Measuring Material Quantities

Before any task can begin, the required materials must be estimated and obtained. Accurately estimating materials is important in order to ensure the perfect amount is acquired - avoiding the additional costs associated with over or under ordering. Here is a brief section on ordering materials, from both a mathematical and construction perspective.  

The majority of material estimations are going to be presented in either square or cubic measurements, with a small handful of materials being measured by weight from the supplier. Understanding the basic formulae for calculating these areas is important when ordering materials.  

Square meterage is calculated by multiplying the length of the surface by the width (L x W). This applies to walls, ceilings, floors, and any other flat surface. Depending on the material we’re estimating, the amount required can be deduced from the total square meterage of the surface in different methods. For sheet goods such as plasterboard(2.4mx1.2m), OSB and Plywood(8’x4’/2440mmx1220mm), and rigid foam insulation(2.4mx1.2m), each sheet covers approximately 3 square metres. To calculate the number of sheets required, simply divide the total square meterage by 3. For medium sized scenarios, unless the area is small and can be very accurately measured (i.e. only a handful are required), an extra sheet should be purchased to account for wastage. For larger jobs, 10% of the estimated quantity should be added on, to account for damage and wastage. 

In a flat roof extension scenario, we can save time calculating all the materials required by simply measuring the floor meterage of the extension. If measured accurately, this meterage displays the meterage required for underfloor insulation, ceiling plasterboard, flat roof ply (2 layers for a warm roof), and the insulation required in the flat roof. Quantities for other sheet materials can be calculated via this method, as long as the formula is known. For example, standard chipboard flooring(600mmx2.4) covers half the square meterage of a standard 4’x8’ sheet. This means to calculate the number of floorboards required, we take the full meterage of the room, divide it by three, then multiply that value by 2. Alternatively, we can just divide the total meterage by 1.5. 

In other instances, we can use the dimensions of an area to calculate the timber required for the installation of carpentry components. The number of joists required for a floor or ceiling assembly, rafters required for a roof assembly, and studs required for a stud partition can all be calculated using the dimensions of the surface area. For joists, it's obvious that the length required is the width of the room in the direction that they will be running. The specific amount required can be calculated by dividing the length of the room by the “on centre” measurements of the joists and adding 1. For example, the number of joists required for a room that is 3.6m long when spaced at 400mm centres is 10 (3.6/0.4 = 9 + 1 =10). This same formula can be used to calculate the number of rafters or studs in any given run. For studs, the length of timber required in terms of height is known by the height measurement of the area intended for the studwork.  

When ordering joists or studs, we also need to account for the solid blocking required for the job. This can be achieved by ordering extra lengths for blocking, or by ordering lengths of timber longer than the required length. For example, the offcut when cutting 3m long studs for a 2.4 height room leaves enough waste per length for one piece of blocking. Alternatively, if we’re ordering 2.4m lengths, then a few extra lengths must be purchased for the blocking. When ordering for joists, it might pay to order a few lengths of a timber the next dimensional width down, to allow for the blocking to sit below and above the floor and ceiling line respectively.

Most finished materials and moulding come in standardised lengths and are measured in linear metres. When ordering these materials, an additional 10% is commonly added to the total amount required to account for wastage, trimming, and defects in the lengths.   

For materials such as cladding, the method of calculation is slightly more involved. Whilst the boards come in lengths in a linear fashion, the cladding may be sold by the manufacturer in square meterage. For example, commonly available cladding comes in lengths of 3.6m or 4.2m, with widths at somewhere around 100-150mm. Whilst these are individual length, when installed in a specific arrangement they come together to form a measurable square area. We can calculate the area required to be clad in square metres and provide this value to the manufacturer in order for them to supply the correct number of boards for that area. Alternatively, if we’re purchasing the lengths directly from the merchant, we may have to calculate the amount required beforehand. To calculate the amount required, the process is as follows.  

First begin by noting the dimensions of the area that is going to be cladded, notably the height and the width. Some cladding styles feature an overlap in their installation, with others slotting into each other. Regardless of the specific style, we need to determine the width of each board that is on show after installation. For featheredge boards, each board laps over the board under it by a certain amount. Therefore, we need to determine the width of the board that is left on show. We can then divide the total height of the area being cladded by the width of the boards on show to accurately calculate how many rows of boards there will be. We can multiply this value by the length of the area being cladded to calculate a total linear meterage for the job. At this stage we can remove the area of any non-cladded areas such as windows, or the offset of a gable. We can divide this total by the length of the boards that we are purchasing to calculate the total number of boards required for that specific square meterage. We should then add 10% to this value to accommodate for any waste, offcuts, or damage to the lengths.

In the context of general building there are a handful of material estimating rules of thumb that can be used to acquire materials to supply other trades with relative accuracy. In terms of plastering, we already know that one full sized sheet of plasterboard (2400mmx1200mm) will cover approximately 3m². When ordering standard multi finish plaster, 1 25kg bag will cover three boards with 2 coats, or 9m². For walls that are dry lined, 1 25kg bag of drywall adhesive is enough to adhere 2 full sized boards. For walls that are not being dry lined, but hard walled instead, 1 bag of hard wall will cover just under 3m², or visually, approximately 1 board full.  

When ordering ready mix concrete for footings, the supplier will need to know the amount required in a fairly accurate estimation. The amount is calculated in cubic metres. To calculate the amount required, we can simply use the formula for calculating volume - length x width x height. For a concrete oversight or raft, measuring this is easy, as the area to be filled is square or rectangular. For example, we can measure a length of 6m, by a width of 4 metres, by a depth of 100mm, to return the required value of 2.4 cubic metres (6mx4mx0.1m=2.4m³).  

Estimating the amount of concrete required for conventional trench footings can seem more complicated but is still just a simple formula of length x width x depth. The easy part here is determining the width and the depth of the concrete, which should be consistent through the trench. Typically, these values are around 600mm and 300mm respectively. To accurately measure the length in a linear fashion, we must measure the length of the central line through every section of the footing trenches.  

Floor screed can be calculated by the cubic metre in the same fashion.  

As an aside here, concrete is sometimes measured or sold in tonnes. Being able to approximate the conversion mentally can be useful. As such, it's relevant to know that 1 cubic metre of concrete is approximately 2.4 tonnes. 

Another major material component in UK construction is masonry - bricks, blocks, and mortar. As a rule of thumb, 1 square metre of brickwork consists of 60 bricks, assuming a standard brick bond with regular 10mm mortar joints. On the other hand, 1 square metre of blockwork only consists of around 10 blocks. We can calculate the total materials required for an area using these values. If we’re building a wall that is 5 metres long by 2.4 metres high, the total area is 12m² (5mx2.4m =12m). If there are 12 square metres of the wall to construct, and each metre contains 60 bricks, we can determine that we’ll need 720 bricks. This same area would only require 120 blocks.

We can accurately estimate the number of bricks or blocks required for a gable by calculating the full square area of the gable using the width and height dimensions, and dividing the total value of either materials or area in half. As with most materials, we must add a specific amount or percentage to the total value to account for damage and waste.  

In terms of ordering sand and cement for mortar, 1000 bricks will require approximately 1 tonne of sand and 6 bags of cement. Accurately calculating the amount is difficult, due to the various criteria such as bond type, wastage, frog depth, joint size etc. As a rule of thumb though, this is somewhere handy.

 [1]measuring cladding