What's Actually in the Bag? A Merchant's Guide to Sands, Ballast and Mortar Mixes

Ask ten people what sand they need for a patio and you will get at least four answers, one of which will be 'just building sand, mate'. That is not because people are careless. It is because the words get used loosely. Building sand, sharp sand, grit sand, ballast, mortar mix, plastering sand — they all sound like variations on a theme until you tip the wrong one into the mixer and watch the job go sideways.

At McPhail & Sons we sell a lot of aggregates, and the questions at the counter are usually the same. So here is the no-nonsense version: what each bag is for, what it is not for, and how to describe the job so you get the right one first time.

Building sand: the soft one for mortar

Building sand is fine, relatively soft and usually has a bit of clay in it. That is not a fault; it is what makes it workable in mortar. It is the sand you want for bricklaying mortar, bedding blocks, pointing and general masonry work. It gives the mix a bit of body and helps it stick to the trowel rather than falling off in lumps.

Where it goes wrong is when building sand gets used for concrete. It is too fine and too soft for a structural mix, and the result can be weak and crumbly. If you are laying a slab, you want sharp sand or ballast, not the soft stuff you use for mortar.

Sharp sand: the gritty one for concrete and screeds

Sharp sand is coarser and more angular. It is the one for concrete, floor slabs, screeds and some paving bedding. The grit helps the mix bind together and gives it strength. If you are mixing concrete by hand, sharp sand plus cement and gravel will do the job, though many people find ballast easier because it is already blended.

Sharp sand is not the one for bricklaying mortar. It makes the mix harsh and difficult to work, and it can be a nightmare to point neatly. If a bricklayer asks for sharp sand by mistake, it is worth double-checking what they are actually doing.

Ballast: the all-in-one for concrete

Ballast is a blend of sand and gravel. It is the straightforward choice for concrete because you do not have to buy and mix the sand and gravel separately. A typical concrete mix is one part cement to four or five parts ballast, depending on the strength you need and what it is for. It is not a mortar mix, and it should not be used for bricklaying. If you put ballast in a mortar mix, you get lumps, poor workability and a finish that looks like it was applied with a shovel.

Mortar mix and plastering sand

Mortar mix is a pre-blended product, usually sand and cement with the ratios already sorted. It takes some of the guesswork out of small jobs and is handy if you do not want to store separate materials. Read the bag and follow the instructions; do not add extra cement 'for strength' unless you know what you are doing.

Plastering sand is finer again and is used for rendering and plastering. Do not substitute building sand for plastering sand just because it is nearby. The finish will not be the same, and you will notice.

The mix-ups we see most often

• 'Sand is sand.' It really is not. The wrong sand can make a mix weak, harsh or unworkable.
• Sharp sand in mortar. It makes bricklaying harder and pointing messier.
• Building sand in concrete. It can leave you with a weak mix that does not set as it should.
• Ballast in mortar. It is the wrong tool for the job and it shows.
• Using damp sand without adjusting. Damp sand bulks up, so measure by volume carefully and keep it consistent.
• Buying a bulk bag for a job that needs three bags. Surplus aggregate is heavy, awkward to store and not always easy to return.

How to describe the job at the counter

You do not need to know the technical name for everything. You just need to tell us what you are doing. Try: 'I am laying a 12 square metre patio with a 50mm bedding layer.' Or: 'I am building a garden wall with concrete blocks and I need mortar.' Or: 'I am mixing a shed base, about 2 metres by 3 metres, 100mm deep.' From there we can work out whether you need building sand, sharp sand, ballast or a pre-blended mix, and how much of it.

Useful details include the area, the thickness, whether it is indoors or outdoors, whether it needs to be strong enough to drive on, and how you are getting it home. A bulk bag is great value, but not if you cannot get it through the gate or lift it off the pallet.

Bags, bulk bags and delivery

Most aggregates come in 25kg bags or bulk bags of around 850kg or 1000kg. Small bags are easier to handle and store, especially for DIY jobs. Bulk bags are better for larger projects, but they need space, access and a firm surface. If you are not sure, ask about delivery and check the access before the wagon arrives.

Getting the right aggregate is not about being fussy. It is about saving yourself a second trip, a wasted mix and a finish you have to explain to the customer. If you are unsure, bring the job details to the counter. We have been helping people get this right since 1952, and we would rather sell you the correct bag than the wrong one twice.