Conservatory Roof Pitch Calculator

This calculator works out the pitch (slope angle) of a conservatory roof from two measurements you can take yourself: the vertical rise and the horizontal span. Enter those two numbers and it returns the pitch in degrees, plus the slope ratio and the roof length, so you know what angle you’re dealing with before you order glass, polycarbonate, or plan a replacement.

Formula

The pitch is the angle between the roof surface and the horizontal plane. It is calculated using the arctangent (inverse tangent) of the rise divided by the span:

pitch (degrees) = atan(rise / span) × (180 / π)

Where:

• rise = vertical distance from the top of the wall plate to the highest point of the roof (mm or inches)
• span = horizontal distance from the outer edge of the wall to the point directly under the ridge (mm or inches)
• atan = inverse tangent function, giving the angle in radians, which we convert to degrees

The slope ratio (1:x) is simply span ÷ rise, and the roof length (the actual sloping surface) is found with Pythagoras: √(rise² + span²).

Worked Example

Using the default values from the calculator: rise = 600 mm, span = 3000 mm.

1. Divide rise by span: 600 ÷ 3000 = 0.2
2. Take the arctangent of 0.2: atan(0.2) ≈ 11.31°
3. Convert to degrees: 11.31° (already in degrees, no further conversion needed)
4. Slope ratio: 3000 ÷ 600 = 1:5
5. Roof length: √(600² + 3000²) = √(360,000 + 9,000,000) = √9,360,000 ≈ 3059 mm

The calculator shows a pitch of 11.3°, a slope ratio of 1:5, and a roof length of 3059 mm.

Why Pitch Matters for a Conservatory Roof

The pitch of a conservatory roof is not just a cosmetic detail. It affects water runoff, snow shedding, glazing bar spacing, and the type of roofing material you can use. Most conservatory roofs have a low pitch, typically between 5° and 15°, because they are designed to fit under existing house eaves. A pitch below 5° is considered effectively flat and will pool water unless special membrane systems are used. Above 15°, the roof becomes more like a standard extension roof and may require different structural support.

How to Measure Rise and Span Correctly

To get an accurate pitch, you need two measurements. The span is the horizontal distance from the outer face of the conservatory wall to the point directly under the ridge (the highest point). The rise is the vertical distance from the top of the wall plate (where the roof meets the wall) to the underside of the ridge. Use a tape measure and a spirit level to ensure the span is truly horizontal. If you only have the roof’s height and the full width, remember that for a symmetrical gable roof, the span is half the total width.

Common Mistakes When Measuring

• Using the full width instead of half – for a symmetrical roof, the rise is measured from the wall to the ridge, so the span is half the total width.
• Measuring along the slope – the span must be horizontal, not the length of the roof surface.
• Ignoring the wall plate thickness – the rise is from the top of the wall, not the floor.
• Mixing units – ensure both measurements are in the same unit (mm, cm, inches).

What Pitch Can Your Roofing Material Handle?

Different roofing materials have minimum pitch requirements to function correctly:

• Polycarbonate sheets – generally need at least 5° to shed water effectively.
• Glass panels – usually require 5° to 10° minimum, depending on the frame and drainage design.
• Solid roof panels (insulated) – often need 15° or more to prevent ponding and ensure proper drainage.
• Bitumen or felt – can work at low pitches but may need additional layers or a torch-on membrane.

If your calculated pitch is below the minimum for your chosen material, water can pool, leading to leaks, moss growth, and premature wear. In such cases, you might need to raise the ridge height or choose a different roofing system.

Limits of the Calculation

This calculator assumes a simple triangular cross-section – a straight line from wall to ridge. Real conservatory roofs often have multiple facets (like a P or T shape), a central valley, or a curved lantern. For those, you would need to measure each slope separately. Also, the calculation does not account for the thickness of the glazing bars or the roof covering, which slightly alter the effective angle. The result is accurate enough for most planning and material selection purposes, but for structural calculations, always consult a qualified engineer.

Frequently Asked Questions

What is the minimum pitch for a polycarbonate conservatory roof?

Most manufacturers recommend a minimum of 5° for polycarbonate sheets to ensure water runs off. Some multi-wall polycarbonate systems can work at 3° if the sheets are laid in the correct direction and sealed properly, but 5° is the safe standard. Check the specific product datasheet for the exact minimum.

How do I measure the rise if I can’t reach the ridge?

Use a laser distance measure or a long straight edge and a level. Measure from the top of the wall plate to the underside of the ridge, holding the level vertically. If you cannot access the ridge, you can measure the height of the wall and the height of the ridge from the ground, then subtract – but ensure both measurements are taken from the same point.

Can I increase the pitch of my existing conservatory roof?

Yes, but it is a major structural change. Raising the ridge or altering the wall plate height affects the entire frame, glazing, and drainage. In most cases, you would need planning permission and building regulations approval. It is usually more cost-effective to replace the entire roof system rather than modify an existing one.

What is the difference between pitch and slope?

Pitch is the angle in degrees, while slope is often expressed as a ratio (1:5) or a percentage (rise ÷ span × 100). For example, a 5° pitch is roughly a 1:11 slope or 8.7%. The calculator gives you both the angle and the ratio.

Does the calculator work for lean-to roofs?

Yes, for a lean-to (single-slope) roof, the rise is the difference in height between the high and low ends, and the span is the horizontal distance between them. The same formula applies. For a gable or hip roof, use half the total width as the span, as explained above.

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in rise | Rise (vertical) | mm | 600 | 0
in span | Span (horizontal) | mm | 3000 | 0
out pitch | Roof pitch | degrees | atan(rise / span) * 180 / pi | 1
out ratio | Slope ratio | 1:x | span / rise | 1
out length | Roof length | mm | sqrt(rise^2 + span^2) | 0