Hipped roof: plan, elevation and a misplaced ridge
A simple rectangular house has external wall faces 10.20 m by 7.80 m. The roof is hipped at a constant pitch of 30°, with eaves level at +3.30 m and a 0.50 m overhang all round. The roof is to be drawn in plan at 1:50 and in four elevations. A junior draughtsperson has drawn the ridge 3.60 m long and placed it at +5.40 m, having scaled the height from a photograph. The planning officer has asked for a roof plan, two elevations and a section, all consistent. There is a chimney on the rear slope, 0.60 m square in plan, whose flue centre is 2.00 m from the left gable and 1.20 m horizontally up-slope from the rear eaves.
1. Calculate the correct ridge height and ridge length, and explain why the junior draughtsperson’s figures are wrong.
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The ridge height is set by the half-span and the pitch. The wall plate rectangle is 10.20 m × 7.80 m, so the half-span is 3.90 m. Rise = 3.90 × tan 30° = 3.90 × 0.577 = 2.25 m. Ridge level = 3.30 + 2.25 = +5.55 m. The overhang does not change the ridge height because the eaves remain at +3.30 m; it only extends the roof outline in plan.
For a hipped roof of constant pitch, the hips run at 45° in plan, so the ridge length equals the difference between the long and short sides: 10.20 − 7.80 = 2.40 m. The junior’s 3.60 m ridge and +5.40 m level are both inconsistent with a 30° pitch. A photograph cannot be scaled reliably because perspective and lens distortion change apparent heights; the section must be set out from the pitch and eaves level.
2. Describe how you would construct the roof plan and transfer it correctly to the elevations and section.
Read the worked answer
Start with the wall plate rectangle, 10.20 × 7.80 m. Offset the eaves line 0.50 m all round to give the roof outline, 11.20 × 8.80 m. Draw the four hips at 45° from the corners of the wall plate rectangle, and draw the ridge 2.40 m long, centred. Mark slope arrows and label the pitch. The chimney is set out from the wall faces, not from the drawing border.
In elevation, project every roof line horizontally from the section or from calculated levels: eaves +3.30, ridge +5.55. The ridge appears as a horizontal line; the hips appear as sloping lines; the eaves and fascia run horizontally. In the side elevation the ridge appears shorter or as a point depending on orientation, so check the plan orientation before projecting. In section, cut through the ridge and show the two slopes, the ceiling construction, insulation and the overhang. Consistency means the same ridge level appears in every view.
3. The chimney is 0.60 m square and sits on the rear slope. Explain how you would locate it in plan, section and elevation and what checks you would make.
Read the worked answer
In plan, set the flue centre 2.00 m from the left gable wall face and 1.20 m horizontally up-slope from the rear eaves line. Draw the 0.60 m square chimney around that centre; check that it clears the ridge and the hip line. The roof surface at that point is approximately 3.30 + 1.20 × tan 30° = 3.99 m, so the chimney must be shown passing through the slope at that level.
In section, cut through the chimney and show the flue, the roof build-up, the flashing and the height above the roof. In elevation, project the chimney’s position and show it above the roof line. Check that the chimney does not sit in a valley or on a hip, that the flashing has a sensible upstand, and that the chimney height satisfies the relevant requirement for smoke clearance.