What are gutters and why do they matter?

Gutters are channels installed along the eaves (lower edges) of a roof to collect rainwater and direct it away from the building's foundation. A properly sized gutter system prevents water damage to walls, foundations, and landscaping. Undersized gutters overflow during heavy rain, while oversized ones are an unnecessary expense.

How is gutter size calculated?

Gutter sizing is based on the volume of water the gutter needs to handle. This depends on two factors: the roof area (which collects the rain) and the rainfall intensity in your region.

Each gutter size has a maximum drainage area rated at a reference rainfall of 1 inch/hr (25.4 mm/hr), based on the SMACNA standard. For example, a 5-inch K-style gutter can drain up to 513 m² at this reference intensity. For higher rainfall, the effective drainage area decreases proportionally: at 50 mm/hr (about 2x the reference), the max area is halved. The calculator compares your roof area against these effective limits to find the smallest suitable gutter.

Rainfall intensity by region

Rainfall intensity varies greatly by geography and climate. Selecting the right category is important for accurate gutter sizing. Here is a general guide:

Category	Intensity	Typical regions
Light (25 mm/hr)	≤ 25 mm/hr	Arid and semi-arid zones: inland Spain, southern Italy, North Africa, Middle East, steppe regions, rain shadow areas
Moderate (50 mm/hr)	25 - 50 mm/hr	Most of Western and Central Europe: UK, France, Germany, Netherlands, Scandinavia, northern US, Canada, temperate coastal areas
Heavy (100 mm/hr)	50 - 100 mm/hr	Mountain regions, subtropical areas: Alps, Carpathians, southeastern US, Japan, southern China, parts of Australia, monsoon-adjacent zones
Extreme (150 mm/hr)	> 100 mm/hr	Tropical and equatorial zones: Southeast Asia, Caribbean, Central America, West Africa, India (monsoon season), Florida, Gulf Coast

K-style vs Half-round gutters

The two most common gutter profiles are K-style and half-round. They differ in shape, capacity, and aesthetics:

• K-style — Flat back, decorative front profile. Higher capacity for the same width. The most popular choice for modern residential buildings. Handles more water and is easier to mount flush against the fascia board.
• Half-round — Classic semicircular profile. Lower capacity but easier to clean due to the smooth shape. Often chosen for historic buildings, Mediterranean-style homes, or for aesthetic reasons.

How are downspouts calculated?

Downspouts (also called leaders or drain pipes) carry water from the gutter down to ground level. The number of downspouts depends on two rules:

• By roof area: one downspout for every 55 m² (approx. 600 sq ft) of roof area. Larger roofs collect more water and need more drainage points.
• By gutter length: one downspout for every 12 meters (approx. 40 ft) of gutter run. Long gutter sections without a downspout will overflow at the far end.

Understanding the materials list

The materials list estimates all the components needed for installation. Gutter sections are typically sold in standard lengths — 2 meters in metric regions or 10 feet in imperial regions. End caps close off the open ends of the gutter — the number depends on the roof type: a shed roof (single slope) has 2, a gable roof has 4, and a hip roof has 0 because the gutter forms a continuous loop. You can also select "Custom" to enter the exact number of end caps for non-standard configurations. Corner pieces (inside and outside) connect sections at roof corners. Hangers are brackets that attach the gutter to the fascia, spaced every 60 cm (24 in) for stability. Each downspout requires an outlet (the hole cut into the gutter), two elbows (one at the top to angle away from the eave, one at the bottom to direct water away from the wall), and pipe sections to reach from the gutter to the ground.

Gutter maintenance tips

Clean gutters at least twice a year — in late autumn after leaves fall, and in spring. Check for sagging sections which indicate broken hangers. Ensure downspouts drain at least 1 meter (3 ft) away from the foundation. In cold climates, consider installing gutter guards to prevent ice dam formation. Aluminum and vinyl gutters are low-maintenance, while steel gutters should be checked periodically for rust.