The Brew Cabin

Tony,

you have a calculator telling me that for a 40m2 patio with a bed of 40mm I need about 3tonnes of grit sand.

I'm trying to work out how much sand, cement and plasticiser I need to create the mortar joints.

I think I have 170m of joints, these will be 12mm thick and about 35mm deep = 0.07m3 of mortar.

Thanks again

PS what am I supposed to do with the old paving slabs I removed from around my house. It seems a shame to throw them in the skip.

The bedding quantity sounds about right at 3 tonnes - a 50mm bed takes around a tonne every 10m²

I don't have an online calculator for jointing mortar; perhaps I should write one, as I have them on my system here. Your calculation is basically correct except for one very important factor - wastage. You need to add 20-60% for wastage, and the reason why there's such a relatively large anmount of wastage depends on just how the flags are being jointed.

If you're buttering and pointing, a significant quantity of the buttering mortar is trapped beneath the flags or is squeezed up and lost. With square-edges flags, this might be as little as 20%, but with riven stone flags or those wet-cast flags with imitation fettled edges, the amount lost to wastage can reach 60% or even more.

If you're pointing only, then allowing 20% for wastage is much nearer the mark.

Anyway, to complete your calculation....

170m x12mm x 40mm x 1.4 (wastage) = 0.11m³

0.11m³ @ 2.4T/m³ = 275kg @ 4:1 = 55Kg cement plus 220kg sand

Plasticiser for such a relatively small amount of mortar is negligible. If you have lots of other mortar work, it might be worth buying a 5 litre pack, but, if this is your only requirement, then the old trick of using a squirt of wash-up liquid to the gauging water is probably a better idea.