Underlayment Calculator

Calculate how many rolls of underlayment you need for roofing, flooring, or waterproofing projects. Works for felt paper, synthetic roofing underlayment, foam underlayment, and vapor barriers.

🧱 Construction🏗️ Rolls = Area / Coverage per Roll x (1 + Waste%)

Area length (ft)

Area width (ft)

Underlayment type

Waste factor (%)

Please enter valid values.

Formula & Reference

Variable	Formula	Units
Underlayment Calculator	Rolls = Area / Coverage per Roll x (1 + Waste%)	rolls

Step-by-Step Examples

Example 1

Roofing Felt (30 lb)

30x25 ft roof area (750 sq ft), 10% waste.

Rolls = 750 x 1.10 / 200 = 4.13 -> 5 rolls
30-lb felt is preferred over 15-lb for most residential roofing
2 squares per roll (1 square = 100 sq ft)

✓ 5 rolls of 30-lb felt

Example 2

Synthetic Roofing

1,200 sq ft roof, synthetic underlayment, 10% waste.

Rolls = 1,200 x 1.10 / 1,000 = 1.32 -> 2 rolls
Synthetic is lighter, stronger, and UV-resistant
More expensive but fewer seams

✓ 2 rolls synthetic

Example 3

Foam Flooring Underlayment

400 sq ft hardwood floor, 10% waste.

Rolls = 400 x 1.10 / 100 = 4.4 -> 5 rolls
Foam underlayment under floating hardwood or LVP
1/8-inch foam is standard; thicker for concrete subfloors

✓ 5 rolls foam underlayment

Real-World Applications

🏠

Roofing

Calculate felt paper or synthetic underlayment for new roofs or re-roofing.

🌳

Flooring Installers

Determine foam or cork underlayment rolls for floating floor installations.

🏗️

Builders

Include vapor barriers and waterproofing membranes in material estimates.

💰

Cost Planning

Underlayment is often overlooked in project budgets but adds up quickly.

Common Mistakes to Avoid

⚠️

Not accounting for overlap in roofing underlayment

Roofing felt is installed with 6-inch horizontal overlaps and 12-inch at ridges. This overlap reduces the effective coverage. Most product labels account for this in the stated coverage per roll.

⚠️

Using thin foam under tile

Do NOT use foam underlayment under tile. Tile requires a rigid substrate. Use uncoupling membrane (Schluter DITRA) or cement backer board.

⚠️

Skipping underlayment on roofs

Never omit roofing underlayment. It's your secondary water barrier and is required by building code and all shingle manufacturers' warranties.

Frequently Asked Questions

What is roofing underlayment for? ▾

The secondary layer of moisture protection under shingles. If shingles fail or blow off, underlayment protects the sheathing. Required by code and all shingle manufacturer warranties.

15-lb vs 30-lb roofing felt? ▾

15-lb felt covers twice the area per roll but tears more easily and is less UV-resistant if exposed. 30-lb is standard for most residential projects and provides better protection during construction.

Synthetic roofing underlayment vs felt? ▾

Synthetic (polyethylene or polypropylene) is lighter, stronger, tear-resistant, UV-resistant for longer exposure, and can stay exposed longer during construction. It costs more upfront but saves labor from lighter weight.

What underlayment for hardwood floors? ▾

Thin foam (1/8 to 1/4 inch) for floating hardwood or LVP over wood subfloors. On concrete, use a moisture barrier or combination underlayment. Glue-down and nail-down hardwood don't use underlayment.

Does LVP need underlayment? ▾

Many LVP products have underlayment pre-attached. If yours doesn't, or for concrete subfloors, a thin 1/8-inch foam with moisture barrier is recommended. Don't double up underlayment with pre-attached products.