About this calculator
A full material list for a gable roof covered in IBR sheeting: trusses, purlins, sheets cut to length, screws, flashings and wall plates.
How it’s calculated
Trusses go at no more than 1,200 mm centres under metal or fibre-cement sheeting (SANS 10400-L table 4), so trusses = building length ÷ 1.2 m, rounded up, plus one. The same table limits these deemed-to-satisfy Howe trusses to pitches of 15° to 30° and clear spans up to 8 m, and sets the bays and bottom chord size.
Each sheet is cut to the rafter length: (half the span + eaves overhang) ÷ cos(pitch). Clotan’s formula gives the sheets per side: (building length + both gable overhangs − 70 mm) ÷ 0.686 m, because an IBR sheet covers 686 mm. Purlins of 50 × 76 mm run at up to 1,200 mm centres. Clotan fixes the sheets through the crests of alternate flutes, which are 171.5 mm apart, so each 686 mm sheet takes two screws at every purlin, plus one at the end of each row.
SANS 10400-L table 1 gives box-rib sheeting such as IBR a minimum slope of 11°; 5° is allowed only on a category 1 building with no hips or valleys and sealed end laps of at least 250 mm (4.2.2.1).
Worked example
With these inputs:
- Building length (along the ridge)
- 10 m
- Clear span (wall plate to wall plate)
- 8 m
- Roof pitch (degrees)
- 17.5
- Eaves overhang (horizontal)
- 500 mm
- Gable overhang (each end)
- 300 mm
the result is:
- IBR sheets
- 32 sheets
- Sheets per side
- 16
- Rafter (slope) length incl. eaves
- 4.72 m
- Trusses at 1,200 mm centres
- 10 (6-bay Howe, bottom chord 38 × 152 mm)
- Purlin rows per side
- 5
Tips from the trade
- Lay sheets so the side laps face away from the prevailing wind and rain.
- Check that the truss supplier’s design matches your covering, pitch and span before ordering sheets.
- Order sheets cut to length to avoid end laps where you can.
Common questions
How many IBR sheets for a 10 m long roof?
With 300 mm gable overhangs at each end: (10.6 − 0.07) ÷ 0.686 = 15.4, so 16 sheets per side and 32 for a gable roof.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.