We want to turn a Hobart attic into a bedroom. The ceiling is 2.4 m at the ridge but rakes down steeply on both sides. The designer says it can still comply. How is ceiling height measured under a sloping roof?

Probably not as described. The rule for an attic bedroom is more generous than for other rooms, but a ceiling that rakes straight down from a 2.4 m ridge still falls well short of it. The designer can only be right if the ceiling is not a simple rake, for example if it has a flat section at or near the top. [1]

The rule for a sloping ceiling in an attic. A habitable room in an attic with a sloping ceiling must have a height of not less than 2.2 m for at least two-thirds of the floor area of the room. [1] A bedroom is a habitable room, so this is the test that applies. It is a concession: a habitable room with a sloping ceiling that is not in an attic needs 2.4 m over two-thirds, and a non-habitable room needs 2.1 m. [1]

How the floor area is counted. When working out the floor area for this test, any part of the room with a ceiling height of less than 1.5 m is not included. [1] So the low edges under the rakes drop out of the calculation entirely, and the two-thirds test runs only across the floor where the ceiling is 1.5 m or higher. [1] [2]

The measurement itself. Height is measured from the floor to the ceiling. Where there is no ceiling lining, the Standard's explanatory information says the measurement is taken from the floor to the underside of the floor or roof above. [2]

Why the 2.4 m ridge is not the answer. The ridge is the highest point, not the compliance measure. What matters is how much of the counted floor sits under 2.2 m or more. With a straight rake on both sides from a 2.4 m ridge:

• the ceiling only has to drop 200 mm from the ridge before it goes below 2.2 m;
• it has to drop 900 mm before it goes below 1.5 m and leaves the count.

Because both lines are on the same slope, the share of counted floor at 2.2 m or more is 200 / 900, or roughly 22%, however steep or shallow the pitch. The rule needs at least 67%. Steeper rakes do not change the ratio; they just make the room narrower.

What would make it work. The profile has to change so that far more of the counted floor sits at 2.2 m or above, for example a flat ceiling section across the middle of the room at 2.2 m or higher. Ask the designer for a section through the room showing the 1.5 m and 2.2 m lines and the floor areas either side of them. That settles the question on paper, and your building surveyor will want to see it anyway.

Tasmanian edition note. Tasmania is on NCC 2022 Amendment 2, and there is no Tasmanian variation to this provision, so the national figures apply. NCC 2025 carries the same 2.2 m, two-thirds and 1.5 m figures, so the answer does not change when the edition moves. [3] If your approval was issued between 1 May and 4 June 2026, while NCC 2025 was briefly in force in Tasmania, confirm your governing edition with CBOS. Ceiling height is also only one of the checks for a new bedroom: light, ventilation and egress need to be confirmed separately.