Gutter and downspout sizing and installation for exterior roof drainage

Gutters, downspouts, and where they fit

A gutter is the trough hung at the eave that catches water running off the roof slope. A downspout, also called a leader or conductor, is the vertical pipe that takes what the gutter collects and carries it down and away from the wall and the foundation. Together they are the exterior way to drain a roof, where the water goes off the edge and outside the building line.

That is the difference from interior roof drains and scuppers, which pull water through the roof and down pipes inside the building. The companion guide on roof drainage covers that interior side, the primary drains, the overflow, and the rain load. The two approaches solve the same problem from opposite directions. Interior drainage is the norm on a large low-slope roof ringed by a parapet. Exterior gutters are the norm on a steep-slope roof and on smaller low-slope buildings with an open eave, and they show up as a supplement even where interior drains carry the main flow.

The job of the system is plain: get the water off the roof fast enough that it never stands in the trough or backs up over the edge, and put it down on the ground far enough from the building that it does not undermine the foundation. Everything else in this guide is how you size and hang it so it does that for twenty years instead of two.

How do you size a gutter?

You size a gutter to two numbers: the roof area that drains into it and the design rainfall intensity for the location. Get the area and the rainfall rate, find the runoff flow, then pick the smallest gutter profile and size that carries that flow at the slope you can build. SMACNA publishes the working charts for this in its Architectural Sheet Metal Manual, and the plumbing code carries gutter sizing tables too.

Rainfall intensity is where the two main sources differ, so know which one your spec calls for. The International Plumbing Code sizes from a 100-year, 1-hour rainfall rate in inches per hour, read off the code's rainfall maps for the location. SMACNA's gutter charts are built on a shorter, sharper 100-year, 5-minute storm, which is the burst that actually overruns a gutter. The 5-minute rate is higher than the 1-hour rate, so the basis you use changes the answer. Both trace back to NOAA rainfall data for the site, which is the same design rainfall the interior roof-drainage sizing runs on. Use the rate your local code and the project spec name, not a number from another region.

Run the flow first, then read the chart. The runoff reaching the gutter is the roof area times the rainfall rate times a conversion constant, and SMACNA's table then gives the maximum roof area a given gutter size and slope can handle at that rainfall. Pick the size off the table for your rate, do not eyeball it from a catalog photo.

Q

Runoff flow reaching the gutter, in gallons per minute

A

Roof area draining to the gutter, in square feet, projected horizontally

i

Design rainfall intensity, in inches per hour, from the code map or NOAA for the site

Getting the drained area right

The area that counts is the horizontal projection of the roof draining to that gutter, not the slope length. A pitched roof catches rain on its footprint, so a 40 ft by 30 ft roof plane feeds the same water whether it is a 4:12 or a 9:12, and you use the 40 by 30 footprint. Crews who measure along the rafter overstate the area and oversize the gutter, which is the harmless error. The costly one runs the other way.

Add the walls that drain onto the roof. A higher wall above a lower roof throws its runoff onto that roof during a wind-driven rain, and SMACNA accounts for it by adding a share of the vertical wall area to the roof area, commonly half the largest adjoining wall for a single wall. Skip that on a building with a tall face above a low canopy and the canopy gutter runs over in the first hard storm.

Split the roof at the ridges and valleys into the areas that actually run to each gutter, and size each gutter run for the area feeding it. One long gutter draining two roof planes carries both. A valley that dumps into a short gutter section concentrates flow there, and that section needs a downspout close by or it overruns while the rest of the run sits half empty.

Field example: 5 in K-style on a 1,400 ft² roof

Take a roof plane of 1,400 ft² projected area in a region with a design rainfall of 4 in/hr. The runoff flow is 1,400 times 4 times 0.0104, about 58 gallons per minute reaching the gutter. That is the water the gutter and its downspouts have to clear.

A 5 in K-style aluminum gutter at a workable slope carries that on a single run when the downspouts keep it emptied, and at 4 in/hr a 5 in K-style covers roughly 1,400 ft² of drained area on the SMACNA basis. Push the rainfall to 7 in/hr, a Gulf Coast burst, and the same gutter covers far less area, so the roof either moves up to a 6 in gutter or gets more downspouts and shorter runs. Same roof, different storm, different answer.

Now the downspouts. At one square inch of downspout for every 100 ft² of roof at 1 in/hr, this roof at 4 in/hr needs about 56 in² of downspout. A 3 in by 4 in downspout is 12 in², so call it five downspouts, or fewer if you go to a larger leader. The gutter size and the downspout count are one decision, not two. A big gutter with too few downspouts still backs up.

How many downspouts do you need?

Plan at least two downspouts on any real run and add one for roughly every 600 ft² of roof a 2 in by 3 in leader serves, or every 1,200 ft² for a 3 in by 4 in, both figured at about 1 in of rain in an hour. Heavier rainfall scales that down. A common cross-check is one square inch of downspout area for every 100 ft² of roof at a 1 in/hr design rate, multiplied up by the actual rainfall intensity.

Spacing along the gutter matters as much as the count. Keep downspouts no more than about 35 to 40 ft apart, and let no single downspout drain more than roughly 50 ft of gutter length. Water in a sloped gutter runs to the low end, so a downspout every 40 ft on a 120 ft run means three lows and three high points, and the gutter empties before any section fills. Put one downspout at the far end of an 80 ft gutter and the near half overruns while the leader at the end runs full.

The minimum useful leader is about 7 in² of cross-section. Anything smaller is for a porch or a canopy, not a roof plane. Two downspouts also buys redundancy: one clogs with leaves and the second still moves water instead of the whole gutter going over the front.

K-style, half-round, and box gutters

Three profiles cover almost everything. K-style has an ogee front face with a flat back, holds the most water for its width, and bolts flat to the fascia, which is why it is the default on residential and light commercial work. A 5 in K-style holds roughly 1.2 gallons per linear foot, about 20 to 30 percent more than a 5 in half-round, so for the same opening it moves more water before it spills.

Half-round is a semicircular trough in standard 4, 5, and 6 in diameters. It carries less than a K-style of the same nominal size and needs brackets rather than a flat back, but it sheds debris and self-cleans better because there is no inside corner for grit to pack into, and it is the profile historic districts and copper installations usually call for. When the spec says half-round, expect to go up a size against the K-style you would have used.

Box gutter, also called commercial or O.G. box, is a large rectangular trough for the high runoff off big roofs and the long eaves on commercial buildings. It is sized and fabricated to the job rather than pulled off a shelf, and it is where the conductor head and the built-in overflow come into play, covered further down. Pick the profile for the building and the runoff, then size within it, do not let the shop's standard stock pick the profile for you.

Material, gauge, and one-piece versus sectional gutter

Aluminum is most of the market because it does not rust and rolls into one continuous gutter on site. Common residential aluminum runs 0.027 to 0.032 in thick, and the heavier commercial work moves up to 0.040 in. The thicker the metal, the better it holds its shape under a ladder, a snow load, or a section of ice, so do not let a bid shave the gauge to win price on a building that sees real winter.

Galvanized and galvalume steel are stiffer and take more abuse, at the cost of eventual rust where the coating wears or a cut edge goes unsealed. Copper is the long-life choice, three to four times the price of aluminum but fifty years or more with little upkeep, and its joints get soldered rather than sealed, which is a different trade and a different bid. Match the downspout and the hangers to the gutter metal. Mixing dissimilar metals, copper gutter on steel straps, sets up galvanic corrosion that eats the cheaper metal at the contact.

One-piece gutter is roll-formed in a continuous length on the truck, so the only joints are at corners and end caps, and there are far fewer places to leak than a sectional run pieced from 10 ft stock. That is the real reason it dominates: every seam is a future leak, and the machine removes most of them. Sectional still has its place for copper, half-round, and any profile you cannot roll on site.

What slope should a gutter have?

Slope a gutter toward its downspout at least 1/16 in per foot, which is about 1/4 in of fall over 10 ft, and many crews run 1/8 in per foot where the fascia line allows it. The gutter is not level. It is pitched so the water has somewhere to go, and the high point is the spot farthest from a downspout.

Lay it out before you hang it. On a run with a downspout at each end, the high point is the middle and the gutter falls both ways. On a run draining to one end, the whole length pitches that way. String a line from the high point to the downspout at the design fall and set the hangers to it, then check it with a level before you call it done. A 40 ft run wants about an inch of total drop at 1/16 in per foot.

Too flat and water stands in the trough, which adds dead weight, breeds mosquitoes, and rots out the metal and the fascia behind it from the constant wet. Too steep and the gutter looks crooked against the eave and, on a hard rain, water can run fast enough to shoot past the downspout opening at the low end. There is room between those, and that room is the 1/16 to 1/8 in per foot range. Standing water in a finished gutter means the slope is wrong, full stop.

Hangers and attachment

Hidden hangers are the standard now: a bracket that clips inside the gutter and screws through the back into the structure, with nothing showing and no spike to work loose. Space them about every 24 in on center in ordinary conditions, and tighten that to 16 to 18 in in snow country, because the load that fails a gutter is rarely the water. It is the ice and snow sitting in it and sliding off the roof onto it.

Screw into solid wood, not just the fascia skin. The fascia board alone splits and pulls; the rafter tails behind it are the structure that holds a loaded gutter, so hit them where you can. A hidden hanger commonly carries an integrated #10 by 2 in screw driven with a 5/16 in hex driver, and the trick is moderate torque. Run it down too hard and you strip the wood, which leaves the hanger holding on nothing.

In heavy snow, hangers rated on the order of 200 lb per linear foot and set at the tighter spacing are what keep the gutter on the building through a winter. Tie that back to where the snow lands. A north slope that builds a drift, or an eave below a long rake that sheds, gets the close spacing whether the rest of the building needs it or not. Long runs also need room to move, so plan the expansion the metal will demand, covered next.

Gutter position and the drip edge

Hang the gutter so its front edge sits below the plane of the roof. Carry a straightedge down the roof slope past the eave; the back of the gutter is high and the front is low enough that a sheet of snow sliding off the roof passes over the gutter instead of catching the lip and tearing it off the wall. Set the gutter too high and proud of that line and the first winter slide takes it down.

Water has to get from the roof edge into the trough, and that is the drip edge and the gutter apron. The roof's drip edge or a gutter apron flashing runs out over the back of the gutter so runoff sheeting off the shingle or the membrane drops inside the gutter, never behind it. Miss that and water wicks back under the edge and down the fascia, rotting the very board the hangers screw into.

The back of the gutter is the detail that leaks quietly. Water that gets behind the gutter has found the fascia and the wall, and you will not see it until the paint blisters or the soffit stains. Flashing the eave into the gutter is the same discipline as flashing any roof penetration, and the companion guide on penetration and flashing details covers how those laps shed water by gravity. Get the metal lapping the right way, high over low, and the water has no path but down into the trough.

Expansion joints on long runs

Metal grows and shrinks with temperature, and a long gutter run that is locked solid at both ends will buckle, oil-can, or split a seam as it works through the seasons. Aluminum moves more than steel for the same temperature swing, and a dark gutter in summer sun moves more than the air temperature suggests. On a long commercial eave the run needs somewhere to take up that movement.

The fix is an expansion joint, a deliberate break in the gutter with a cover or a slip detail that lets the two lengths move past each other while staying watertight. A common spacing keeps expansion joints no more than about 48 ft apart, and the high point of the gutter slope is a natural place to put one because the water runs away from it on both sides. Coordinate the joint with the downspouts so each segment between joints still drains.

Skip the joints on a long run and the gutter tells you in a year or two: a seam that opens and drips, a hanger that has pulled, a section that has oil-canned into a wave. That is not a workmanship defect in the seam itself. It is the run fighting its own thermal movement with nowhere to put it.

Box gutters and the conductor head

A box gutter is the large rectangular trough used where a big roof or a long commercial eave throws more water than a stock K-style or half-round can take. It is fabricated to the runoff, lined or formed in heavier metal, and it carries the volume a shelf gutter cannot. With that volume comes a different set of details at the outlet.

A conductor head, also called a leader head or a collector box, is the open funnel that sits at the top of a downspout where the gutter or a scupper discharges into the leader. It catches the concentrated flow off a box gutter or a through-wall scupper and feeds the downspout, and because it is open at the top it doubles as overflow relief: if the leader clogs or the storm beats the design, the head spills over its own front instead of backing the whole gutter up. On a building with parapet scuppers, the conductor head is where the exterior leader picks up the flow the scupper sheds.

Build the overflow into the box gutter itself on a big run. An overflow slot or a low front section, set above the normal water line, gives a clogged gutter a place to dump that is not behind the roof edge. That is the same logic as the secondary overflow on an interior-drained roof, handled in the roof-drainage guide, moved to the outside of the building.

How do you keep a clogged gutter from backing up under the roof?

Give the water a way out the front of the gutter before it can rise to the back. A gutter is a dam waiting to happen. Fill it with leaves at the downspout, hit it with a storm, and the water climbs until it either pours over the front, which is fine, or wicks over the back and behind the fascia, which is the leak that rots the eave and the wall.

The cheapest overflow is the front of the gutter itself, hung so the front lip is lower than the back. When the trough fills, it spills forward and down, away from the building, instead of backward into the structure. That one detail, front lower than back, is what makes a clog a nuisance instead of damage. Get it backwards and every overflow event runs inside the wall.

On box gutters and any run where a clog could trap water against the roof edge, add a real overflow: a slot, a scupper through the front, or an open conductor head that spills before the water reaches the deck. Treat it the way interior roofs treat the secondary drain. The primary path moves the design storm; the overflow path handles the day the primary is blocked, and it is sized and placed so the water leaves the building rather than entering it.

The downspout, the elbow, and the discharge

The downspout takes the gutter's water to the ground, and the part people get wrong is the bottom. Run the leader down the wall, strap it about every 6 to 10 ft so it does not rattle loose or pull away, and use elbows to offset it tight to the building and to kick the discharge out at the bottom. The straps tie back to the same structure logic as the hangers: into solid backing, not just siding.

Where the water lands is the whole point of carrying it down. A downspout that dumps right at the foundation feeds water into the soil against the wall, which is how you get a wet basement, a heaved slab, and settlement. Get the discharge away from the building. A splash block or a boot at the bottom spreads the flow and directs it out, and the ground should fall away from the foundation so the water keeps going. The grading that carries it the rest of the way is its own subject, but the downspout's job is to start the water moving away, not to drop it at the wall.

On a building with people walking the eave line, the leader and its elbows are also a trip and a snag, so route them tight and strap them clean. A downspout that has pulled loose at the bottom elbow is the most common gutter callback there is, and it is a two-screw fix that should have been done right the first time.

Tying the downspout into underground storm

On a commercial site and on a lot of residential work now, the downspout does not spill onto a splash block. It drops into an underground conductor that carries the water to a storm sewer, a drywell, or a daylight outfall away from the building. The leader connects to the buried pipe through an adapter or a boot at grade, and from there it is a storm-drainage run governed by the plumbing code.

Put a cleanout where the vertical leader meets the horizontal underground run. That transition is exactly where leaves, grit, and roof debris pack in and plug, and without a cleanout the only way to clear it is to dig. A cleanout at the base, accessible from grade, turns a buried clog into a ten-minute job with a snake instead of an excavation.

Coordinate the tie-in with the site civil drawings, because the underground conductor sizing and the connection to the storm system are the civil and plumbing scope, not the gutter installer's guess. Size the buried pipe for the same design storm the gutter was sized for. A downspout sized right that feeds an undersized buried line just moves the backup underground, where you cannot see it until it surfaces at the base of the wall.

Ice, snow, and the cold-climate detail

In a cold climate the gutter's enemy is not the rain. It is the ice. Heat escaping through the roof melts the snow above the warm field, the meltwater runs down to the cold eave and the gutter, and it refreezes there into an ice dam. The dam grows, backs water up under the shingles, and the whole frozen mass loads the gutter with weight it was never sized to hold.

That load is why the hanger spacing tightens in snow country, 16 to 18 in on center instead of 24, and why the gutter sits below the slide line so a sheet of roof snow passes over it rather than ripping it down. A gutter packed with ice can weigh more per foot than the water it carries, and a gutter full of ice with a slab of snow sliding onto it is what tears gutters off buildings every winter.

Heat trace, a self-regulating heating cable run in the gutter and down the downspout, keeps a drain path open through the ice so meltwater has somewhere to go instead of damming. It manages the symptom. The real fix for ice dams is up on the roof, air-sealing and insulating so the roof deck stays cold and the snow does not melt from below in the first place. Heat trace in the gutter is a patch on a roof that is losing heat, and it is worth saying so to the owner before they think a heated gutter solved their problem.

Gutter guards and screens

Gutter guards keep leaves and debris out of the trough so it does not clog at the downspout and back up. They range from a cheap drop-in screen to a fitted mesh to a surface-tension hood that lets water curl in while leaves fall past. They all help and none of them make a gutter maintenance-free, which is the claim to be skeptical of.

Under a heavy tree, a guard turns a twice-a-year cleanout into a once-a-year one and keeps the worst of the clog out of the downspout, which is where clogs do the damage. In a hard rain, some guard designs shed water along with the leaves, so the gutter that never clogs also never fills, and the runoff sheets over the front. Size the guard to the rainfall the same way you size the gutter. A guard that defeats a 7 in/hr burst is a guard that caused an overflow it was sold to prevent.

The honest pitch to an owner: a guard reduces how often someone is on a ladder and reduces downspout clogs, and it still needs a look every year, more under pine and oak. Pine needles and shingle grit get through most screens and build a mat on top of others. The maintenance does not disappear. It gets smaller.

Seams, end caps, and why gutters leak

Gutters leak at the joints, almost never in the field of the metal, which is the same lesson the roof itself teaches. The places to watch are the inside and outside miters at the corners, the end caps, the downspout outlets, and any seam in a sectional run. A continuous, machine-rolled gutter exists precisely to delete most of those joints.

Sealed joints are only as good as the prep. The metal has to be clean and dry, the sealant has to be one rated for gutters and the metal in question, and the lap has to shed water by gravity, high piece over low piece, so the sealant is backup and not the only thing holding the water. A miter sealed over dirty metal lets go in a season, and then you are chasing a corner drip that stains the wall below it.

Copper is the exception that proves the rule: its joints are soldered into one continuous metal, not sealed with a bead, which is why a soldered copper gutter outlasts everything and why it costs what it costs. That is a different skill and a different crew. For aluminum and steel, the watchpoints are the corners and the end caps, and the fix when one leaks is to cut it out, clean it back to bright metal, and reseal or re-rivet, not to smear more sealant over a failed joint.

Flashing the gutter into the roof edge

The gutter only works if the water gets into it, and that handoff from the roof to the trough is a flashing detail. On a shingle roof, the drip edge metal runs out over the back of the gutter so runoff drops inside. On a low-slope membrane roof drained to an exterior gutter, a gutter apron or edge metal carries the membrane's runoff out past the fascia and into the gutter, lapped so water cannot get behind it.

This is the same gravity-lap discipline as any roof edge or penetration, and the companion guide on roof penetration flashing details lays out how those laps work and where the field crews get them backwards. The rule does not change at the gutter: the upper piece always laps over the lower piece, so water running downhill stays on top of the metal the whole way into the trough.

On a re-roof, the existing drip edge and the gutter rarely line up the way the detail assumes, and that mismatch is where water sneaks behind the gutter. Check that the new edge metal actually delivers water into the gutter and not behind it before the gutter goes back up. A perfect gutter hung under a drip edge that dumps behind it leaks anyway, and it leaks into the fascia where you cannot see it.

ANSI/SPRI GT-1 and the structural side

A large exterior gutter is part of the roof edge, and the roof edge is what wind tries to peel first. ANSI/SPRI GT-1, the Test Standard for External Gutter Systems, is how a commercial gutter system is tested to stay attached. It was approved as an American National Standard in 2016 and brought into the International Building Code in the 2021 edition, so on commercial work it is increasingly a code-driven requirement, not just a manufacturer claim.

GT-1 runs three tests on a full-size gutter sample, between 8 and 12 ft long with its real brackets, straps, and fasteners. G-1 loads the face of the gutter outward, the way wind pulls it off the wall. G-2 loads the bottom of the gutter upward, the way wind gets under it. G-3 loads the bottom downward with the weight of ice and water, the cold-climate case. A gutter system that passes all three at the project's design wind pressure is one that has been shown to hold, not just one that looks stout.

What this means in the field is that the attachment is engineered, not improvised. On a building where GT-1 applies, the bracket type, the fastener, the spacing, and the substrate are part of a tested assembly, and substituting a cheaper hanger or stretching the spacing breaks the listing. Confirm the gutter system's GT-1 compliance and the design wind pressure against the project documents and the adopted code edition before the metal is ordered.

Large commercial and data-center roofs

A big low-slope roof, the kind over a warehouse, a distribution center, or a data hall, is usually drained through interior drains and overflow scuppers, the system the roof-drainage guide covers. Exterior gutters still earn a place at the perimeter, on the eaves of attached canopies, loading docks, and equipment yards, and on the lower roofs that catch runoff thrown off a taller face.

The exterior numbers do not get gentler at scale, they get less forgiving. The drained areas are large, the design storm is the same hard burst, and the consequence of a backup is water against a building full of equipment that cannot get wet. Size the perimeter gutters and the conductor heads for the real area and the real rainfall, including the wall that drains onto the lower roof, and tie the leaders into a storm system sized by the civil drawings rather than spilling at the base of a wall that has a service entrance or a generator yard behind it.

On the high-value building, the overflow is not optional thinking, it is the design. Every exterior gutter that could trap water against the roof edge gets a front-spill detail or a real overflow, and the leaders get cleanouts so a clog is cleared from grade. The whole point is that a plugged downspout on a Saturday is a maintenance ticket, not a flooded room.

The upkeep after handoff

A gutter system is the one part of the roof the owner has to maintain, and most do not until it fails. Clean the troughs and the downspout outlets at least twice a year, more under trees, because the clog at the outlet is what backs the whole run up. Run water down each downspout and watch that it comes out the bottom, which is the only way to catch a buried-line plug before a storm finds it.

Check the slope and the hangers on the same visit. A gutter that has started to hold standing water has either clogged or had a hanger let go, and a sag pulls the slope flat and starts the cycle of standing water and rot. A hanger backing out of the fascia is a five-minute fix now and a torn-off gutter after the next ice load. Look at the seams and the end caps for the early drip stain on the fascia below them.

Hand the owner the basics in writing: clean it twice a year, more in fall, keep the downspouts clear to grade, and look at the hangers and the seams while you are up there. The system that gets that ten-minute attention twice a year lasts decades. The one that gets ignored fails at the fascia, and the fascia repair costs more than every cleanout that was skipped.

What to document

Write down what was installed so the next person can check it against the storm it was sized for. A gutter that fails in five years is often a gutter that was never sized to the area and rainfall it actually drains, and without a record there is no way to tell whether it was undersized or just neglected.

Capture each gutter run with its drained area, the profile and size, the slope, the downspout count and size, the design rainfall used, and the sizing basis, whether IPC or SMACNA. Note the hanger type and spacing, and on a commercial job the GT-1 compliance and the design wind pressure. Record where the leaders discharge and whether they tie into an underground storm line with a cleanout. That record is what an owner, an inspector, or the next contractor reads to know the system was sized on purpose.

Common mistakes

• Sizing the gutter for an average rain instead of the local design storm, so it overruns in the burst it was supposed to handle.
• Too few downspouts, or downspouts too far apart, so the gutter backs up between them while one leader runs full.
• No slope or the wrong slope, leaving standing water that adds weight, breeds mosquitoes, and rots the metal and the fascia.
• Hangers spaced too far apart for the snow and ice load, so a winter slide sags or tears the gutter off the building.
• Hanging the gutter above the roof slide line so sheeting snow catches the lip instead of passing over it.
• No overflow path and a front edge higher than the back, so a clog sends water behind the fascia instead of over the front.
• Dumping the downspout right at the foundation instead of carrying the water away with a splash block or a tied-in line.
• Substituting a cheaper hanger or wider spacing on a GT-1 commercial system and breaking the tested assembly.

Field checklist

Standards and references

Several documents govern exterior drainage, and they each own a piece. SMACNA's Architectural Sheet Metal Manual is the working reference for gutter and downspout profiles, fabrication, and the sizing charts that give the maximum roof area per gutter size and rainfall. The International Plumbing Code, in its storm drainage chapter, also sizes gutters, leaders, and conductors from the roof area and a design rainfall, and the International Building Code carries the roof-assembly and edge requirements. The residential equivalent lives in the IRC.

ANSI/SPRI GT-1, the Test Standard for External Gutter Systems, governs the wind and load resistance of commercial gutter systems and was added to the IBC in the 2021 edition. NRCA's manuals give the roofing-side details for tying the gutter to the roof edge. The design rainfall itself comes from NOAA precipitation data for the site, the same source the interior roof-drainage sizing uses.

Section numbers and adopted editions move between code cycles and between jurisdictions, so confirm the article against the edition the authority having jurisdiction has actually adopted, along with any local amendments, before citing it on a submittal. Where the project specification or a manufacturer's listing is stricter than the code minimum, the stricter requirement controls.

Units, terms, and conversions

Exterior drainage uses a handful of terms that change name across a drawing set, a catalog, and a spec, so the same part can read three ways.

A downspout is also called a leader or a conductor. A gutter is a trough or, on commercial work, an eaves gutter. Rainfall intensity is in inches per hour and traces to a return period, the 100-year storm being the common design basis. Roof area for sizing is the horizontal projection in square feet, not the sloped surface. Downspout capacity is often expressed as roof area served per inch of rainfall per hour, and gutter capacity in gallons per minute or gallons per linear foot of trough.

Downspout / leader / conductor

The vertical pipe carrying water from the gutter down and away

Conductor head / leader head

The open collector box at the top of a downspout that catches and funnels flow and spills overflow

Rainfall intensity

Design rain rate in inches per hour for a return period, from the code map or NOAA

Drained area

The horizontal roof projection feeding a gutter, in square feet, plus any wall that drains onto it

Ice dam

Refrozen meltwater at the cold eave that backs water up and loads the gutter

GT-1

ANSI/SPRI test standard for the wind and load resistance of external gutter systems

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• How do you size a roof drain?
• How high should base flashing be on a commercial roof?
• What is a SMACNA pressure class?
• What is a duct leakage test?
• What is a ballasted roof?