For making masonry wall in a building we normally use bricks or concrete blocks. The surface of the concrete blocks and bricks are uneven. And the size of bricks or concrete blocks isn’t always same. So, after constructing walls we see uneven surfaces. To make the wall surfaces smooth and even we make plaster on it.
Due to this unevenness of the wall, the plaster thickness often differs. The thickness of plaster on RCC surface isn’t same as the thickness on brick walls. So, when we calculate the cement and sand quantity for plastering we need to consider this thickness.
So, what should be the thickness of plaster on various types of surfaces?
The Thickness of Plaster on Different Surfaces
The standard thickness of plaster on any surface is ½″ (12mm). But, due to the uneven surface of walls and weak workmanship, this standard plaster thickness varies.
When we calculate the quantity of cement and sand for plastering, we use different thickness of plaster for different surfaces. The thickness of plaster is as below –
- For RCC surface – ½″ (12 mm)
- For flashed brick wall surface – ¾″ (20 mm)
- For unflashed brick wall surface – 1″ (25 mm)
When we make brick walls, we try to keep all the brick face at the same alignment on one side of the wall. This side of the wall is called flashed side. Due to the different size of bricks, the brick face of the other side of the wall will not be in the same alignment. This is called unflashed side of the wall.
How to Calculate The Cement Sand Quantity for Plastering
For calculating the cement and sand quantity for plastering, you first need to calculate the required quantity of mortar.
For calculating the required quantity of mortar you need to get the area for different types of plastering surface.
So, let’s see step by step process of calculating the cement and sand quantity for plastering.
Step 1: Get The Surface Area for Plastering
As different surfaces required different thicknesses of plaster, we need to get different surface area separately.
For getting surface area, you just need to get the length and width (height) of the surface and then multiply both dimensions.
Let’s assume we got,
- RCC surface area is 100 sq.ft.
- Flashed brick wall surface area is 100 sq.ft, and
- Unflashed brick wall surface area is 100 sq.ft.
Step 2: Get The Required Mortar Quantity for Plastering
As different plastering surfaces require different thicknesses of plaster, the required mortar volume will be different for various surfaces.
The required mortar quantity for RCC surface is,
=Area of RCC surface x thickness of plaster
= 100 sq.ft x ½″ (½″= 0.042 ft)
= 4.17 cubic feet.
Similarly, the required quantity of mortar for flashed brick wall surface is,
= 100 sq.ft x ¾″ (¾″ = 0.063 ft)
= 6.25 cubic feet.
And, the required mortar quantity for unflash brick wall surface is,
= 100 sq.ft x 1″ (1″ = 0.083 ft)
= 8.33 cubic feet.
So, the total required quantity of mortar is,
= 4.17 + 6.25 + 8.33
= 18.75 cubic feet.
Step 3: Get The Dry Quantity of Mortar
To Calculate the cement and sand quantity for plastering you need to get the dry quantity of required mortar.
Dry quantity of mortar is,
= wet quantity of mortar x 1.27
= 18.75 x 1.27
= 23.81 cubic feet.
Now the question is, what is the 1.27. It’s a constant we normally use to get the dry volume of wet mortar. I’ve talked about this before on “how to get the quantity of cement and sand in Mortar”.
Step 4: Knowing The Cement and Sand Ratio in Mortar
For calculating cement and sand quantity in Mortar, we need to know the proportion of cement and sand in Mortar.
For plastering, we normally use 1:4 ratio of cement and sand. 1:4 ratio means, one unit cement is mixed with four unit sand.
So, the total unit of cement and sand in mortar for 1:4 ratio is,
whenever you want to get the volume of a particular ingredient in Mortar, you just need to divide the dry volume of mortar by the total unit and then multiply that with the proportioning unit of that particular ingredient.
Step 5: Calculating Cement Quantity in Mortar
Now we have everything to calculate the cement quantity in Mortar. Let’s do this.
From the above discussion,
The dry volume of mortar = 23.81 cubic feet.
The ratio of cement sand= 1:4
So, the cement quantity in mortar is,
= 23.81 ÷ 5 × 1
= 4.76 cubic feet.
Cement is sold in the market as bags. So, we need to convert the volume of cement into bags.
We know, the density of cement = 40 kg/ft3 [1440 kg/m3].
So, the weight of our required cement quantity is,
= 4.76 x 40
= 190.40 kg.
A bag of cement normally contains 50-kilogram cement. So, the required bag of cement for our calculated mortar quantity is,
= 3.81 bags
Say 4 bags.
Step 6: Calculating Sand Quantity in Mortar
As the cement sand ratio in mortar is 1:4. So, the sand quantity in mortar is four times of cement quantity. As sand is found in the market as volume, so we don’t need to convert the volume into kilogram or bags.
From the step 4, we found the cement quantity is 4.76 cubic feet.
So, the sand quantity is,
= 4.76 x 4
=19.04 cubic feet
Say, 20 cubic feet [Density of sand = 1920 kg/m³].
For calculating cement and sand quantity for plastering, you need to measure the area of plastering surfaces. Then get the mortar volume by multiplying the plaster thickness with the area. After that, you need to convert the wet mortar volume into the dry volume. From the dry volume, you can calculate the quantity of cement and sand.