Offsets

Seam heights and filler pieces for parallel offsets, two-elbow 90s and offsets off a floor or wall.

Parallel offset

Job examples

Jog around a beam (BCIT C4)8" round, 8" throat, two 45s, offset 24 7/16, overall 48 7/8.Filler #1 24 9/16, Filler #2 14 7/16.
Drop under a sprinkler main14 × 10 (14 in the bend), square throat, two 22.5s, drop 12".Filler #1 28 5/8, needs at least 31 3/4 of run.
Rolling offset6" round, 6" throat, 45s, up 18" and over 12".True offset 21 5/8, roll the elbows 33.7°, Filler #1 23 1/8.

How it's calculated

Centreline radius (CL) = throat radius + ½ the duct size in the bend. Seam height (SH) = CL × tan(elbow angle ÷ 2), rounded to the nearest 1/16". Travel = offset ÷ sin(angle), run = offset ÷ tan(angle). Filler #1 = travel − 2 SH. Filler #2 = overall length − run − 2 SH, with the overall length measured from the start of the first elbow. Rolling offset: true offset = √(offset² + roll²).

Two-elbow 90° field measure

Job examples

Join two pipes (BCIT slide)10" pipes at 90°, 10" throat, 60" and 50" to the far edges.Filler about 50".
Rect tie-in (BCIT)14 × 14, square throat, 140 1/2 and 84 to the far edges.Filler 144 9/16.
Measured to the near edges12 × 8 (12 in the bend), 6" throat, 40 and 25 to the near edges.Filler 38 7/8 (BCIT) / 38 15/16 (exact).

How it's calculated

BCIT method: get centre distances first. Measured to the far edge of the other duct (like the BCIT slides: 60" and 50" become 55" and 45" on 10" pipe), take ½ the duct size off. Measured to the near edge, add ½ the duct size. Elbow angles come from those two distances (tan θ = offset ÷ overall). Seam height = CL × tan(elbow angle ÷ 2), rounded to the nearest 1/16". Take each seam height off its leg, find the hypotenuse, then filler = hypotenuse − both seam heights. CL = throat radius + ½ the duct size in the bend.

Exact fit: the filler is tangent to both elbows, so it equals the straight distance between the two elbow radius centres: √((overall − CL)² + (offset − CL)²), with angles from those same two numbers. It lines up without adjusting the elbows.

Two elbows off a floor or wall set elbow angle, Filler #1 and #2

Job examples

Up through the floor into a wall (BCIT Q4)14 × 10, CL 21, 45s, 25 3/8 to the bottom of the wall duct, 58 1/2 to the wall.Filler #1 16 1/8, Filler #2 26 1/8.
Floor riser to a wall opening10" round, CL 15, 45s, opening centre 40" up, 60" over.Filler #1 35 7/16, Filler #2 20.
30° and 60° pair12" round, CL 18, 30° off the floor, centre 36" up, 50" over.Filler #1 20 13/16, Filler #2 21 5/8.

How it's calculated

Get centre distances first: measured to the near edge of a duct, add ½ the duct size; to the far edge, take ½ off. Seam height (SH) = CL × tan(elbow angle ÷ 2), rounded to the nearest 1/16". The first elbow starts at the floor, so take one SH off measurement 1 to get the rise between the elbows. Travel = rise ÷ cos(angle). Filler #1 = travel − both seam heights. Filler #2 = measurement 2 − rise × tan(angle) − the second seam height.

For reference only. Verify against project drawings, specifications and manufacturer data. Spotted something off? Let me know.

← All tools and charts