Flat Roof Outbuilding Calculator

Calculate precise material quantities for your timber-frame garden building

Architectural sketch of a flat roof garden room with front overhang
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Dimensions
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Openings
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Coverings
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Advanced

📐 Building Dimensions

Start by entering the size of your building. These are the key measurements we'll use to calculate all your materials.

🚪 Doors & Windows

Add your door and any windows. You can always add more windows later or skip if you prefer a solid structure.

Door

Windows

⚠️ Opening Placement Issues

    🏗️ Coverings & Finishes

    Choose your roof covering, external cladding, and internal lining. These affect both the look and weatherproofing of your building.

    ⚙️ Advanced Options

    Fine-tune roof settings, insulation, and timber sizes. The defaults work for most buildings, so you can skip to Calculate if you're unsure.

    Roof optionsClick to expand

    Roof Options

    Firring strips create the slope on your flat roof

    Warm roof: insulation sits above the roof deck. Cold roof: insulation sits between joists with ventilation.

    ⚠️ Warm roof requires roof insulation
    • Select roof insulation to enable Warm roof option.
    InsulationClick to expand

    Insulation

    Roof insulation is required for warm roof build-up

    TimberClick to expand

    Timber Spacing

    Timber Sizes

    Complete the steps above, then click Calculate Materials to see your shopping list