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Soffit Ledger Drop

Calculate the vertical measurement from your top plate down the wall to set your soffit ledger board based on roof pitch and H.A.P.
/12
inches
inches

Standard Check

Calculates the vertical distance from the wall top plate down to the ledger board.

What is H.A.P.? 📐

In framing, H.A.P. stands for Height Above Plate. It is the vertical distance measured along the rafter from the top of the house wall plate to the top of the rafter. This measurement is critical because it determines how high the entire roof sits. If H.A.P. changes, the fascia and soffit lines change with it.

Soffit Ledger Drop

11.50"
Measure DOWN from wall plate

Rafter Tip Drop

16.00"
Total vertical drop of tail
OVERHANG RUNDROP
For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →
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What is Soffit & Fascia Geometry?

Setting the soffit line correctly requires translating the roof pitch and rafter thickness (H.A.P.) into a vertical measurement on the house wall.

Mathematical Foundation

Ledger Drop Formula
D=(O×P12)−HD = (O \times \frac{P}{12}) - H
DD= Ledger Drop (inches from plate line)
OO= Overhang Run (wall to fascia)
PP= Roof Pitch (e.g., 8/12)
HH= H.A.P. (Height Above Plate)

Laws & Principles

  • H.A.P. is the vertical distance from the plate to the top of the rafter at the seat cut.
  • If the ledger drop is positive, you measure DOWN from the top plate line.
  • If the ledger drop is negative, you measure UP from the top plate line (common on low pitches).
  • The 'Overhang Run' is the horizontal distance from the wall to the inside of the fascia board.
  • Consistency in H.A.P. across all rafters is critical for a straight soffit line.

Step-by-Step Example Walkthrough

" 8/12 pitch, 24" overhang, and generic 4.5" H.A.P. "

  1. 1. Find raw vertical drop: 24 * (8/12) = 16 inches.
  2. 2. Subtract H.A.P.: 16 - 4.5 = 11.5 inches.
  3. 3. Result: Measure down 11.5" from the top plate line to set the ledger.
Final Result: 11.5 inch Ledger Drop.

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How to Use This Calculator

  1. 1.Enter your values into the input fields above. All inputs are labelled with the expected unit of measurement.
  2. 2.Results update instantly as you type — no submit button required.
  3. 3.Use the Share button to copy a direct link to your current inputs for later reference.
  4. 4.Scroll to the Knowledge Hub below for underlying formulas, worked examples, and reference values.

Pro Tips

Best Practices

  • ✓Double-check your units. Ensure all inputs use consistent units. The calculator labels indicate which unit is expected.
  • ✓Add a safety margin. For materials and structural calculations, design slightly above the calculated minimum.

Common Mistakes

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  • ✗Don't ignore edge cases. Extreme input values may exceed the formula's valid range. Check the Knowledge Hub for valid input bounds.

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