Estimating
Specification
Coverland Taunus tiles at 22,5° roof pitch.
Technical data:
Batten centre = 320mm
Linear cover per tile = 300mm
Tiles per m2 = 10,42
In general three methods are in use to estimate roof tile quantities from a plan:
a) Rafter/eaves length estimating method
b) Trigonometrical method
c) Roof area method
We recommend method a) as follows:
Rafter/eaves length estimating method
This method is to be preferred as it indicates the quantity of tiles that will be used in practice to comply with the minimum laying and fixing specifications. It takes into consideration the extra courses and rows of tiles that will be necessitated by rafter and eaves lengths that are not an exact multiple of the required rafter gauge or tile linear cover respectively. (Refer to Estimating tables.)
Note:
a) The only dimension which has to be measured and scaled is the rafter length.
b) The estimating tables (rafter length) include for a 3mm per course tolerance in the gauge.
c) The estimating tables (eaves length) include for a full tile with a left hand lock on the left hand verge (330mm). These tables assume the tile being laid at the mid lock shuffle position.
d) No allowance has been made for wastage. A 3% wastage factor is the building industry norm.
Example
1. Divide the roof plan into regular sections,
i.e. Rectangles A and B.
2. In each section measure the rafter length from the elevation of that section (roof apex to outside edge of fascia board.) Using the estimating tables, note the number of courses
for that rafter length, i.e. Table 1 (100mm headlap at 320mm batten centres.)
3. Read off the eaves length (including the verge overhangs from the plan) in each section and using the estimating tables, note the number of tiles required for this length,
i.e. Table 2 (300mm linear cover). Section A 20m = 66 tiles Section B 6m = 20 tiles
4. Multiply the number of courses obtained in 2 by the number of tiles along the eaves obtained in 3 to obtain the nett quantity of tiles for that side of the roof. Multiply by two to obtain the nett quantity for both sides of each section,
i.e. Section A = 21 x 66 x 2 = 2 772
Section B = 21 x 20 x 2 = 840
5. Calculate the extra tiles at hips and valleys by multiplying each hip and/or valley by the number of courses involved on the rafter length, then by 2:
6 Hips = 6 x 21 x 2 = 252
2 Valleys = 2 x 21 x 2 = 84
Nett quantity of tiles = 2 772 + 840 + 252 + 84 = 3 948