A building comprises of different component ranging from block work to roofing. The joining of the components is what makes up a complete building. In this write up, we will concentrate on roofing materials as a part of building material estimation.

Over time, it has been a big problem to many in the building industry to foreknow what it will take from them when the building comes to the roofing stage. This is because they hardly determine the total material that will complete the work since many don’t like employing the service of a quantity surveyor or an engineer who suppose to carry out the building material estimation.


Many engineers are not good in this area. But as a sit engineer, you should be able to give the client such estimate. The first thing to look at in a roof is the frame work otherwise called cankers. It is made up of wood that are joined with the aid of nails. The roofing woods are of difference sizes depending on the designer specification. The most commonly use sizes of roofing woods are:

  1. 2x6use for kingpost
  2. 2 x 4 (rafter) use for strut
  3. 3 x 4 wall plate use for tie beam
  4. 2 x 3 (purlin) use for bracing gabble slop to place the roofing sheet
  5. 2 x 2 (purlin) use for noggin for ceiling

These woods come in different sizes in terms of their length depending on the area of sourcing. The commonly use length are 12fts, 16fts and 18fts. The type length of available in the area will help to determine the quantity that will be required.

Another important parameter is the length and breadth of the building to be roofed. The height of the roof and the shape of the building is also a factor that needs to be considered in estimating the roofing material.

NOTE:  The plan designer always give some specification for carrying out the roofing. Some of the specification includes:

  1. The kingpost spacing
  2. The roofing sheet purlin spacing
  3. The eave length
  4. The height of the kingpost.

Once the above parameters are known the calculation of the material can be easily carried out.


Calculate the needed material in terms of the roofing wood that will cover a rectangular building measuring 23630mm x 11030mm with a kingpost height of 3600mm, 600mm eave, 1200mm c/c kingpost spacing, 900mm c/c roofing sheet purlin spacing, 1200mm c/c tie beam spacing and 1200mm c/c rafter spacing. See the figure below.


From the dimensions given in the question,

Building length = 23630mm

Roofing length = 600 + 23630 + 600 = 24830mm

Building breadth = 11030mm

Roofing breadth = 600 + 11030 + 600 = 12230mm.

The length of the wood to be used is 12fts.

Apart from the major length and breadth, there are other dimensions from partition in the building as indicated in floor plan bellow on which the wall plates are laid before the tie beam comes on it. On this note, all the wall that will carry the wall plate dimensions must be calculated.

The calculation there follows thus:

Out length = 23630 x 2 = 47260mm

Inner length = 21230 x 2 = 42460mm

Outer breadth = 11030 x 2 =22060mm

Inner breadth = 8030 x 2 =16060mm

Long Partitions (2) length = 8030 x 2 =16060mm

Short partition (1) length = 3000 x 1 = 3000mm

Total walls parameter =     146900mm

Length of the wood for wall plate) = 3600, therefor, the number of wall plate needed = 146900/3600 =40.8 pieces approximately 41 pieces


To get the kingpost position, divide the outer breadth by 2. That is: 11030/2 = 5515mm. this value will be subtracted from both side of the length to get the position of the first kingpost in either side. After the subtraction, the remaining value is the ridge length which is equal to 23630 – (5515 x2) =12600mm.

To get the number of kingposts, divide then ridge (gabble) length by the gabble spacing value = 12600/1200 = 10.5 approximately 11 kingposts. The length of the tie beam to which the kingpost will be fixed is 11030mm and the length of the wood is 3600mm. Therefore, the number of wood that will cover this length will be 11030 / 3600 = 3 length. To get the number of woods; this value will be multiplied by the number of kingposts. Other shorter tie beam in the triangular part will also be calculated. Therefore, the number of wood for the tie beam will be 3 x 11 = 33 + 13 for the triangular sides plus the kingpost now give us 11 +33 + 13  = 55 rounded up to 60 pieces of 2 x 6. see the figure below.



Applying Pythagoras theorem let the hypotenuse side be X. therefore,

 X2 = 61152 + 30002. This implies that,

X2 = 37393225 + 900000.

 X2 = 46393225

X = √46393225

X = 6811.25 ≈ 6812mm. But the spacing of the purlin is 900. To get the number of wood that cover the span thus: 6812/900 = 7.5 ≈ 8pieces of purlin will cover the span from the ridge to the eave. The building is rectangular in shape which now comprises of two trapeziums and two triangles from the roof plan. The lengths of the ridge which form the upper part of the trapezium, increases as it move down to the eave. So also the triangle. By calculating this varying length, each trapezium take a total of 70 2 x 3 purlin and each triangle take a total of 30 2 x 3. This gives a total of 200 pieces for the entire roof. Below is the summary of the wood calculation.

  1. 2 x 6 hard wood = 60 pieces
  2. 3 x 4 hard wood = 45 pieces
  3. 2 x 3 hard wood = 200 pieces
  4. 2 x 4 hard wood = 50 pieces
  5. 2 x 2 soft wood = 400 pieces


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