Roof drainage material estimator

Downspout Extension Calculator

Estimate downspout pipe sections, elbows, straps, extension length, waste, and rough material cost from downspout count, wall drop, outlet extension, and stock section length.

Units

Downspout runs Vertical wall drop (ft)Outlet extension length (ft)Stock section length (ft)Elbows per run Straps per run Waste / cuts (%)Cost per section ($)Cost per elbow ($)Cost per strap ($)

Sections to buy

Elbows to buy

Order length

Rough cost

Drainage capacity and outlet location matter

This calculator estimates downspout materials only. Roof area, rainfall intensity, gutter and outlet size, slope, splash blocks, buried drain capacity, daylighting, icing, and local stormwater rules can change the real layout.

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Gutter Calculator

Estimate gutter sections and downspout count first.

Soffit and Fascia Calculator

Estimate exterior trim along the same roof edge.

Roof Area Calculator

Check roof area before sizing drainage capacity.

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Downspout extension formula

Measured length = downspouts × (vertical drop + extension length)

Order length = measured length × (1 + waste %)

Sections = order length ÷ stock section length

Elbows = downspouts × elbows per run

Sample takeoff

Four downspouts with 10 ft wall drops, 6 ft outlet extensions, 10 ft stock sections, two elbows per run, and 10% waste need about eight sections and eight elbows.Four downspouts with 3 m wall drops, 1.8 m extensions, 3 m stock sections, two elbows per run, and 10% waste need about eight sections and eight elbows.

Downspout Extension Calculator FAQ

How do I calculate downspout extension material?+

Multiply each downspout by the vertical drop plus outlet extension, add waste, then divide by the stock section length. Add elbows and straps per run.Multiply each downspout by the vertical drop plus outlet extension, add waste, then divide by the stock section length. Add elbows and straps per run.

Does this size the downspout for rainfall?+

No. It counts material after the drainage layout is chosen. Roof area, rainfall, gutter profile, outlet size, and stormwater rules decide capacity.No. It counts material after the drainage layout is chosen. Roof area, rainfall, gutter profile, outlet size, and stormwater rules decide capacity.

Should I include buried drain pipe?+

Only include it if the same pipe material and section length apply. Buried drainage usually needs a separate layout, slope, cleanouts, and outlet check.Only include it if the same pipe material and section length apply. Buried drainage usually needs a separate layout, slope, cleanouts, and outlet check.