By The Roofing Brief Team. Last reviewed: October 2026.
Downspout installation runs top to bottom: fasten a drop outlet at the low end of the gutter, connect two elbows and an offset to bring the pipe back to the wall, strap the vertical run with 2 bands per 10 ft section (Amerimax’s metal gutter guide), and discharge at least 5 feet from the foundation, the distance the DOE Building America Solution Center gives for EPA Indoor airPLUS homes. Size the drops first: the same DOE guide allows 1 square inch of downspout cross-section per 100 square feet of roof and places downspouts 20 to 50 feet apart. For where each drop goes, see downspout placement and sizing.
Downspout installation decisions: sizes, spacing and distances
Check these numbers on site before cutting the first piece of downspout.
| If you need to decide | Use this threshold | Example | Source |
|---|---|---|---|
| Downspout size for the roof area | 1 sq in of downspout per 100 sq ft of roof | 2×3 in (6 sq in) serves about 600 sq ft; 3×4 in (12 sq in) about 1,200 sq ft | DOE Building America Solution Center |
| Peak flow a leader can carry | 2×2 in: 30 gpm; 2×4 in or 3 in round: 92 gpm; 3×4-1/4 in or 4 in round: 192 gpm | A 74 gpm 5 in half-round run needs a 2×4 or 3 in leader | 2021 IPC Table 1106.3 |
| When to add a second drop | One downspout per 40 ft of gutter maximum; drops 20 to 50 ft apart | A 60 ft run gets a drop at each end | Amerimax vinyl gutter instructions; DOE |
| Gutter slope toward the outlet | At least 1/16 in per ft (DOE); Amerimax metal: about 1 in per 40 ft | 40 ft run: 1 in (Amerimax) to 2-1/2 in (DOE) of fall | DOE; Amerimax metal gutter guide |
| How often to strap the vertical run | 2 bands or clips per 10 ft section | A 20 ft two-story drop gets 4 bands | Amerimax metal and vinyl guides |
| Where the pipe must end | At least 5 ft from the foundation; 10 ft if it feeds an underground catchment | A 3 ft splash block alone falls short | DOE (EPA Indoor airPLUS); 2021 IRC R801.3 on expansive soils |
| Buried drain line slope | Non-perforated ABS or Schedule 40 PVC at 5/8 in per ft (5 percent) or more | A 10 ft line drops at least 6-1/4 in | DOE Building America Solution Center |
| Grade around the house | 6 in of fall in the first 10 ft | Discharge onto ground that keeps falling | 2021 IRC R401.3 |
What you need before you start the downspout install
Before attaching a downspout to gutter, confirm the gutter run is hung with the correct slope and the outlet holes are marked. A downspout only drains well if the gutter pitches toward it: Amerimax’s metal gutter guide drops the guide string about 1 inch per 40 feet (1/4 inch per 10 feet), while the DOE Building America guide calls for at least 1/16 inch per foot. Gather the elbows, downspout stock, brackets, and fasteners for each drop so you assemble top to bottom without stopping.
| Item | Typical spec | Notes | Source |
|---|---|---|---|
| Downspout stock | 2×3 in or 3×4 in, 10 ft lengths | 3×4 in has twice the cross-section of 2×3 in (12 vs 6 sq in) | DOE sizing rule; Amerimax |
| Outlet (drop outlet) | Matches downspout size | Fastened with four pop rivets or screws | This Old House |
| Elbows | Two at the top (front elbows), one at the bottom | Front and side styles, explained below | Amerimax metal guide |
| Brackets or bands | 2 per 10 ft section | Metal or plastic bands, nails or screws into the wall | Amerimax metal guide |
| Fasteners | 1/2 in self-tapping, rust-resistant screws, or pop rivets | At each elbow and downspout joint | Amerimax vinyl instructions |
| Sealant | Gutter sealant (Amerimax SeamerMate or mastic) | Outlet flange, mitres and end caps, not slip joints | Amerimax metal guide |
Amerimax lists the tools for its metal system as hammer, screwdriver, tape measure, string, pliers, hack saw, tin snips, level and ladder; add a drill for self-tapping screws and a rivet gun if you rivet. A downspout crimper saves the most time on 2×3 or 3×4 inch stock because it swages the end of each cut section so it slips inside the next piece.
How to install a downspout: the outlet comes first
Installing a downspout starts at the gutter, not the wall. Cut or punch the outlet into the low end of the gutter run where the slope drains to, then seal the flange and fasten it with four pop rivets or screws, the count This Old House gives, before any elbow goes on. On Amerimax’s vinyl system the drop outlet is fastened to the fascia 1/2 inch below the sloping chalk line, before the gutter is cut to final length.
- Mark the outlet. Center it over the drop location at the lowest point of the gutter slope. Trace the outlet tube.
- Cut the hole. Drill a 1/2 in starter hole, cut to the line with tin snips or an outlet punch, and file the edge.
- Seal the flange. Bead gutter sealant around the flange, seat it from inside the gutter, press flat.
- Fasten it. Set four pop rivets or short screws evenly around the flange; wipe the squeeze-out smooth.
If your gutter run uses a pre-punched outlet section, skip the cutting. Amerimax’s vinyl instructions offer two layouts: a 6 inch gutter stub with an end cap slid into the drop outlet, or the last full piece run through the outlet with a hole cut directly over the downspout.
Downspout elbow types: A elbow vs B elbow and the offset
A downspout elbow changes the direction of the drop, and the two common styles bend on different planes. An A elbow (front elbow) bends toward or away from the wall; a B elbow (side elbow) bends left or right along the wall. Amerimax’s metal guide uses two front elbows and a short section of downspout at the top to bring the pipe from the outlet tube back to the wall, then a front or side elbow at the bottom to turn water away from the foundation.
| Elbow | Bends | Where it goes |
|---|---|---|
| A elbow (front/back) | Toward or away from wall | Top of drop, to jog in toward siding |
| B elbow (side) | Left or right along wall | Routing around a corner or bay |
| Bottom elbow | Away from foundation | Base of drop, into extension |
The offset is the short piece of downspout between the two top elbows that spans the gap from the gutter face to the wall. Amerimax’s method: screw one elbow to the drop outlet, hold the second elbow against the wall, measure the distance between them and cut a downspout length to connect them, adding the length that slips into each elbow. Example: with 6 inches between the elbow ends and 1-1/2 inches of insertion at each end, cut the offset to 9 inches. The screws at the offset stay visible from the ground, so place them on the faces that point toward the wall where you can reach.
Crimp direction matters at every joint. Amerimax’s guide notes that each downspout and elbow has one crimped end and that parts join by fitting the larger end of one over the smaller end of the other, so the crimped (smaller) end always points down and water runs inside the 2×3 or 3×4 inch pipe, never out a seam.
Attaching the downspout and joining sections
To attach the downspout to the gutter, slide the top elbow onto the outlet tube and fasten it, then hang the vertical run off that assembly. Push each crimped end fully into the larger end below it and set a 1/2 inch self-tapping, rust-resistant screw at the joint, the fastener Amerimax specifies; a second screw on the opposite face stops the joint from twisting. Overlapping the wrong way sends water down the outside of the pipe. Any piece you cut to length needs a fresh crimp from a gutter crimping tool before it fits.
- Connect to the outlet. Slip the first A elbow over the outlet tube and secure it with a 1/2 in self-tapping screw on each side face.
- Measure the vertical run. Measure from the wall elbow to 6 to 12 in above the ground (Amerimax) and cut to that length.
- Crimp and join. Swage the cut end, join larger end over smaller, and use a connector past 10 ft (Amerimax).
- Fasten every joint. One or two 1/2 in screws per joint; no sealant on slip joints.
Downspout bracket spacing: how often to strap the run
Downspout brackets hold the vertical run tight to the wall so wind and ice cannot pull it loose. Amerimax’s metal gutter guide fastens downspouts against the wall with 2 bands per 10 ft section, using nails or screws, and its vinyl instructions call for two clips for every 10 ft of downspout. On a 20 ft two-story drop that means 4 bands, one near the top and one near the bottom of each 10 ft section.
| Drop height | Bands or clips | Placement | Source |
|---|---|---|---|
| 10 ft (one story) | 2 | Near the top and near the bottom of the section | Amerimax metal guide |
| 20 ft (two stories) | 4 | 2 per 10 ft section, plus a connector or slip joint at 10 ft | Amerimax metal and vinyl guides |
| 30 ft (three stories) | 6 | 2 per 10 ft section | Amerimax metal guide |
| Bottom extension | 1 clip to hold it upright when lifted | Flip-up extensions only | Amerimax vinyl instructions |
Fasten brackets into solid backing: studs, blocking or masonry anchors, not just siding. On vinyl or fiber-cement, drill a 1/8 in pilot into the sheathing or framing behind so the bracket screw bites something structural. A 10 ft section of 2×3 downspout full of ice carries far more weight than the empty pipe, and a bracket that only grips siding is the first thing to let go.
Finishing the bottom: elbow and extension
The bottom of the downspout gets an elbow that turns the water away from the foundation, then an extension that carries it clear. A downspout that dumps at the base of the wall drives water toward the footing, so move discharge at least 5 feet out, the DOE Building America figure for EPA Indoor airPLUS homes, or 10 feet if it feeds an underground catchment. Where expansive or collapsible soils are known, 2021 IRC R801.3 requires roof drainage to discharge at least 5 feet from foundation walls or into an approved drainage system.
Set the bottom elbow so it points along the grade fall, which 2021 IRC R401.3 sets at 6 inches within the first 10 feet, not into a walkway. Then connect an extension, a splash block, a roll-out extender or a buried drain line; a buried line should be non-perforated ABS or Schedule 40 PVC pitched at least 5/8 inch per foot, per the DOE guide. For the trade-offs, see downspout splash blocks vs extensions and, for longer runs, gutter downspout extensions and how far to run them. To store the water instead, see how to hook up a gutter rain barrel.
Once the drop is fastened top to bottom, run a garden hose into the high end of the gutter and watch the whole path. Water should enter the outlet cleanly, stay inside every joint and leave the extension at least 5 feet from the wall. A drip at a joint means a crimp points up or a 1/2 inch screw is loose; fix it before the first storm.
Where the downspout fits in the whole gutter job
Downspout installation is one stage of a larger gutter job that starts with hanging and pitching the gutter run: Amerimax’s metal guide sets one hanger every 24 inches and a drip edge that projects about 1/2 inch beyond the fascia. Work through the gutter run first, then the outlets, then the downspouts. On accent corners some homeowners swap the closed pipe for a decorative rain chain, which guides water down a visible path instead. The complete sequence, including sizing, pitch, and hanger spacing, is covered in the rain gutter installation guide, and the broader process, timeline, and common mistakes are in gutter installation: process, time, and what could go wrong. Start any research at the Learn hub for how the parts connect.
Frequently asked questions
What order do you install a downspout in?
Install from the top down. Fasten the drop outlet with four rivets or screws, attach the top A elbow, add the offset piece, then the second elbow to bring the run to the wall. Drop the vertical downspout to 6 to 12 inches above the ground with the crimped ends pointing down, strap it with 2 bands per 10 ft section, then finish with the bottom elbow and an extension that reaches at least 5 feet from the foundation.
How do you attach a downspout to a gutter?
Attach a downspout to a gutter through the outlet. Cut or punch a drop outlet into the low point of the gutter, seal the flange with gutter sealant, and fasten it with four pop rivets or screws. Slide the first elbow onto the outlet tube and secure it with 1/2 inch self-tapping, rust-resistant screws. The rest of the downspout hangs off that elbow, so the outlet joint carries the whole drop.
How far apart should downspout brackets be?
Amerimax’s gutter guides call for 2 bands or clips per 10 ft section of downspout, so a 20 ft two-story drop gets 4. Put one near the top and one near the bottom of each section, and fasten each bracket into framing or masonry, not into siding alone.
What is a downspout offset and how long should it be?
A downspout offset is the short section of downspout between the two top elbows that spans the gap from the gutter face to the wall. Screw one elbow to the outlet, hold the second against the wall, measure between them and add the length that slips into each elbow: 6 inches between elbows plus 1-1/2 inches of insertion at each end means a 9 inch cut.
Do downspout joints need sealant?
No. The outlet flange, mitres and end caps get sealant, but the slip joints between elbows and downspout sections do not. Amerimax joins those parts larger end over smaller end, with the crimped end pointing down so water stays inside as it falls, and fastens them with a 1/2 inch self-tapping screw instead of sealant.
How far should a downspout extend from the house?
Discharge water at least 5 feet from the foundation, or 10 feet if it feeds an underground catchment, per the DOE Building America Solution Center guidance for EPA Indoor airPLUS homes. On expansive or collapsible soils, 2021 IRC R801.3 also requires at least 5 feet or an approved drainage system. A buried line should fall at least 5/8 inch per foot.
How many downspouts do I need?
Allow 1 square inch of downspout cross-section for every 100 square feet of roof and keep drops 20 to 50 feet apart, per the DOE Building America Solution Center. A 2×3 inch downspout (6 sq in) covers about 600 sq ft of roof and a 3×4 inch (12 sq in) about 1,200 sq ft. Amerimax’s vinyl system allows one downspout per 40 feet of gutter at most.