Garden Room Frameworks
Published: 4 July 2024
Reading Time: 4 minutes 46 seconds
You will often hear us talk about how garden rooms are multi-layer structures, with each layer playing a crucial role in the strength and durability of the building. The most important layer in this build-up is the framework. Just as our skeleton keeps our bodies strong and stable, the framework ensures that a garden room remains sturdy and resilient, capable of withstanding daily use and the forces of British weather.
There are several different construction systems used in garden room design. It’s difficult to say that one is better than the others, as they all have their unique merits and produce strong cores for the building. Garden room designers often have specific criteria for their choices, such as speed of installation or the ability to outsource the production of the frame, as seen with options like Structural Insulated Panels (SIPs) and timber cassette systems. Companies with skilled carpenters on their team may prefer the flexibility of traditional timber framing, while those looking to enhance the eco-credentials of their buildings might opt for steel framing systems, which can be fully recycled at the end of their lifespan.
We are going to look at the four construction systems regularly used to create the core structure of a garden room. It’s worth noting that while the principles of these systems are common throughout the industry, companies do take different approaches to how they are implemented. For example, in traditional timber framing, many companies use 4”x2” timbers to frame their rooms, while others use 6”x2” or 8”x2”. Designers might choose thicker timbers to create a more robust structure and allow for more insulation.
The same variation is true for Structural Insulated Panels (SIPs) and insulated timber cassette systems. A company might advertise a SIPs framework, but upon closer inspection, you may find that company A uses a 75mm SIP, company B uses a 150mm SIP, and company C a 200mm SIP.
When comparing quotes, a higher price might be due to the use of thicker timbers or panels, accounting for the additional cost.
Traditional Timber Frames
Traditional timber framing techniques, where frames are made up of timber headers and footers joined together with studs and strengthened with noggins slotted between each of the studs, have been popular since the early days of garden room design.
A carpenter can quickly create the frame, offering the flexibility to create openings where needed and to overcome site challenges such as angled walls and difficult access with relative ease. Some companies pre-make their frames off-site and bolt them together on-site, while others deliver the timber straight to the site and build the frames from scratch in the garden.
4”x2” timbers are commonly used for the wall frames, with 6”x2” timbers for the floor and roof structures. Once the frames are made, a layer of timber sheathing board, such as OSB or plywood, is fixed to the outer surface. These boards help strengthen the frames and prevent racking – twisting of the frame.
Light Gauge Steel Frames
Some garden room designers have moved away from using timber and instead use light gauge steel framing systems. These systems offer many benefits: they are lightweight and easy to manoeuvre, quick to assemble, and can be recycled at the end of their useful life. Steel framing systems also have A1 fire resistance ratings.
Light gauge steel framing shares much in common with traditional timber framing, as the metal studs slot between header and footer plates. Some designers strengthen the frame with horizontal noggins fitted between the studs, while others use diagonal braces.
Metal framing systems have a C-shaped profile ideal for slotting insulation into. The metal studs often have holes, allowing for electrical cabling to be easily run around the building without cutting into the frames, further enhancing installation speed.
Like timber frames, light gauge metal frames usually have a sheathing layer to enhance strength and stability. OSB or plywood is often used, but designers aiming for a wood-free design or Building Regulation compliance may use a cement particle board instead.
Structural Insulated Panels (SIPs)
Structural Insulated Panels (SIPs) are widely used in garden room design. Designers like them because they are delivered to the site pre-cut and ready to install, creating a highly insulated envelope for the building. When installing a SIP, you install the core framework, sheathing layer, and insulation in one go, significantly reducing on-site build time and saving money.
A SIP is a sandwich of rigid insulation and OSB boards, creating a structurally robust, highly insulated panel. The panels are joined together in a tongue-and-groove system using insulated spline joints. The top and bottom of the panels are tied together with timber plates.
If your design incorporates large glazed openings like bi-fold doors, the designer will incorporate steel I-beams.
Typically, SIPs garden room specifications use 122mm SIPs, which have 100mm of insulation and 11mm OSB skins on each side. Some companies use thinner 75mm SIPs, while others use thicker SIPs – the thicker the SIP, the more insulation.
The best-performing SIPs are filled with Polyisocyanurate (PIR) and Polyurethane (PUR) insulation, but Expanded Polystyrene (EPS) and Extruded Polystyrene (XPS) are also used. These options are more cost-effective but less efficient, so bear that in mind when comparing specifications.
Insulated Timber Cassette Panels
Timber cassette panels share much in common with SIPs. They can be manufactured to size off-site and delivered with the insulation in place for quick assembly. Like SIPs, a garden room built using insulated timber cassette panels can have the framework completed in just a few hours.
With insulated timber cassette panels, a box is constructed using a sheet material like plywood or OSB and a timber perimeter frame. Insulation is fitted into the void in this box and then topped off with another layer of sheet material. This combination is very strong and robust. Like SIPs, this system means you are installing the core framework, sheathing, and insulation layers in one go.
The thickness of timber cassette panels varies between companies. Many use 4”x2” timbers paired with 18mm sheet materials, but some have created lighter-weight versions using 2”x2” timbers with 12mm sheet materials. These thinner walls result in slightly more interior space.
Conclusion
When choosing a garden room, it's easy to get focused on the interior and exterior finishes. However, it’s important to look beyond these layers and explore the core of the building – the framework. The framework is the backbone of your garden room, dictating its strength, durability, and overall performance.
We encourage you to ask companies about the construction systems they use. Understanding whether they opt for traditional timber frames, light gauge steel frames, Structural Insulated Panels (SIPs), or insulated timber cassette panels can provide insight into the quality and longevity of the garden room you are investing in. Additionally, make sure to ask about the dimensions of the timbers, steel frames, or panels used. This will help you compare your options accurately and ensure you have the full picture.
By prioritising the structural integrity of your garden room, you ensure that it will not only look fantastic but also stand the test of time, providing a reliable and comfortable environment for years to come.
