To determine the quantity of cement (or lime) and sand, required for plastering unit square metre of the area of various thickness is to work out the volume of mortar required per sq.m of plaster by multiplying the area to be plastered by its thickness.
In order to allow extra mortar for raked out joints, cavities, uneven surfaces, etc. the above worked out quantity is to be increased approximately by 30%.
Further to convert the wet volume of mortar into its corresponding dry volume, it should be increased by about 30%.
Then the quantities of cement and sand required can be determined by dividing the total dry volume of mortar by the sum of the numerical figures of proportion or mix of the mortar and multiplying it by the individual numerical figures.
e.g. To determine the quantities of cement and sand for 12 mm thick plaster in cement mortar (1:4), the procedure would be as follows:
Considering the area to be plastered as 10 sq.m, with a thickness of 12 mm, the quantity of wet mortar required = 10 (sq.m) x 1.2 / 100 (m) = 0.12 cum.
Therefore, Adding 30% extra for filling joints, and uneven surface, etc., quantity of wet mortar required = 0.12 + (0.3 x 0.12) = 0.156 cum.
Therefore, Quantity of dry mortar required = 0.156 + (0.3 x 0.156) = 0.156 + 0.0468
= 0.2028 cum.
Further, allowing for wastage, etc.
The total quantity of dry mortar required = 0.203 cum.
Therefore, Quantity of cement required = 0.203 / (1+4) = 0.041 cum.
= 0.042 x 30 = 1.23 bags
and Quantity of sand required = [0.203 / (1+4)] x 4 = 0.162 cum.
Approximate method
The approximate method of determining the volume of dry mortar required is to multiply the quantity of wet mortar required by a factor 1.8
i.e. Volume of wet mortar required = 0.12 cum.
Therefore, Volume of dry mortar required = 0.12 x 1.8 = 0.216 cum.
Therefore, Quantity of cement required = 0.216 / (1+4) = 0.043 cum.
= 0.043 x 30 = 1.29 = 1.30 bags
and, Quantity of sand required = [0.216 / (1+4)] x 4 = 0.0173 cum. which is same as determined above.
Neeru finish coat of 1.5 mm thickness.
If the inside face is to be plastered further with a Neeru finish of 1.5 mm thickness then the quantity of Neeru required for 10 sq.m surface
= 10 (sq.m) x 1.5 / 1000 (m)
= 0.015 cum.
i.e. 0.015 x 30 = 0.45 bags = 0.5 bags of Neeru which is available as ‘Sagol’ or ‘Sunala’ as brand name in the market.
Basics for Site Engineer 