The example: a simple house
All dimensions below are assumed, so you can check every step. The house is 8.0 m wide and 6.0 m deep, the eaves height is 5.5 m and the ridge sits 2.0 m above the eaves (pitched roof, gable ends on the sides).
Step 1: gross area per façade
gross front or rear facade = width x height = 8.0 x 5.5 = 44.0 m²
gross side facade = depth x eaves height + 0.5 x depth x gable height
= 6.0 x 5.5 + 0.5 x 6.0 x 2.0 = 33.0 + 6.0 = 39.0 m²
total gross = 2 x 44.0 + 2 x 39.0 = 166.0 m²
Step 2: deduct the openings
window 1.2 x 1.5 = 1.80 m² window 0.9 x 1.2 = 1.08 m² door 1.0 x 2.3 = 2.30 m² patio door 2.4 x 2.3 = 5.52 m²
The front has 3 windows and 1 door (5.40 + 2.30 = 7.70 m²), the rear has 2 windows and the patio door (3.60 + 5.52 = 9.12 m²), the left side has 2 small windows (2.16 m²) and the right side has 1 small window (1.08 m²). That is 20.06 m² of openings, so 166.0 minus 20.06 = 145.94 m².
Step 3: add the reveals
If the cladding returns into the reveal, that adds area. It depends on the detail, so check the drawing and the specification. Assumption: reveal depth 0.15 m, cladding on the jambs and the head; the bottom is a sill or threshold and is not counted here.
reveal = (2 x height + width) x depth window 1.2 x 1.5: (3.0 + 1.2) x 0.15 = 0.63 m² door 1.0 x 2.3 (no bottom): (4.6 + 1.0) x 0.15 = 0.84 m² patio door 2.4 x 2.3: (4.6 + 2.4) x 0.15 = 1.05 m² window 0.9 x 1.2: (2.4 + 0.9) x 0.15 = 0.495 m²
Step 4: net area to clad
| Façade | Gross m² | Openings m² | After openings m² | Reveals m² | To clad m² |
|---|---|---|---|---|---|
| Front | 44.00 | 7.70 | 36.30 | 2.73 | 39.03 |
| Rear | 44.00 | 9.12 | 34.88 | 2.31 | 37.19 |
| Left side | 39.00 | 2.16 | 36.84 | 0.99 | 37.83 |
| Right side | 39.00 | 1.08 | 37.92 | 0.50 | 38.42 |
| Total | 166.00 | 20.06 | 145.94 | 6.53 | 152.47 |
From net area to order quantity
Now comes the part a drawing does not show. Assumptions: a board with a covering width of 130 mm in stock lengths of 4.8 m, battens at 600 mm centres, two screws per board and batten crossing, and a cutting loss of 8 per cent on boards and 5 per cent on battens. Those two percentages are examples. Cutting loss depends on product format and on the layout of the façade, and it must come from your supplier or your own experience.
boards: 152.47 / 0.130 = 1,172.8 m, plus 8% = 1,266.7 m
1,266.7 / 4.8 = 263.9, so 264 boards
battens: 145.94 / 0.600 = 243.2 m, plus 5% = 255.4 m
255.4 / 3.0 = 85.1, so 86 lengths of 3.0 m
screws: (1 / 0.130) x (1 / 0.600) x 2 = 25.6 per m²
25.6 x 152.47 = 3,910, plus 5% = 4,105
4,105 / 500 per box = 8.2, so 9 boxes
The same 152.47 m² therefore becomes 264 boards, 86 batten lengths and 9 boxes of screws. Extra battens around the openings and corner profiles are not in here; work those out per opening and per corner. Always round up to what you can order: whole lengths, whole packs, whole boxes.
Why you calculate per façade
Calculate per façade and keep the intermediate steps. If the architect moves a window, you change one line instead of redoing the whole total. Note for each façade which revision of the drawing you took off from, and which assumptions you used for the reveal depth. That way a colleague can follow your calculation without you being there.
Where it often goes wrong
- Forgetting the reveals, or counting them when the detail names a different material.
- Calculating the gable triangle as a rectangle.
- Counting corner profiles and sills in m² when you order them in running metres.
- No fixed rule for small openings. Agree one and write it down.
- Not comparing dimensions in the specification and on the drawing. If they contradict each other, read Specification and drawing contradict each other.
How Modou helps
Modou takes the net areas and openings from the IFC model and calculates per build-up from your own library through to boards, battens, screws and rounding to stock lengths. For every line you see how it was calculated and which assumptions it rests on, and you approve each assumption. Read how it works or start with your project.