Basic principles for the estimation of wood requirements and roofing sheet requirements for building works

A roof is a covering over a building that protects the elements and the inhabitants from rainfall, sunshine, ice, snowfall and other adverse effects of weather etc.

A roof comprises of two sections: the wood or steel section skeleton and the roof covering itself that consists either of aluminum sheets, galvanized iron sheets, zinc or asbestos sheets. Aluminum sheets is most common in Nigeria today because of many desirable properties such that it does not corrode like steel sheets and does not break like asbestos. Common aluminum roofing sheet brands in Nigeria are the: Long span aluminum, Step-tile aluminum and Metcoppo aluminum roofing sheets. A roof can be simple (See Figure 1) or complex. The more complex a roof is, the more difficult it is to quantify (See Figure 2). The complexity of roof is usually affected by the shape of the building it is covering. Estimation of wood components of the roof is a daunting task especially for complex roofs. It is ideal to consult Professional Quantity Estimator for such estimation. This guide is basically for learning purposes for students and professionals.

Figure 1; Simple Roof

Figure 2; Complex Roof

The wood components of a roof include (See Figure 3):

Figure 3; Roof Truss

1. Wall plate: This part of the roof usually comes on top of the blocks or roof beams and transfers the weight of the entire roof skeleton to the blocks/roof beams. It usually comes in sizes of 75 mm x 100 mm. To get the number required for this part, calculate the total length of the walls/roof beams that would carry the wall plate and divide by the length of one of such woods as available in the market. The length is usually 3600 mm.
2. Facia board: Concrete facia is common in Nigeria recently. However, in situations where facia board applies, get the perimeter of the entire roof section, after incorporating a usual overhang of 600 mm or whatever is applicable. Facia board is usually made of 25 mm x 300 mm planks. To get the number required, divide the total perimeter by the length of one plank, usually 3600 mm.
3. Tie beam: This is a horizontal member that transfers the load of the roof truss to the wall plate. It usually spans the open spaces of the building, spaced at 1200 mm and spans across the shorter span of the building. It is usually made of 50 mm x 150 mm wood. To get the number required, first divide the total length it would span by the spacing to get the number required, then multiply the number required by the length of one. To get the number to buy in the market, divide the total length by the length of one, usually 3600 mm.
4. Struts: These are members placed diagonally and used to connect the tie beams to rafters. The calculation of this is a bit complex and may require proper analysis of truss. However, for simplicity, we can multiply the number of tie beams by 2 to get the number of struts required. Structs usually comes in sizes of 75 mm x 100 mm.
5. Rafter: The rafters are the woods that lie across the falling portion of the roof. It is necessary to determine the slant height of the roof by the use of Pythagoreans theorem. Just as tie beams, they are also spaced at 1200 mm because they are nailed to connect each other, i.e. tie beam to rafter to struct. They are usually made of 75 mm x 100 mm wood. Having gotten the length of one and the number required, multiply the length by the number required to get the total length required. Then divide the total length by the length of usually 3600 mm as sold in the market.
6. Purlins: These are the part of the roof skeleton that run across the rafters and on which the roof sheets are nailed. Purlins come in sizes of 50 mm x 50 mm. They are usually spaced based on design and type of roofing sheets. However, the spacing of 900 mm is common. To get the number of purlins required, add the total slant length of the rafter also incorporating the overhangs and divide by the spacing. Get the total length of one purlin and multiply by this number to get the total length of all purlins required. Then divide this total length by the length of one in the market, usually 3600 mm to get the number to buy in the market.
7. Kingpost: This is the piece of wood and vertical member that stands at the middle of two rafters falling on opposite sides. It usually connects the rafter joints to the tie beam. The height is usually same as the vertical height of the roof and the numbers are same as the number of rafters/tie beams. It usually comes in sizes of 50 mm x 150 mm.
8. Queen Post: This is similar to the king post because it is also a vertical member. However, it connects other parts of the rafter to the tie beams. It is spaced at 1200 mm and comes in sizes of 50 mm x 150 mm.

Estimation for roof sheets requires the following:

1. Effective area coverage (m²): The estimation of the area coverage of roof is easy for simple roofs and difficult for complex roofs. For simple gable roof for instance, the height of the roof must be known because slant height is usually higher than horizontal height. With the height, the rafter length can be determined using the Pythagorean theorem as follows: Rafter length (slant height) = √ (1/2 span of roof)² + (height of rise)^2. For complex roofs, we can do a quick estimate by dividing the roof into series of horizontal squares, triangles or rectangles, summing the areas and adding 10% of the total sum of areas obtained to carter for the height of the roof. This is not an accurate measure because the higher the roof, the more is the number of plates required and the more complex the roof, the higher the number of offcuts required which also increases the number of sheets/plates required.

2. Number of sheets in a bundle (m²): Zinc requirement is usually specified by zinc gauge. The material is available in plain sheet form or corrugated sheet. The common market sizes are (a) 2134 x 914 mm and (b) 2438 x 914 mm. The estimator allows for 0.2 m end laps and 0.1 m side lap which gives an effective coverage area of 1.57 and 1.82 m² Twenty (20) sheets are usually in one bundle. With this knowledge and knowledge of area of roof in question, it is possible to determine the quantity required. Long span aluminum roofing sheet are supplied in any length required up to 20 m. The sheets can be supplied in widths/rolls of about 600 mm to 1300 mm and thickness (gauge) of 0.45 m to 0.9 m but the commonest are 0.45 m and 0.55 m. The effective coverage of the aluminum roof sheets depends on the side laps which varies according the width of the roof sheet.

3. Packets of nail required: Nails are commonly used for zinc roofs and asbestos roof. Galvanized roofing nails are usually sold in packets. A packet of galvanized roofing nail contains approximately 500 pieces of nails. About 6 number of galvanized roofing nails and washers are used per sheet.
4. Bolt requirement: Bolts are commonly used for aluminum roof sheets. 3 bolts per square metre is ideal. Knowing the total square metre, one can determine the number of bolts required.
5. Ridge capping/ Flashing apron: These are accessories used in aluminum roofing sheets. They are measured in linear metres with a standard length of 3.0 m long. Effective length coverage depends on end laps given, usually 150 m is allowed.
6. Waste: Usually 5% for nails and washers and 2.5 % for roof sheets itself.