By The Roofing Brief Team. Last reviewed: October 2026.
A gutter elbow is the angled fitting that redirects water where a downspout has to change direction, and the two gutter elbow types you will actually buy are the A elbow and the B elbow. An A elbow turns water toward or away from the wall (front to back). A B elbow turns water left or right along the wall. Amerimax’s Installing Metal Gutter Systems sheet labels them exactly that way, “Front A Elbow” and “Side B Elbow”, each sold in 2×3 and 3×4 inch sizes.
For the full run from outlet to extension, see our step-by-step downspout gutter installation. For count and location, see downspout placement and sizing.
A elbow vs B elbow: what is the actual difference?
The difference between an A elbow and a B elbow is the direction the water turns relative to the wall. An A elbow (front elbow) turns the flow front to back, toward or away from the wall. A B elbow (side elbow) turns the flow left or right, running the downspout sideways along the wall. Both are the same crimped-end fitting in the same 2×3 and 3×4 inch sizes and the same 30 to 90 degree angles; only the turn axis changes.
The fastest way to tell them apart on the shelf: set the elbow on its back the way it would sit on the wall. Alsco Metals describes the A style as wider than it is tall, with its wide side against the building; looking down the downspout, it discharges toward you. The B style is taller than it is wide and turns to your right or left.
| Elbow | Turns water | Looking down the downspout, it discharges | Typical use | Common placement |
|---|---|---|---|---|
| A elbow (front) | Toward or away from the wall (front to back) | Toward you | Bringing the downspout back to the wall at the top, then out at the bottom; building offsets | Top and bottom of most standard downspouts |
| B elbow (side) | Left or right along the wall | To your right or left | Routing around a corner, a door, a walkway, or a paved area | At the outlet or base when the run has to shift sideways |
A standard, no-obstruction downspout uses three A elbows: two at the top to step the pipe from the outlet back to the wall, and one at the bottom to kick water out away from the foundation. The Amerimax Installing Metal Gutter Systems sheet calls for “a front or side elbow at the downspout’s bottom to direct water away from the foundation”, so the bottom piece is an A elbow when the drop points straight out and a B elbow when the yard slopes sideways.
What ranking guides say versus the maker sheets
Online guides do not all use the same labels. Check these common claims against the maker documents before you buy.
| Claim you may read | What the maker documents say | Source |
|---|---|---|
| “The A elbow is the side elbow” | A is the front elbow, B is the side elbow | Amerimax install sheet |
| “A B elbow is always a sharp 90 degree bend” | Both A and B styles come in 30, 45, 75 and 90 degree angles; 75 degrees is the most common | Alsco Metals |
| “The B elbow is the bottom elbow” | Either a front (A) or side (B) elbow goes at the bottom; pick by the direction the water must leave | Amerimax install sheet |
How do you choose between an A and a B elbow?
Choose the elbow by the direction the downspout must move at that joint. If the pipe needs to come toward the house or push away from the foundation, use an A elbow. If it needs to travel left or right along the wall, past a corner or an obstacle, use a B elbow. When a run has to move both sideways and out, you combine them: a B elbow to shift sideways, then A elbows to offset in and out, and Alsco Metals notes the two styles can be mixed in the same downspout leg.
| If the downspout needs to | Use | Pieces for a 2×3 system |
|---|---|---|
| Step from the gutter outlet back to the wall | 2 A elbows | 2 front A elbows, plus a short filler if the gutter sits more than a few inches out |
| Discharge straight out from the wall at grade | 1 A elbow at the bottom | 1 front A elbow, then an extension to reach 5 ft where IRC R801.3 applies |
| Discharge sideways along the wall (driveway, walkway, side slope) | 1 B elbow at the bottom | 1 side B elbow |
| Skirt a corner, door or window on the way down | 2 B elbows with a filler | 2 side B elbows, 1 cut downspout section |
| Clear a soffit, bay or brick ledge, then return flush | 2 A elbows with a filler | 2 front A elbows, 1 cut section sloped downhill |
| Turn where a 75 degree elbow will not fit | 90 degree elbow | Accept more turbulence and a higher clog risk at that joint |
Gutter elbow types beyond A and B
Past the A/B distinction, gutter elbows are categorized by shape and by the pipe they fit. Rectangular 2×3 and 3×4 inch elbows fit standard rectangular downspouts and come in A and B styles. Round 3 and 4 inch elbows fit round downspouts, common on half-round and copper systems, and turn in any direction because the pipe is symmetrical. Alsco lists rectangular, round and square shapes, with rectangular the default on most homes.
- Rectangular A and B elbows: the default for K-style aluminum and steel gutters, in 2×3 and 3×4 inch.
- Round elbows: for round downspouts on half-round gutter systems and copper; no A/B split because the round pipe turns any way.
- Offset elbows: a pre-bent single piece that replaces two A elbows and a filler where the jog is short and fixed.
- Downspout outlets and end drops: not elbows, but they connect the gutter to the first elbow. the Amerimax Installing Metal Gutter Systems sheet uses either an outlet cut into the gutter or an end with drop joined by a seamer.
Gutter elbow sizes: 2×3, 3×4, and round
Gutter elbow sizes match downspout sizes: 2×3 inch and 3×4 inch rectangular, plus 3 and 4 inch round. The elbow must match the downspout it joins. A 2×3 elbow cannot slip over a 3×4 downspout, and a forced or mismatched joint either leaks at the seam or works loose.
| Size | Shape | Open area (computed) | Roof area drained, average rainfall | Source for drainage |
|---|---|---|---|---|
| 2×3 in | Rectangular | 6 sq in | About 800 to 1,000 sq ft | Alsco Metals |
| 3×4 in | Rectangular | 12 sq in | About 1,600 to 2,000 sq ft | Alsco Metals |
| 3 in round | Round | 7.1 sq in | Match to the downspout diameter | Area: pi x 1.5 in squared |
| 4 in round | Round | 12.6 sq in | Match to the downspout diameter | Area: pi x 2 in squared |
If your existing gutters overflow at the downspouts during hard rain, upsizing the downspout and its elbows from 2×3 to 3×4 doubles the open area from 6 to 12 square inches, and Alsco’s figures double the roof area each downspout serves. Worked example: a 1,800 sq ft roof plane draining to one corner is past the 1,000 sq ft Alsco gives for a 2×3, so either add a second 2×3 drop or switch that drop to 3×4. Sizing the whole system is covered in our roof rain gutter system design guide.
Gutter elbow angles: why most are 75 degrees, not 90
Alsco lists elbows in 30, 45, 75 and 90 degree angles and calls 75 degrees the most common choice, with 90 degrees reserved for tight areas that need a sharper bend. A gentler turn keeps water moving through the curve instead of hitting a hard corner, and a softer bend gives leaves and shingle grit less of a ledge to catch on.
- 75 degrees: the standard elbow; used for top and bottom turns and for building offsets.
- 90 degrees: a tighter turn for tight clearances, at the cost of more turbulence and clog risk.
- 30 and 45 degrees: gentler transitions for shallow offsets around trim or a gradual sideways shift.
What is a downspout offset (kick-out) and how far does it reach?
A downspout offset, also called a kick-out, uses two A elbows and a short straight section to jog the pipe out past an obstruction and then back flush to the wall. The first A elbow pushes the downspout away from the wall, a filler piece of 2×3-inch or 3×4-inch downspout spans the gap, and the second A elbow brings it back in. Two 75 degree elbows give a smooth S-curve that drains well.
The reach of an offset equals the filler length between the two elbows, so measure it on site and pitch the filler downhill so water never ponds.
- Fit the first A elbow to the outlet or pipe above, angled toward the wall.
- Hold the second A elbow against the wall where the vertical run will sit and measure the gap between the two elbow ends.
- Cut a filler to that gap plus the lap at each end, crimping the cut end if needed, sloped down toward the wall.
- Fasten each joint with two #8 sheet metal screws and check the vertical run sits flush to the wall.
How to install a gutter elbow so the joints do not leak
Install a gutter elbow by fitting the larger end of each piece over the crimped end of the piece below, then securing each joint with sheet metal screws, per the Amerimax Installing Metal Gutter Systems sheet. Every downspout and elbow has one crimped end. The rule that prevents leaks is to point each crimped end downhill so it drops inside the next piece, letting water run inside the connection rather than dribbling out a seam. When a field-cut end will not slip in, taper it with a gutter crimping tool.
- Match the sizes. Elbow and downspout must be the same size and shape, 2×3 to 2×3 or 3×4 to 3×4. A 3×4-inch elbow will not seat on a 2×3-inch pipe.
- Point every crimp downhill. The crimped end of the upper piece slides inside the larger end below, keeping water inside.
- Dry-fit the whole path first. Assemble the top elbows, any offset, and the bottom elbow before fastening, so you can adjust before committing screws.
- Screw each joint. Use two #8 zip screws, 3/8 or 1/2 inch long, on opposite faces of the joint, or pop rivets per Alsco. Longer screws poke into the water channel and snag debris.
- Strap the downspout. Amerimax calls for 2 bands per 10 ft downspout section, fastened with nails or screws, so the elbows are not carrying the weight of the pipe.
- Seal only if a joint weeps. Add a bead of gutter sealant only where a joint still drips after a hose test.
Two #8 screws per joint on opposite faces hold a 2×3 or 3×4 elbow without crushing the pipe. Screws longer than 1/2 inch, or screws driven through both walls, create snag points at exactly the turn where water is already changing direction. Once the elbows and downspout are set, carry the discharge away with a downspout extension. In areas with expansive or collapsible soils, IRC R801.3 requires roof drainage to discharge at least 5 feet from foundation walls or to an approved drainage system.
Common gutter elbow mistakes
Five mistakes cause most leaks and clogs at the turn, and each one means taking the joint apart later.
- Using a B elbow where an A elbow belongs (or vice versa): the downspout points the wrong way.
- Crimp pointing uphill: water leaks out the seam at every joint.
- Screws longer than 1/2 inch: they protrude into the flow path and catch leaves and grit.
- Skipping the pitch on an offset: a flat filler ponds water and can freeze solid in winter.
- Bottom elbow dumping at the wall: a bare 75 degree elbow releases water within inches of the foundation, short of the 5 ft R801.3 distance where that rule applies.
To see where elbows sit in the full drainage path, read what a gutter is and its parts.
Frequently asked questions
What is the difference between an A elbow and a B elbow?
An A elbow (front elbow) turns water toward or away from the wall, and a B elbow (side elbow) turns water left or right along the wall, per Amerimax’s install sheet. They are the same fitting in the same 2×3 and 3×4 inch sizes and 30 to 90 degree angles; only the direction of the bend differs. Most downspouts use A elbows at the top and bottom and add a B elbow only when the run must move sideways.
What size gutter elbow do I need?
Match the elbow to your downspout: 2×3 inch elbows fit 2×3 downspouts, which Alsco rates at about 800 to 1,000 sq ft of roof each, and 3×4 inch elbows fit 3×4 downspouts rated at about 1,600 to 2,000 sq ft. Round elbows come in 3 and 4 inch for round downspouts. A mismatched elbow will not seat, so measure the downspout face before buying.
Why are most gutter elbows angled 75 degrees instead of 90?
Alsco calls 75 degrees the most common elbow angle and reserves 90 degrees for tight areas that need a sharper bend. The gentler 75 degree turn keeps water moving through the curve and gives debris less of a ledge to catch on. Elbows are also sold in 30 and 45 degrees for shallow offsets around trim.
How do you keep a gutter elbow joint from leaking?
Point the crimped end of each piece downhill so it slides inside the larger end of the piece below, then fasten with two #8 sheet metal screws, 3/8 or 1/2 inch long, on opposite faces. Match 2×3 to 2×3 and 3×4 to 3×4, strap the downspout with 2 bands per 10 ft section as Amerimax specifies, and add sealant only if a joint still weeps after a hose test.
What is a downspout offset and when do you need one?
A downspout offset, or kick-out, uses two A elbows and a cut 2×3-inch or 3×4-inch filler section to jog the pipe out past a soffit, bay or brick ledge, then back flush to the wall. You need one whenever the gutter outlet sits farther out than the wall below it. Cut the filler long enough to slip fully over both crimps, and pitch it downhill so water does not pond.
Can you use an A elbow and a B elbow together?
Yes. Alsco notes that A and B styles can be mixed in the same downspout leg. When a downspout has to move both sideways and in or out, a B elbow shifts the run left or right along the wall, then A elbows offset it out and back. A common case is a 2×3 drop that must clear a window, then discharge straight out at grade toward a 5 ft extension.