How much overlap for underlayment?

Typically 4 in on horizontal laps and 6 in on end laps.

Synthetic is lighter and tougher; #30 felt is cheaper.

Do I need ice and water shield?

Many codes require it at eaves in cold climates and in valleys.

Synthetic or felt underlayment?

Synthetic is lighter, tougher and safer to walk on.

How long can it stay exposed?

From weeks to months depending on the product.

Can I walk on synthetic underlayment?

Most have a non-slip surface, but use roof jacks and harnesses on steep pitches.

Roof underlayment calculator

Roof area, roll size and overlap: you get usable area per roll, rolls with waste and boxes of cap nails.

Purchase quantity

120 m²

Net measurement: 120 m² · Waste 5 %

Underlayment rolls2 roll
Usable area per roll70 m²
Cap nails or staples (10 per m²)2 box

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Calculate labour, margin & GST

How it's calculated (visible formula)

usable per roll = (width − overlap) × length; rolls = ceil(area × (1 + waste) ÷ usable)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Work out true roof area.
Enter roll width, length and overlap.
Work out usable area per roll.
Divide area with waste by usable area.
Add cap nails or staples and tape for details.

Worked example: 1,290 sq ft with 4.9 × 164 ft rolls (1.5 × 50 m), 4 in overlap

Usable: (1.5 − 0.1) × 50 = 70 m² (753 sq ft)
126 m² ÷ 70

→ 2 rolls and 2 boxes of cap nails.

Common mistakes

Ignoring overlap.
Laying from ridge down (start at the eave).
Not lapping at valleys and walls.

Pro tips

Lay from eave to ridge so water sheds over each lap.
Reinforce valleys and walls with an extra strip.
Don't leave underlayment exposed longer than allowed.
Turn it up walls and chimneys 4–6 in.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & GST

FAQs

How much overlap for underlayment?: Typically 4 in on horizontal laps and 6 in on end laps.
Synthetic or felt?: Synthetic is lighter and tougher; #30 felt is cheaper.
Do I need ice and water shield?: Many codes require it at eaves in cold climates and in valleys.
Synthetic or felt underlayment?: Synthetic is lighter, tougher and safer to walk on.
How long can it stay exposed?: From weeks to months depending on the product.
Can I walk on synthetic underlayment?: Most have a non-slip surface, but use roof jacks and harnesses on steep pitches.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.