Understanding your result
The headline is the ridge height above the floor or grade, with the rise, the HAP and the wall height that make it up. The stats show the rise, the ridge above the plate, the fascia height and the tail drop. The breakdown goes from the bottom up: span, pitch, run, rise, HAP, ridge above plate, ridge above floor, then the overhang drop and the fascia height.
The number to remember depends on the job. For framing it is the ridge above the plate: that is where the ridge board’s top edge goes, and it is what the rafter length calculator’s HAP feeds into. For a height limit or a chimney clearance it is the ridge above grade. For soffits and gutters it is the fascia height.
The gable wall under this ridge is sized by the gable area calculator from the same span and pitch, and a trussed version of the roof, with HAP set to zero, is counted by the truss count calculator.
How we calculate this
The rise is the straightforward part: for every foot of run the roof goes up the pitch’s rise per 12. What the formula adds is the HAP, because the rise is measured along the top edge of the rafter, and that edge starts at the HAP above the plate, not at the plate. On a 2×8 at 6/12 with a 3½-inch seat the HAP is 6.36 inches, which is why a 6-foot rise becomes a ridge 6 feet 6 inches above the plate.
For an off-centre ridge the two runs differ, and since the rise is the same on both sides, the shorter side is steeper: rise ÷ short run × 12. The calculator keeps your pitch for the long side and reports the other. The guide to rafter maths draws the rise, run, HAP and tail drop on one rafter.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Span | 24 ft | A small-house span; enter yours |
| Pitch | 6/12 | A typical US pitch |
| Wall height | 8 ft | Standard plate height for an 8-ft ceiling; 9 ft is common in newer houses |
| HAP | 6⅜ in | A 2×8 rafter at 6/12 with a 3½-in seat (from the rafter length calculator) |
| Overhang | 12 in | Common US eave overhang |
| Ridge offset | 0 | A symmetrical gable; enter the offset for a saltbox or an off-centre ridge |
Assumptions last reviewed October 7, 2026.
The calculator does not add the roofing thickness (about 1 to 1½ inches for shingles on plywood), does not know your floor’s height above grade, and does not apply any zoning definition of building height. The pitch calculator converts a pitch measured in degrees, and why a roof is bigger than a house explains the overhang and rise together.
Two worked examples
A 24-foot span at 6/12 on 8-foot walls
Span 24 feet, pitch 6/12, wall 8 feet, HAP 6⅜ inches (2×8), 12-inch overhang.
- Run: 12 ft; rise: 12 × 6 ÷ 12 = 6 ft
- Ridge above plate: 6 ft + 6⅜ in = 6 ft 6 in
- Ridge above floor: 8 + 6.53 = 14 ft 6 in
- Tail drop: 1 × 6 ÷ 12 = 6 in; fascia height: 8 ft + 6⅜ in − 6 in = 8 ft 0 in
The fascia top lands almost exactly at plate height, which is why a 12-inch overhang on a 6/12 roof with 2×8 rafters is so common: the soffit framing is level with the ceiling.
A 28-foot span at 8/12 on 9-foot walls with a wider overhang
Span 28 feet, pitch 8/12, wall 9 feet, HAP 7½ inches (2×10), 18-inch overhang.
- Run: 14 ft; rise: 14 × 8 ÷ 12 = 9 ft 4 in
- Ridge above plate: 9 ft 4 in + 7½ in = 9 ft 11 in
- Ridge above floor: 9 + 9.96 = 19 ft
- Tail drop: 1.5 × 8 ÷ 12 = 12 in; fascia height: 9 ft + 7½ in − 12 in = 8 ft 8 in
Nineteen feet to the top of the rafters, 20 feet with the roofing: on a crawl-space house that is about 22 feet to the ridge above grade, which some zoning limits for a single storey do not allow.
Where to find your inputs
Span. Outside of wall to outside of wall across the building, from the plans or a tape at the top plate.
Pitch. From the plans, or from a level and ruler on an existing rake board or rafter.
Wall height. Floor to the top of the double top plate: 8 ft 1 in for a “pre-cut 92⅝ stud” wall, 9 ft 1 in for a 9-foot ceiling. Use the height above grade if you want the ridge above the ground.
HAP. From the rafter length calculator for your rafter size, pitch and seat cut, or from the plans. Zero for trusses dimensioned to the top chord at the heel, or enter the truss heel height.
Common mistakes
- Forgetting the HAP. The ridge is higher than wall plus rise by the rafter’s height at the plate, 5 to 9 inches on most houses.
- Using the ceiling height for the wall. The plate is an inch or more above the finished ceiling. Measure to the top of the plate.
- Measuring the rise from the full span. Rise is per foot of run, and run is half the span on a symmetrical roof.
- Ignoring the roofing thickness. Sheathing and shingles add an inch or so at the ridge, and a ridge vent adds another.
- Treating the fascia as plate height. The tail drops at the pitch; a wide overhang on a steep roof can put the fascia a foot below the plate.
- One height for an off-centre ridge. The rise is the same both sides, but the short side is steeper. Enter the offset.
Questions people ask
- How do I calculate the ridge height of a roof?
- Rise = half the span × rise per 12 ÷ 12: a 24-foot span at 6/12 rises 12 × 6 ÷ 12 = 6 feet. Add the rafter's height above the plate (about 6⅜ inches for a 2×8 at 6/12) for the top of the rafters at the ridge, 6 feet 6 inches above the top plate, and add the wall height for the ridge above the floor: 14 feet 6 inches on an 8-foot wall. The sheathing and roofing add an inch or so more.
- How high is the ridge of a house above the ground?
- Wall height plus rise plus HAP, measured from the floor; from grade, add the height of the floor above the ground. A single-storey house with 8-foot walls, a 24-foot span and a 6/12 roof has its ridge about 14½ feet above the floor and 16 to 17 feet above grade with a crawl space. A two-storey house adds the second floor's height, usually 9 to 10 feet.
- What is HAP and why does it matter for ridge height?
- HAP, height above plate, is the vertical distance from the top of the wall plate to the top edge of the rafter at the birdsmouth. The rafter's top edge, not the plate, is where the rise starts, so the ridge is higher than wall plus rise by the HAP. The rafter length calculator gives it for your rafter size and seat cut; for trusses dimensioned to the top chord, use zero.
- Why is the fascia lower than the top plate?
- The rafter tail continues past the wall at the roof pitch, so it drops as it goes: 6 inches per foot of overhang at 6/12. A 12-inch overhang puts the top of the rafter at the fascia 6 inches below where it is at the wall, and with a 6⅜-inch HAP the fascia top ends up just about level with the plate. The calculator reports it so soffit and gutter heights can be planned.
- How do I find the ridge height for a roof with the ridge off centre?
- Enter the offset: the distance the ridge sits from the centre of the span toward one wall. The longer side keeps the pitch you entered and sets the rise; the shorter side has a steeper pitch, which the calculator reports. A 4-foot offset on a 24-foot span at 8/12 gives a 10-foot 8-inch rise, with the short side at 16/12.
- Does zoning measure height to the ridge?
- It depends on the jurisdiction. Many measure from average grade to the ridge, others to the mean height between eave and ridge, and some to the highest point of the roof surface. The calculator gives the geometry to the top of the rafters; add the sheathing and roofing thickness and check which datum your code uses.