Calculating Areas

The long term performance of any paint film is largely related to the amount of paint on the surface, in effect the thickness of the dry paint material.

Most paints contain water or solvents which when they evaporate, allow the wet paint film to “dry out” thus forming a dry layer of paint over the surface. Many paints have around 50% volume that needs to evaporate, before the film “dries” and thus the wet film looses half of its original thickness, through this evaporation.

Thus if the wet thickness is say 1 mm (which is thick) has 50 % water/solvent, then when it dries, it will be 0.5 mm thick.

(There are some paints/coatings which have very low solvents, Epotec being one with almost 100 % non volatiles/solvents. With these the wet thickness is almost the same as the dry film thickness).

Thus getting the right amount of dry paint on a surface is important if you want the best long term performance, especially so in demanding situations, like roofs and walls.

Internal Surfaces:

Ceilings (Flat): Measure the length and width of the room, (can pace out*)
Length x Width in metres will give the Sq M of the surface.

Internal Walls: Measure length, width and height of walls in Metres. The area is the addition of the length x 2 and the width x 2, added together and multiplied by the height. That is (2 x L) + ( 2 X W) x H

Or you can measure the total perimeter and multiply be height.

Most rooms in NZ are 2.4 – 2.8 M high.

You should deduct doors and windows, so allow for a normal door 1.5 Sq M per side and Windows are 1-2 Sq M on average, but measure to be sure.

This will give you the total interior surface area, for which you can calculate the amount of paint. See the relevant Data Sheets for correct coverage rates.

* Pacing out, If you use a “normal walking step” and are of average height, then each step will be 1 Metre, approx.

External Surfaces:

Roofs:
Flat (Deck, patio): break the roof into rectangles, measure length and width in metres to get sq M and add all together to get total.

Pitched/Sloping:
Depending on the complexity of the roof design, however the easiest way is to break it into rectangles and triangles.

For Rectangles, length multiplied by width in Metres, will give Sq M.

For Triangles, length multiplied by width, divided by 2, will give Sq M.

Add these together to get total area.

NOTE: If climbing on roofs be careful. Maybe easier to pace /measure exterior perimeter of house and guesstimate height of roof. Convert to rectangles/triangles and calculate area.

Tiles: (Pressed Metal, Concrete, Clay)
These have a profile such that the actual surface area is greater than the over all dimensions of each tile. Allow and extra 10-15% of measured area to get the net flat area for paint consumption.

Long Run, Tray Roofing:
The profile means the area of the roof is greater than measured due to the ridges. Allow 10 -15% over measured area for the ridges.

External Walls:

To calculate the wall areas, measure the perimeter of the building in meters and multiply by the height. (For single storey houses 2.4 – 2.8 M, and 5 – 5.5 M for 2 storey houses).

Delete areas of doors and windows. Measure the windows ( 1-2 Sq M on average).

Wall finishes have an impact on the amount of material needed for correct film build.
(See relevant Data Sheets for more information)

Rough Cast, Heavy Texture: allow an added amount of material, usually 50-100 % more than for flat surfaces, see respective product Data Sheets.

Medium Textures: allow an extra 20-50 %.

Weather boards, and similar: allow an extra 5-10 % .

Summary:

In order to get the longest life from your paint, correct surface preparation and film build are two of the most important aspects. It is better to have slightly more paint material than not enough and in general it will give a longer life. The amount of time and effort in surface preparation and application labour is the same, no matter whether you put on a little more (or less paint) and thus get extra life from the paint.