Which sand or aggregate do I need?

The differences between building sand, sharp sand, ballast, MOT type 1 and gravel, and where each belongs.

Difficulty: beginner
Time: 15 min
Read: 3 min

Short answer

Soft building sand for mortar and pointing, sharp sand for concrete and screeds, all-in ballast for general concrete, MOT type 1 for compacted sub-bases, and decorative gravel for surfaces. They are not interchangeable.

Aggregates all look like piles of stone and sand, and using the wrong one produces mortar that will not work or a base that will not compact.

Step by step

Use soft building sand for mortar.Fine and slightly clay-bearing, which makes it workable and sticky. It is what gives mortar its buttery quality for bricklaying and pointing.
Use sharp sand for concrete and screeds.Coarser, angular grains that interlock and give strength. Sharp sand in mortar makes a harsh mix that will not spread.
Use all-in ballast for general concrete.Sand and aggregate already blended, so you only add cement. Convenient for footings, post holes and paths.
Use MOT type 1 for sub-bases.A graded crushed stone with fines that lock together when compacted. It is the correct base under a patio, drive or path — gravel will not compact the same way.
Use single-size gravel for drainage.Uniform stone with no fines, so water passes freely. Right for French drains and soakaways, wrong under a paving slab.
Choose decorative gravel by size and use.10mm for paths that will be walked on, 20mm for beds and borders where it stays put. Very small gravel migrates into shoes and tyres.
Use grit sand for laying slabs.For a sand and cement bed under paving. Building sand is too fine and holds too much water.
Check whether it is washed.Washed sand has had the clay and silt removed. Unwashed sand can stain and can weaken concrete.
Work out quantity by volume.Area times depth, plus about 10% for compaction and waste. Aggregates settle noticeably once compacted, and running short mid-job is a real nuisance.

Tips

Soft sand for mortar, sharp sand for concrete. Getting this the wrong way round is the classic error.
MOT type 1 compacts and locks; gravel does not. Sub-bases need type 1.
Add about 10% to your quantity calculation for compaction and waste.

Common mistakes

Using sharp sand for mortar — The angular grains give a harsh mix that will not spread or hold on the trowel, making bricklaying and pointing very difficult and the joints poor.
Using rounded gravel as a sub-base — Rounded stone with no fines will not lock together under compaction, so the surface above it settles and moves. MOT type 1 is graded specifically to compact.

If it doesn't work

Mortar harsh and unworkable

Cause: Sharp sand instead of building sand — Fix: Use soft building sand, and add plasticiser.

Patio settling unevenly

Cause: Wrong sub-base material or insufficient compaction — Fix: Lift and re-lay on properly compacted MOT type 1.

Gravel migrating everywhere

Cause: Size too small for a trafficked area — Fix: Use 20mm rather than 10mm, and edge the area properly.

Concrete weak or staining

Cause: Unwashed aggregate with clay content — Fix: Use washed aggregate for anything structural or visible.

Questions people ask

What is the difference between sharp sand and building sand?

Grain shape and size. Building sand is fine and rounded, which makes mortar workable. Sharp sand is coarse and angular, which gives concrete and screeds their strength.

What is MOT type 1?

A graded crushed stone containing fines, specified for compacted sub-bases under paving and drives. The mix of sizes is what allows it to lock solid when compacted.

How much do I need?

Area multiplied by depth gives volume, then add around 10% for compaction and waste. Suppliers sell by tonne or by bulk bag, so check the conversion for the material.