We looked at the basic principles of estimating primer and underlayment quantities in the unit: Planning and costing.

You'll recall that the simple formula for working out the amount of primer needed to cover a floor is:

        Area of floor ÷ coverage of product = volume required

The units of measure that these figures are generally expressed in are:

• Area of floor - square metres (m²)
  
  
• Coverage of product - square metres per litre (m²/L)
  
  
• Volume required - litres (L)

The same principle is used for working out underlayment quantities.

However, this time the coverage is generally shown in square metres per bag or kilogram at a given thickness (m²/bag or m²/kg at whatever thickness the manufacturer nominates).

This means that if your finished thickness is going to be different from the manufacturer's nominated thickness, you'll need to make an adjustment to the quantity.

For example, say you were going to spread a levelling compound at an average thickness of 3 mm on the subfloor, and the coverage rate printed on the bag reads:

        Coverage: 13 m²/bag at 1 mm thick

To work out the total quantity of compound required, you would need to use the same formula and multiply your answer by 3.

If you need to revise these basic principles before we move on to some more complex examples, go to the following lessons in Planning and costing:

• Primers and adhesives
  
  
• Levelling compounds.

You may also want to revise the maths basics that underlie these calculations in the unit: Making measurements.

Ramps

In geometric terms, ramps are actually triangular prisms. That is, they are a normal square or rectangular shape on top, but their depth is triangular.

Let's say you needed to build a ramp between two rooms to accommodate a floor height difference of 20 mm. You have chosen a slump-free repair mortar with a coverage rate that reads:

        Coverage: 12 m² at 1 mm thick (per 20 kg bag)

The steps involved in working out the quantity are as follows. Remember that we have to convert all measurements from millimetres to metres for these calculations.

1. Find the surface area

Area = width x length = 1.5 x 1.2 = 1.8 m²

2. Find the coverage for the given thickness

The nominated coverage is 12 m² per 20 kg bag when spread at an average thickness of 1 mm. So if the ramp was only 1 mm thick, you would need:

        1.8 m² ÷ 12 = 0.15 bags

3. Find the total number of bags

In our case, the ramp thickness goes from 0 mm at one end to 20 mm at the other. So the average thickness is 10 mm. In other words:

Average thickness = thickness at start + thickness at end ÷ 2

        = 0 + 20 ÷ 2 = 10 mm

This means we need to multiply the Step 2 figure by 10:

        0.15 x 10 = 1.5 bags

Diluted primer

Some primers are diluted with water when the substrate surface is very porous.

Let's say that you have diamond ground the concrete subfloor shown in this diagram and are about to coat it with primer.

The container says that the coverage rate is 3.5 m²/L, but that for porous concrete the primer should be diluted with water at a 2:1 ratio.

This means that 2 parts water will be added to every 1 part primer. In other words, if there was a total of 3 litres required for the floor, 2 litres would be water and 1 litre would be primer.

How much water and primer will you need for this job?

Step 1: Find the area

        Area = 3.5 x 6.2 = 21.7 m²

Step 2: Find the total volume at the normal coverage rate

Volume (L) = area (m²) ÷ coverage (m²/L)

        = 21.7 m² ÷ 3.5 = 6.2 litres

Step 3: Find the water and primer volumes

A water/primer ratio of 2:1 means: 2 parts water + 1 part primer = 3 parts in total. So:

Volume of water	= total volume ÷ total no. of parts x no. of parts water
	= 6.2 litres ÷ 3 parts (in total) x 2 parts (water)
	= 4.1 litres
Volume of primer	= 6.2 litres ÷ 3 parts (in total) x 1 part (primer)
	= 2.1 litres

You can check your answer by adding the two components together:

        4.1 litres water + 2.1 litres primer = 6.2 litres in total

Wastage factors

Depending on the type of job and the products you're using, you may want to build a wastage factor into your calculations. This helps to compensate for slight variations in estimates or small amounts of wastage.

Installers often allow for 5 to 10% wastage on primers and levelling compounds. For example, if you wanted to add 10% wastage to the primer calculation above, the total volume required would be:

        6.2 litres + 10% (0.62 litres) = 6.8 litres

If you need more practise on using percentages, go to the 'Decimals and percentages' lesson in Making measurements.