatures. Because of the low temperature you may want to add a cement additive with frost protection, or mix a curing accelerant in with the mortar, although be aware many anti freeze additives are not recognised by British or European building standards. If I do have any large scale mortar work to be done, a cold spell may mean I would sit on that job and wait for the weather to improve, although with small repair work I will often speed up the cure with fast drying cement during the warmest part of the day.(more cement additives further down)
Which sand is best for strong mortar
This is an area that is so often overlooked by DIY’ers, and some of the trade occasionally. All building sands are not created equal. Quite often these days a bag of off the shelf building sand here in the UK will be required to meet a British standard, which can mean that the particles of sand are a specific size and shape. Common building sand, often referred to as a ‘soft building sand’ may make the mortar nice to use for trowel work, and it can often feel a bit like mud, so it sticks to a trowel great. Unfortunately if you use this type of sand for every job, it may not be fit for purpose.
If you think of concrete as a good example of a strong mix, and you examine the ballast that is mixed with cement to form the concrete itself, you would notice a huge variation in aggregate size and shape. Some stones are large 25mm+, some stones are much smaller, grit is also an element, and finally finishing with fine grain sand to bond it all together. All these smaller particles come together to fill in the gaps around the larger ones, and when they have a even coating of cement, that combination gives it its strength. But of course it would be almost impossible to work it using a trowel, or lay any bricks with.
Fine building sand – A highly processed fine sand like those often found in large DIY chains can be just the job for brickwork pointing, brickwork repairs, or thinly re-bedding ridge tiles onto slate as an example. Often though, mortar has to be used on a roof in a larger mass, this could be large gaps in brickwork for pointing, deep wide chases, or re-laying ridge tiles onto a thick bed of mortar or onto small profiled roof tiles.
The trouble is often the building sand has been processed and all the jagged irregular shaped particles have been removed, or it may contain elements of silt or clay that haven’t been washed out either. If used in a large mass you risk cracking as the mortar hardens when used for pointing large gaps, or bedding ridge tiles onto profiled tiles as an example.
Harsh building sand – Getting to know which merchant sells what type of building sand can be the key to a nice job with a building sand mix. Often I will travel across town to pick up a bag of building sand where I know it will be a little harsh (sharp), or have irregular particles in it.
Sharp sand – Sometimes, when for instance flaunching the top of a chimney stack or bedding tiles or slates to a Verge, a large mass of very strong and weather resistant mortar is required. Sharp sand, or washed sand as it is often known fits the bill here. It has a varied particle size and shape without silt or clay, and it doesn’t have the very large stones found in ballast and used for concrete.
Often a mix made with sharp sand is used for other building jobs such as screeding floors, but it can be unusable and too harsh for any sort of detail work, like the pointing or relaying of ridge tiles. However, if you mix washed sand into a finer builders sand it can become usable for trowel work like cement flashings, big deep chases, and re-bedding ridge tiles on profiled roof tiles.
Mixing building sands – I often use this when reseating ridge tiles on profiled roof tiles… A ratio of 1 Wash sand, 2 Building sand and 1 Cement. Or very occasionally 1 Wash sand, 1 Building sand, and 1 Cement.
It is still however a 3 : 1 mix, add a good plasticiser and you’re done…
I know you may be thinking does anyone really go to this much trouble over sand for goodness sake? Well, yes. I have know builders to sieve their own sand by hand, and even ‘turn’ washed sand dry in a cement mixer for 10 – 15 minutes to knock some of the particles smaller or smoother. Personally I don’t do that, but you have to respect that kind of dedication.
Mortar mass and how to stop cracking
This is a subject I have touched on above. As the thickness of your mortar increases so will the chances of cracking with ordinary building sands alone. Let’s take the re-bedding of ridge tiles onto a roof as a prime example. When a ridge or hip tile is bedded onto the roof, the thickness of the mortar bed determines how high the ridge tile will sit above the roof tiles, the bigger the distance is between the ridge and the roof, the more mortar mass it will take to fill the gap. This means a mix made with a standard building sand with small fine particles in a large dollop, will want to shrink and crack as it cures. With large thick mortar, standard fine building sand does not always have the varied particle size and shape to give it enough strength, and will benefit from being a sharper building sand or having sharp sand added if this is the case.
Cracks in ridge tile cement – I know this is nitpicking, but it illustrates the principle perfectly. This new roof is 2 months old, and you can see shrinkage cracks on the joints. This is the thickest part of the mortar where fine ‘soft builders sand’ couldn’t cope.
Chimney Flaunching – This is a good example of the wrong mix for the job. Here the large mass that is the ‘Flaunching’ to hold the pot in place has been done with standard building sand. Even though it’s a very strong mix, shrinkage cracks have ruined the finished job.
One positive aspect with large masses of mortar is that it will want to dry slower as it contains more water, and a nice slow drying mortar will shrink and crack less resulting in a better job. Beware of relying on this though, if the weather is hot or dry, the roof tiles or bricks you are working with may want to suck the moisture from your mix, resulting in a damaged, weakened mortar and possible cracks.
A good key for pointing
The phrase ‘a good key’ is often referred to when trying to get any sort of render or pointing to stick to a substrate like a wall, brickwork or ridge tiles. If you were to consider an old fashioned lock and key, now you imagine the key entering the lock and turning 90º left or right into a position where if you tried to pull the key out it would be stuck, if you can achieve that basic principle with a nice strong mortar it will be very hard to remove, even if the adhesive bond isn’t as good as it should be.
A ‘key’ – Here you can see the depth that the cement mortar has receded, if you can poke your fingers in 5 – 10mm + often that would be considered a large enough gap to accept fresh mortar and provide ‘a good key’ without additional measures, such as grinding out the old mortar with a chasing tool.
This is also true for other jobs like gaps in paving slabs, although the depth may need to be deeper.
Often a good key with ridge tiles will mean that you have visible gaps between both the ridges tiles where they touch each other, or the ridges and the roof itself. If not, any old mortar should be pecked out (removed). If this isn’t the case, often after the subsequent pointing of the ridges without a good key or quality sand cement mix, the result will be it falling out and down the roof very prematurely. The quality of the substrate underneath the ridges should really also be in good condition for a good job.
Ridge tiles – Here are two separate properties with the same roof tiles, and roughly the same year. In the large picture you can clearly see there was no ‘key’ for any pointing to grip onto. In the smaller picture you can see that the large gaps will provide much better adhesion, once the mortar hardens into the nooks and crannies.
The substrate & what not to point up
This is the thing you are actually trying to stick the mortar to, so as you can imagine it is a little important. Normally when working with a sand and cement mortar this would mean something with as little flex and movement as possible, preferably with a rough or porous surface to provide the cement with some grip. If you think of a plastic surface as a very bad example of a substrate, initially it may provide a bond to the mortar, but when it dries and you were to flex the plastic in any way it would quickly peel off and fall to the floor. Obviously we don’t bond mortar to plastic as a general rule in the building trade, but a situation where a bad substrate may arise could be the following…
A sandy substrate can occur very often with brickwork or ridge tiles. Sometimes if the original mortar mix has been cement weak, compromised by poor additives or other factors, it may not only be weak, but start to crumble and return back to a sand like state. In the case of pointing ridge tiles not only will the fresh mortar not bond to the sandy face properly, but often the sand will get between the roof surface and mortar making that bond to the surface of the roof ineffective. This may well cut short the life of the pointing unless an excellent key is made, possibly leaving re-bedding as a more sensible option.
Sandy substrates – In the main picture we can see the mortar bed of a removed ridge tile has turned back into sand. No cement no matter how strong will stick to this substrate. The small picture shows the exact same thing in an old wall.
With brickwork, like the pointing of a wall or chimney, an old sandy mix will compromise the bond between the top surface of the bricks and the old mortar itself. In these circumstances you may need to make sure you have plenty of depth in the chase to make any pointing repair effective, and larger chases may benefit from a sharper type building sand also.
A dry substrate more often than not occurs during the summer months in the UK. A dry surface like bricks or concrete tiles will draw moisture from your mortar (sucking off), and this in normal conditions may be bearable. In the summer however dry arid air combined with heat can compromise the strength of the mortar and cause cracking, as well as cracking away from the surfaces you are trying to bond to, especially in exposed places like the top of a roof. What happens is that not only has the brick or tile become very dry making it absorb water like a sponge, but the hot sun and ambient heat evaporates the water away from it and into the air, leaving it thirsty for more water, and so on.
Sometimes work schedules or a long hot summer (occasionally they do happen) means that work has to press on regardless. A good technique to use is often to wet the substrate repeatedly with water. In the case of working on a roof this can mean sitting ridge tiles in bucket of water for 5 minutes before use and wetting the surface of the roof where the tiles will mate repeatedly with a bucket of water and a wet brush, or a hand held water sprayer. I know bricklayers have occasionally sat their bricks in a plastic bath filled with water just before laying to let the water absorb as much as possible into porous bricks. I also once saw a plasterer repeatedly wet a south facing wall he was about to render with a garden hose 13 times! I can tell you it definitely worked as the render is still there and the bond is fantastic 10 years later.
Glazed substrates are what they sound like, if the surface is glazed or fired smooth any minute roughness in the surface is normally lost. If this is on ridge or roof tiles, it makes achieving a long term bond very problematic even with additives.
Always a problem – A good mortar mix with fine and coarse particles, cement additive for increased bond, a nice key and a cool day. Combining these things doesn’t guarantee success with hard glazed surfaces, but it sure improves your chances.
Use the best cement additives
The most common being a plasticiser like ‘febmix’, this works by creating millions of tiny bubbles in the mix, this process is called ‘entraining’. The main benefits are that the mortar becomes more workable with a trowel and flows better allowing it to squeeze into finer details whilst using less water.
There are also many other mortar additives and combinations of various properties such as waterproofing, accelerants, (speed up drying) retardation, (slow drying) frost proofing, salt inhibitors, bonding agents, cement dyes and frost proofing, to name but a few.
Plasticiser – Febmix , there’s a good reason it’s referred to as the original… its been around ages and it works.
All rounder – Evo-Stik Seriously Better Cement – This is great. It’s light weight for keeping handy at all times, comes in sachets, and is perfect for small jobs. Plasticises, improves bond and frost proofs all in one.
Bond and Flex – For increased adhesion, durability and flexibility consider an S.B.R additive (Styrene Butadiene Rubber) like S.B.R. bond. It has a great list of positives but will reduce your working time to about 10 – 15 Minutes at a time. So big mixes are out when working with this.
Waterproofer – If you’re looking for a waterproofing agent for mortar, have a look at ‘Sika Waterproofer’. It’s not cheap but is highly rated by the trade.
A quick word of caution, using an additive isn’t a guarantee of an magically improved mix or an excuse to be negligent with the basics. I cement additives myself especially in winter to guard against light frosts, but I would not rely on them 100%. Many frost proofers alone are not yet recognised by British or European building standards, this doesn’t mean they don’t work… Just saying.
Using cement dyes in a mortar mix
I will lay my cards out on the table right now, I’m not a fan of cement dyes or colourants in mortar when used on a roof or ridge line. I cannot say for sure that it weakens the mix prematurely, and would probably be sued if I did. What I will say is that over my extensive roofing career, I have been to repair the roofs of a disproportionate amount of new build houses that are either just outside of the NHBC guarantees, or have had failures within them. Well over 90% of these had cement dye used in them on the roof that was starting to crumble. I can’t say whether this is installer error or product fault, but I don’t use them in my projects unless specifically instructed to.
How to use quick drying cement
If you’re using this in any capacity it normally means you are a professional, by that I mean that time is money and you can’t wait, or it’s an emergency. The main reason for using quick drying cement in roofing is to avoid inclement weather like rain showers or impeding frost, in which case it can be a real boon if your back is to the wall. If you use pure quick drying cement in the usual ratio of 3 : sand and 1 : cement (quick dry) it may result in a mix that dries in 10 – 15 mins, so fast that even in very cold conditions you get very little time to use it.
Mixing quick drying cement – This is also a favoured tool of landscapers, particularly when added to a concrete mix for sinking wooden or concrete posts for fencing. Often this is used in a diluted form as mentioned below or seen in the diagram.
A better method may be to mix quick drying cement with standard cement to allow more working time for small repairs. An example of this would be for instance if we were doing a small bucket mix with a brick layers trowel… 1 third trowel of quick drying, 2 thirds standard OPC and three full trowels of sand, which still gives a ratio of 3 to 1, but depending on the weather a drying and working time of 20 mins +. Playing with the amount of quick drying and standard cement will accelerate or delay the drying process. Not only does this give you greater control, but is cheaper too.
If you’ve just read this article and you aren’t connected with the building trade you may be thinking this is a hell of a lot to take in, or it’s really complicated. The truth is most of it is just really common sense, it just looks like a big list when written down. A bit like making a cup of tea could be if you did the same.
These of course are my personal tips learnt over the years, or gleamed from others in the trade, both success and failure have shaped this article. Another roofer, builder, plasterer or renderer may have their own opinions, or other great nuggets to share particular to their trade. Never stop learning.