The measurement seen from above, not along the slope.
Thirty to forty-five degrees is usual on a house; a lean-to is often much shallower.
The projection beyond the wall, at the eaves and at the verge.
How this calculation works
Why the footprint is always too small
The plan area is the shadow the roof casts, and the roof itself is the slope. At thirty degrees the real area is about fifteen per cent more; at forty-five it is over forty per cent more. Ordering off the footprint is the single most common way a roofing order comes up short.
What this does not include
Dormers, valleys, hips and abutments. Those add area and they add considerably more labour than area — a hipped roof takes longer per square metre than a plain gable, and the cuts waste material.
Frequently asked questions
Is this the area for tiles or for insulation?
Both — it is the geometry of the surface. What you then need per square metre depends on the product, and the tile figure also depends on the batten gauge.
How do I handle a hipped roof?
Put in four pitched sides. The total area comes out about right; the labour and the waste on the hips do not, so price those separately.
What if I do not know the pitch?
Measure the rise over a known horizontal run and take the angle from that, or take it off a rafter. Guessing the pitch is guessing the order.
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