How Much Underlayment and Ice Shield Do You Need?
Underlayment: quantity from roof squares
Start from the roof area the roof pitch calculator gives you — footprint times slope factor, plus waste. Then account for the lap:
| Product | Roll marked as | Realistic coverage after lap |
|---|---|---|
| Synthetic underlayment, wide roll | 10 squares | ~9–9.5 squares |
| #15 felt | 4 squares | ~3.5–4 squares |
| #30 felt | 2 squares | ~1.8–2 squares |
| Ice and water shield | 1.5–2 squares | Sold and used by linear foot of eave |
The stated coverage on most rolls is the gross area of material. Side and end laps — typically 2 to 6 in on the side and 6 in on the end, marked on the roll — consume part of it. Buying one roll over the calculation is the normal way to handle both the lap and the offcuts at hips and valleys.
Pitch changes the requirement
| Pitch | Underlayment |
|---|---|
| 4/12 and steeper | Single layer, per manufacturer and code |
| 2/12 to 4/12 | Doubled underlayment typically required for asphalt shingles |
| Below 2/12 | Shingles not appropriate — membrane roofing system |
That doubling is easy to forget in an estimate and it is not a small item: a low-slope roof needs roughly twice the underlayment of the same area at 6/12.
Ice and water shield is a linear measurement
Self-adhered ice barrier goes where water can back up, not across the whole roof (unless a low-slope detail calls for it). Measure:
- Eaves: the full linear length of every eave, in a band wide enough to extend from the edge to a point specified by code — commonly at least 24 in inside the interior wall line, which often means two courses of a 36 in wide roll on a low-pitch roof with a deep overhang.
- Valleys: the length of each valley, centred on it.
- Around penetrations and at wall intersections, per the manufacturer's detail.
Requirements are code-driven and climate-driven, so the width and the number of courses at the eave are set locally rather than by a rule of thumb.
Everything else that is not shingles
| Item | Measured by | Rough figure |
|---|---|---|
| Drip edge | Linear feet of eaves and rakes | Sold in 10 ft lengths; allow overlap at joints |
| Starter strip | Linear feet of eaves, and rakes if specified | |
| Hip and ridge cap | Linear feet of hips and ridges | See the hip vs gable guide |
| Valley metal | Linear feet of valley | Plus overlap at joints |
| Step flashing | Per course along a wall | One piece per shingle course |
| Pipe boots | Each | Count the penetrations |
| Ridge vent | Linear feet of ridge | Balance with intake at the soffit |
| Roofing nails | Pounds | Roughly 2.5 lb per square, more for high-wind patterns |
A sensible ordering sequence
- Get roof area and squares from the roof pitch calculator, waste included.
- Underlayment: squares ÷ realistic roll coverage, rounded up, plus one roll. Double it below 4/12.
- Walk the roof edges with a tape and write down linear feet of eave, rake, ridge, hip and valley. Every accessory above comes from those five numbers.
- Count penetrations and wall intersections for boots and step flashing.
- Check the shingle manufacturer's installation instructions and your local code before finalising — the warranty depends on the specified underlayment and ice barrier, not on what is cheapest.
Convert between rise-over-run and degrees, then turn a footprint into real roof area — with the slope factor, rafter length, squares and shingle bundles you need to order.
Frequently asked questions
How many rolls of underlayment do I need?
Divide your roof area in squares by the roll coverage after laps — a synthetic roll marked 10 squares realistically covers about 9 — then round up and add one roll for offcuts at hips and valleys.
Do I need two layers of underlayment?
Between 2/12 and 4/12 pitch, doubled underlayment is typically required for asphalt shingles. At 4/12 and steeper a single layer is normal. Below 2/12, shingles are not appropriate at all.
How much ice and water shield do I need?
It is a linear measurement, not an area one: measure the full length of every eave and valley. The required width at the eave is set by local code and climate and often needs two courses of a 36 inch roll where overhangs are deep.
- How to Measure Roof Pitch (Safely, From the Attic)
- How Many Shingles Do I Need? Squares, Bundles and the Slope Factor
- Hip Roof vs Gable Roof: Calculating the Area
- What Is a Good Roof Pitch?