Skip to main content

How To Practically Calculate The Quantity (m2) Of Roofing Sheet Required

I've been wanting to do this topic but couldn't come up with the easiest way to teach it; while it is possible to measure the quantities of roofing sheets needed for your project even before you set the first block using the Architectural drawings; it requires a lot of experience especially in trigonometry and site visualization. I have however found an easy way around it; as would be discussed below.

I have carefully designed and attached a roof plan for this purpose, (feel free to use should it fit your building design... lol)

Calculating the quantity of roofing sheet needed is done is meter square (m2), same parameters used for selling and purchasing roofing sheets hence we would expect to use the AREA FORMULAS (i.e Area formulas for Rectangle / Square, Triangle, Trapezium and Rhombus), sounds easy right? Yes it is!!!

The most difficult aspect of calculating Area of Roofing is FINDING THE TRUE LENGTH (Technical Drawing students should understand this) and that is infact why I opted for the practical way to calculate because I expect the roofing carcass to have been done on site before you can apply this teaching by asking your carpenter to measure key length as we would explain soon. (We would also discuss finding TRUE LENGTH in brief at the end of this tutorial)


Diagram 1: shows the roof plan and the front elevation of a building while
Diagram 2: shows the division and label to guide us in our calculation.
Diagram 3: shows the dimensions of lengths that are ALREADY IN THEIR TRUE LENGTHS (those not labeled would have to be measured on site or calculated)

We would calculate the roof per section eg R1, R2, etc and add up later. (PRINT OUT THE ATTACHED DIAGRAMS)

Let's start by calculating R1.

The shape R1 is a triangle, so we would use the triangle formula (½BxH) where B = (A+B) in Diagram 2 = 9m and H is the true length of C. (ask your carpenter to measure (A+B) and C on site and write down the distance in meters). Measuring from my drawings C = 6.15m

Hence area of R1 = (9m x 6.15m) / 2
Area of R1 = 27.68m² (what this means is that minus waste, we need 27.68m² of roofing sheet to roof R1 alone) we would add waste after the entire exercise. 

Now let's calculate R2:

The shape R2 is a trapezium hence the formula would be ½x(A+B)xH - (notice i divided R2' out) here the carpenter should measure the following distances and you can note them down; R, (F-K) and D in true length. R = 7m, F-K = 18.4-2.4 = 16m and D = 6.15m (measured from my drawings).

Hence area of R2 = ½ (7+16) x 6.15 = (23m x 6.15m) / 2
Area of R2 = 70.73m²

I separated R2' to make the calculation and formula application simplier. R2' wld now be calculated as a Rhombus using formula L x B. 

Hence area of R2 = O x O' = 2.4m x 3.05m (measured from my drawings as I expect you to measure from site)
Area of R2' = 7.32m² also note that R2' = R7 hence R7 = 7.32m²


Calculating R3:

R3 = R2 - (the small triangle in R4&R5) so I'll calculate the small triangle and subtract from R2.
hence; the area of the smaller triangle using same principle from R1 = (7m x 4.7m ) /2 = 16.45m²

R3 = 70.73 - 16.45 = 54.28m²

Now let's calculate R4 & R5: The trapezium formula applies here also; measure F, I and Q where F is the true length and F = H in the formula ½x(A+B)xH
= ½ (3.99+7.49) x 4.8 = (11.48m x 4.8m) / 2
Area of R4 = 27.55m²

Area of R5 = 27.55m² also

Now let's calculate R6 and R8: If you look at the roof plan critically, you would notice that R6 + R8 = A perfect Triangle; so instead of calculating them separately, draft it into a complete triangle and calculate using the triangle formula. Coincidentally also, R6 +R8 = R1 so lets just pick the figures from R1.

Hence R6+R8 = R1
Area of R6+R8 = 27.68m²

= 27.68 + 70.73 + 7.32 + 7.32 + 54.28 + 27.55 + 27.55 + 27.68
= 250.11m²

Now lets add waste of 10% - 20% depending on the roofing material e.g. Gerard roofing sheet would generate more waste than Long Span roofing sheet.

ADD 20% = 250.11 + 20% of 250.11 = 250.11m² + 50.02m²


Multiple 300.13 by the price of 1m² of roofing sheet eg N3200 (Gerard) = 300.13 x 3200 = N960,416:00k (Minus Accessories)

I'll briefly explain calculating TRUE LENGTH of C in R1 below.

HOW TO CALCULATE THE TRUE LENGTH OF C IN R1 (diagram 1,2,3 above) also see attached diagram below

From the Architectural Drawings you can get the HEIGHT AND BREATH OF THE ROOF, what the drawings won't tell you is C (I dimensioned it thou for learning purpose in the attached diagram)

= 4.2² + 4.5² = C²
= 17.64 + 20.25 = C²
C² = 37.89

C = Root of 37.89 
C = 6.15548

C = 6.155 (same with the dimension from AutoCAD)



Popular posts from this blog

Cement, Sand And Granite (concrete) Mix Explained

Good Day:
Lets explain concrete mix and how your bricklayer is expected to mix your next foundation / decking.

Most time as Non-Construction experienced house builders, we put our trust on the bricklayers to mix our concrete rightly, however very few of them can be trusted to do the right thing - If your bricklayer negotiated for material and labour (its even more difficult to trust him) because he might be maximizing gain and reducing materials; often times even when you buy your materials; they tend to put in excess sand, granite ratio to minimize time spent in doing the job.


Alternative Building Material (expanded Polystyrene)

With the current increasing cost in building a house in Nigeria, and with the Nigeria Culture/Mentality
“I must have my own House” a lot of Nigerians have been looking for various building material options which are relatively cheaper and perform the same function as the conventional ones.

One of such material options which I will be focusing on this thread is Expanded Polystyrene (EPS), This material is strong, durable, light weight, water resistance with excellent thermal insulation which is very popular in Environmentally “Green” Homes.
Expanded Polystyrene is an affordable and incredibly sustainable choice in the construction circle due to versatility and performance.
It can be used in construction as a structural base infill for example in floor Slab, Roads, Bridges Railway infrastructure, or as a structural element of its own, Roof Eaves, wall (partition), D├ęcor Materials.

Choosing the right foundation for that project type & soil.

Wikipedia defines foundation as: A foundation (or, more commonly, base) is the element of an architectural structure which connects it to the ground, and transfers loads from the structure to the ground. Foundations are generally considered either shallow or deep.
It is advisable to know suitability of each types of foundation before deciding which best suit your design type and soil. We would discuss them briefly below: 
Types of Foundation and their Uses
The following are different types of foundations used in construction:
1. Shallow foundation Strip foundationPad or Individual footing foundationRaft or Mat foundation 2. Deep Foundation Pile foundation We would only focus on the shallow foundation in our discussion here; 
1a. Strip foundation
Strip footings are commonly found in load-bearing masonry construction, and act as a long strip that supports the weight of an entire wall. These are used where the building loads are carried by entire walls rather than isolated columns.
This type …