Garden office roof structure
There are four key systems used to create the roof structure of a garden office. Which is used will very much depend on the preference of the company you work with and your budget.
The four systems commonly used for garden office roofs are:
- Steel insulated roof panels
- House quality SIP's
- Cold roofs
- Warm roofs
Steel insulated roof panels
Many of the cheaper garden offices on the market, make use of steel insulated roof panels. These panels have been used in commercial building for decades and have become popular with garden office designers who like the speed of installation they offer and their material efficiency.
Steel insulated roof panels are commonly used on modular garden offices. The steel layers form both the internal and external finishes. The structural panels, don't require rafters to support them. This makes them very quick to install.
Insulated roof panels are a type of structural insulated panel (SIP's). They are made up from two layers of colour coated steel with a layer of rigid insulation in-between.
As well as offering strength, the steel layers form both the interior and exterior finishes. Both layers have a protective colour coated finish and don't require any further finish.
The interior side is finished in a brilliant white colour. Depending on the manufacturer, this surface will either be smooth or have grooves in it. There are choices for the exterior colour, but typically, we see bottle green or dark grey used.
The panels interlock and because of the steel structure need minimal framing to support them. Depending on the size of the office, you may find that the walls are support enough, but on big spans, a beam may run through the centre of the room.
Removing the need for a framework means that the white side of the panel can act at the ceiling. This cuts down on material costs, a key reason why this system is popular on cheaper garden office designs.
The downside is that these panels reduce your options when it comes to lighting. Because there is no cavity in the ceiling, you do not have the option to recess downlights. A look that many buyers like. Instead, you are limited to using spotlights or track lighting. If fixed into the ceiling, you will find that trunking with the cables in is visible.
Insulated roof panels come in various thicknesses. The thicker the panel, the more insulation, the more efficient it will be. If the offices you are considering use this system, it would be worth comparing the thickness of the panels.
House quality SIP's
Structural insulated panels (SIP's) have become commonplace in garden office design. The best designs on the market, use the same SIP's panels as used in modern housebuilding. These panels can form the roof structure as well as the walls and floor.
The house quality SIP's used, are a sandwich of structural OSB and rigid insulation. The layers are bonded together at high pressure.
A SIP's roof will go together really quickly. It will be fixed into the wall panels without the need for an additional framework. On wide spans, you may find that a supporting beam is run through the centre of the room. To avoid this, many garden office ranges come with a maximum depth of about 4 meters.
House quality SIP's panels are often chosen when a highly insulated garden office is required. Because they are used in house building, it is easy to specify the right thicknesses to achieve or exceed the Building Regulation targets for insulation in a new house.
With the sandwich like buildup of a structural insulated panel. You don't have a void for cabling and recessed lighting to be incorporated. Garden offices with SIP's roofs, do typically have recessed downlights in the ceiling. This is possible because a service void is created between the SIP's panel and the plasterboard which forms the internal lining.
Cold roofs have been used since the early days of garden office design. They are commonly used on timber frame designs. They are made-up with several layers of material.
Cold roofs where the insulation sits between the rafters needs a ventilated airspace above. They are starting to go out of fashion in garden office design with SIP's panels and warm roofs superseding them.
Rafters are used to create the roof framework, and insulation is fitted snuggly between the timbers. We talk about the different insulations used in garden office design, in greater detail here.
It is vital that there is a continuous ventilated airspace above the insulation. This means there need to be soffit vents at each end of the roof.
If a roof covering such as EPDM is going to be used. A roof deck is fixed over the airspace into the top of the rafters. This deck will be either plywood or OSB. This deck helps strengthen the roof framework and forms an excellent basis for the roof covering.
If you have chosen a tiled roof covering such as Cedar shingles. This deck will be replaced with treated battens.
Over the deck or under the tile battens, depending on the roof design. A breathable membrane will be laid. This layer will make the roof temporarily weather tight prior to the final roof covering being fitted.
Cold roofs like this, have largely gone out of fashion because of having to maintain the ventilated airspace. Also, because the insulation is fitted flush with the bottom of the rafters. It can limit your options when it comes to choosing recessed lighting in the ceiling. This is because of the risk of fire. It all depends on the mix of insulation used and the downlight system selected.
Warm roofs have superseded cold roofs in quality timber frame garden office designs. Unlike cold roofs, they do not need a ventilated air space, as the insulation sits on top of the rafters.
Warm roofs where the insulation sits above the rafters has become popular in garden office design. Warm roofs are popular as the insulation covers the whole roof, so there are no 'cold spots'.
A warm roof is made-up from several layers of material. A frame of timber rafters has a deck of OSB or plywood fixed into the top. This helps to strengthen the structure and provides a sound basis for a vapour membrane layer. This layer is essential as it helps control condensation.
A rigid insulation such as Celotex is them laid over the vapour membrane. Because the insulation covers the whole of the roof, rather than being fitted in sections between the rafters. There are no cold spots.
A second deck of OSB or plywood is then fitted over the insulation and fixed into the rafters. A specialist long, twisted fixing is used for this, to ensure a secure fixing is made into the rafter. A breathable membrane is then fitted over this second deck, followed by the roof covering.
Because the insulation is fitted on top of the rafters, there is no problem in fitting recessed downlights in a warm roof build-up.
This guide has looked at the different roof structures used in garden office design. There are then several different roof coverings which can be fitted on top. We explore them in our roof coverings guide.