Gutter guides

Sizing. This Old House's method combines three factors: the roof drainage area feeding each gutter section, a pitch factor (about 1.0 for a flat roof up to 1.3 for a 12-in-12), and your regional peak rainfall intensity in inches per hour. A 5-inch K-style gutter covers roughly 5,520 square feet of drainage area; 6-inch takes it to about 7,960.

Downspouts. The rule of thumb is about one square inch of downspout cross-section per 100 square feet of roof. A 2x3 rectangular downspout serves roughly 600 square feet, a 3x4 about 1,200, a 3-inch round about 706 and a 4-inch round about 1,255. Chronic overflow at one corner is very often a downspout count problem.

Measuring. Walk the perimeter and measure every eave that will carry a gutter. That is your linear footage, and it is the number every quote should be divided by.

Hiring. Give every bidder the same written scope: footage, material and gauge, gutter size, profile, number and size of downspouts, discharge locations, any fascia repair, guard type if any, and haul-away. Confirm licensing and insurance through your state or local licensing authority — requirements differ by state and by city, so check your own jurisdiction rather than trusting a contractor's summary of the rules.

How to choose

Start with drainage area, not price

An undersized system installed beautifully still overflows every storm.

Count downspouts deliberately

Adding a downspout is one of the cheapest capacity upgrades available.

Standardize the scope before bidding

Identical written scope to every bidder is the only way to compare prices honestly.

Verify insurance with the carrier

Ask for the certificate and call the insurer. Ladder work on your property is a real liability question.

Ask where the water goes

Discharge placement is part of the design. Dumping into a splash block next to the foundation is not a plan.

Get the payment schedule in writing

Large up-front deposits on a one-day job are a warning sign.