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Warm and Cold Deck Roof

Single storey garages usually have softwood exposed rafters and if open bays, the space between the rafters is usually also left open. If the garage is to be enclosed with garage doors then we would usually supply eaves closures between the rafters to stop small birds being able to enter the garage bays. If the roof space is to be insulated such as an office above a garage or an annex, then there are a couple of options - Insulating above the rafters (warm deck) or insulating between the rafters (cold deck) Insulating above the rafters usually involves oak rafters with the rafters exposed below the plasterboard, whilst with softwood rafters the plasterboard is fitted underneath the rafters after the insulation is fitted.

Warm Deck Roof

Insulation above the roof deck How it’s built (layers from inside → outside): • Ceiling (plasterboard) • Joists • Roof deck (OSB/plywood, oak boards or plasterboard) • Insulation laid on top of the deck (usually PIR boards) • Waterproof membrane (EPDM, GRP, felt, etc.) Why it’s called “warm”? Because the roof deck stays warm — the insulation is above it. Advantages • Minimises condensation risk • Roof deck stays in the warm zone • Better for longevity • Easier to meet building regs • Simple design with no ventilation required Best for: Modern flat roofs, garden rooms, studios, oak building extensions. Warm Deck structures usually have French heeled rafters cut over the inside face of the eaves beam with oak sprockets which look like oak rafters from the outside and oak eaves closure kit. Cold deck will usually softwood rafters with a birdsmouth cut over the outside face of the eaves beam with softwood eaves closures.

Cold Deck Roof

Insulation between the joists, below the roof deck How it’s built (inside → outside): • Ceiling • Insulation between joists • Ventilated air gap above the insulation (usually 50 mm) • Roof deck • Waterproof membrane Why it’s called “cold”? Because the roof deck is above the insulation and becomes the coldest part of the structure. Advantages • Cheaper materials • Works on simple small roofs Disadvantages • High condensation risk if ventilation is not perfect • Hard to ventilate properly in real builds • Deck can rot if moisture is trapped • More fiddly to build correctly • Lower thermal performance Best for: Older roofs being retrofitted, simple sheds, basic non-habitable structures. ✅ Which is better? For almost every modern garden building, workshop, office or oak frame structure: Warm deck is the preferred and recommended method. It avoids condensation problems, is easier to get right, and keeps the structure healthier over time.

Roofworks

In most cases we supply the roof structure as part of the timber kit - this can be softwood rafters for garages and outbuildings and usually oak rafters and trusses for garden rooms, orangeries and conservatories. Roofworks generally refers to everything else in the roof structure.

Warm Deck

Cold Deck

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