Plastering Material Estimation
Understand the 'Dry Volume' conversion and the high wastage factors associated with vertical plastering.
The 1.33 Factor in Plastering
When you mix dry cement and dry sand with water, the volume of the resulting wet paste is significantly smaller than the original dry ingredients. For plastering and mortar, engineers use a multiplier of 1.33 ($33\%$ increase) to convert the required wet volume of the wall covering into the dry material you need to purchase.
Wastage and Void Filling
Plastering has a higher wastage factor than concrete ($15\%$-$25\%$):
1. Joint Filling: Mortar joints between bricks are often recessed and absorb a significant amount of plaster.
2. Droppings: A substantial amount of material falls to the floor during application.
3. Uneven Walls: If the masonry is not perfectly plumb, the plaster thickness will vary.
Standard Thicknesses
- Ceilings: $6$mm. Thin is better to prevent peeling under gravity.
- Internal Walls: $12$mm. One coat is usually sufficient.
- External Walls: $18$mm to $25$mm. Usually applied in two coats (rough and smooth) for waterproofing.
Mix Ratios
The strength of plaster depends on the cement ratio:
- 1:3 / 1:4: Rich mix used for external walls or damp-prone areas.
- 1:5 / 1:6: Standard mix for internal dry walls and ceilings.
Frequently Asked Questions (FAQ)
Should I use Fine Sand or Coarse Sand?
For a smooth finish, fine sand (river sand or crushed stone sand passed through a $2.36$mm sieve) is used. Coarse sand leads to a stronger plaster but is difficult to finish smoothly and can cause "grazing" or fine cracks.