How many whirlybirds do I need for my house?

The standard rule is one 300mm whirlybird per 50–55m² of ceiling area. A 2-bedroom home (~90m²) needs about 2 units, a typical 3-bedroom home (~150m²) needs 3, and a 4-bedroom home (~220m²) needs 4. Add one extra unit for a dark-coloured roof and one more for a steep roof pitch, as both increase roof-cavity heat and volume.

How much do whirlybirds cost to install in Brisbane?

In Brisbane, whirlybird installation costs from $299 for a single unit and $699 for two units, fully installed and including GST. Each additional unit beyond that adds around $400. A typical 3-bedroom home needing 3 units costs around $1,099 installed.

Where should whirlybirds be positioned on a roof?

Whirlybirds should be installed near the ridge line — the highest point of the roof cavity — where the hottest air collects. On long roofs, space units evenly along the ridge rather than clustering them. On hip roofs, place units on the two longest roof faces for even airflow.

Calculator 5 min read 9 June 2026

How Many Whirlybirds Do I Need? Free Brisbane Calculator (2026)

Name: Mike's Whirlybirds
Address: Brisbane, QLD, 4000, AU
Telephone: 0447 890 868
Price range: $$
Rating: 5

Free whirlybird calculator for Brisbane homes — enter your house size and instantly see how many whirlybirds you need and the fixed installed price. Plus the full formula Mike uses.

Mike — Mike's Whirlybirds

Brisbane Roof Ventilation Specialist

One of the most common questions Mike gets before a job is: "How many whirlybirds do I actually need?" Get it wrong — too few and the ventilation is inadequate; too many and you've spent more than necessary. Use the free calculator below for an instant answer with a fixed Brisbane price, then read on for the full formula.

Whirlybird Calculator

Enter your home details for an instant estimate and fixed Brisbane price.

Approx. ceiling / floor area (m²)

Not sure? A 3-bed house is ~150m², a 4-bed ~220m². Use the floor area under the roof only.

Roof colour

Roof pitch

Estimate only — based on 1 unit per 55m². Fixed Brisbane pricing: $299 for one, $699 for two, then +$400 per extra unit (inc. GST). Mike confirms the exact price over the phone.

The Standard Formula

The widely accepted industry standard for residential roof ventilation is:

One whirlybird per 50 m² of ceiling area

This assumes a pitched roof with standard ceiling insulation and adequate eave venting.

This formula comes from ventilation research including CSIRO studies on residential roof spaces in Australian climates. It provides sufficient air changes per hour to meaningfully reduce roof cavity temperatures.

Quick Reference by Home Size

Home Type	Approx. Ceiling Area	Recommended Units	Fixed Installed Price
2-bed cottage	~80–100 m²	2 units	$699
3-bed house	~130–160 m²	3 units	$1,099
4-bed house	~180–220 m²	4 units	$1,499
Large home / Queenslander	250+ m²	5+ units	From $1,899

* Fixed Brisbane pricing, inc. GST: $299 for one unit, $699 for two, then +$400 per additional unit installed on the same visit.

Factors That Increase the Number of Units Needed

High Roof Pitch

A steeply pitched roof creates a larger air volume in the roof cavity than a low-pitched roof over the same floor area. More volume = more hot air to extract. If your roof pitch is 30° or more, consider adding an extra unit.

Dark Roof Colour

Charcoal, dark grey, and black roofs absorb significantly more solar radiation than light-coloured roofs. A dark Colorbond roof on a 35°C Brisbane day can reach 80°C — versus 65°C for the same roof in cream. If your roof is dark, add one extra unit to the formula.

Poor or No Eave Venting

Whirlybirds extract air from the roof cavity, but they need cooler air to flow in from the eaves to replace it. If your eaves are closed (no vents, or vents blocked by insulation), the whirlybirds won't work to their full potential. Mike will ask about your eave vents when you call for your free quote.

Hip Roof vs Gable Roof

A hip roof (four sloping sides) typically has better natural air cross-flow than a gable roof. Gable-end roofs can develop dead spots — areas where hot air gets trapped. If you have a long gable roof, Mike may recommend positioning units at each end rather than clustered together.

Multi-Storey Homes

Only the roof cavity needs to be ventilated — the ceiling area above the top floor. A two-storey home with a 150 m² footprint has roughly 150 m² of roof cavity, not 300 m².

Placement Matters Too

The right number of units is only part of the equation — placement is equally important. Whirlybirds should be installed near the ridge line (the highest point of the roof cavity) where the hottest air accumulates.

For long roofs, spread units along the ridge rather than clustering them together. For hip roofs, Mike typically places units on the two longest roof faces.

Can I Have Too Many Whirlybirds?

Not practically speaking — more ventilation is generally better. The main downside is the unnecessary cost. In extreme cases, excessive ventilation can cause minor moisture ingress if your eave venting is very limited (the slight suction can draw humid air in), but this is uncommon in practice.

If you're unsure, start with the recommended number — Mike will give you a clear recommendation and a fixed price over the phone before any work begins.

The Best Approach: Get a Free Phone Quote

Mike's Whirlybirds offers a free phone quote to every Brisbane homeowner. He'll ask about your roof size, pitch, and existing vents, then give you a clear recommendation and a fixed price — all over the phone. No guesswork, no overselling, no site visit needed just for a quote.

Call 0447 890 868 for a fixed price, or request a free quote below and Mike will call you back.