Every outdoor deck relies on a subframe. Beneath the boards sits the supporting act: the unseen structure that shapes and stabilises the design above.
Choosing the right subframe for your projects means balancing performance, compliance and ease of installation. Done well, it creates the level, reliable platform that the entire deck system depends on.
Regardless of your project type or scale, the core subframe principles stay the same: joists combined with additional supports, such as pedestals, creating a stable, level base for securing deck boards.
However, the exact design will vary with anticipated use. Higher footfall areas, for example, need closer joist centres to maintain strength.
Ground-level installations offer the greatest flexibility in surface finish materials. If the system is installed directly onto the ground, a substructure is required. This frame will act as a ground stabiliser and moisture barrier beneath the subframe, and it's often made from timber appropriate for ground contact.
For complex or irregular footprints, a bespoke timber substructure system can be a good fit, with durable softwoods like Southern Yellow Pine being used. Make sure any wood has been treated to protect it from water exposure and insect damage. Once installed, it can be combined with a metal or timber subframe to support the chosen surface finish.
For projects building onto a hard standing, such as a concrete base at ground level, timber joists paired with adjustable pedestals make a suitable subframe, providing precise height control and straightforward slope correction. Both are essential for perfectly aligned surfaces.
As the height of the project increases, so do regulatory demands…
Balconies on residential buildings over 11 metres in height require all components to be Class A1 or A2 fire rated. A non-combustible subframe with a compliant surface is therefore essential.
A fully compliant build-up can be achieved by pairing these porcelain or mineral composite surfaces with adjustable pedestals and aluminium joists.
For roof terraces, BROOF(t4) regulations require the entire build-up – from membrane to surface finish – to be tested and approved as one system. Therefore, components cannot be substituted. Several of our finishes, including Zerodeck Porcelain, Zerodeck Composite and Silver FR, have been tested with polypropylene pedestals and roof systems from leading manufacturers.
The choice of subframe will often be determined by the surface finish.
For when a clean, screw-free surface is the goal, a subframe with a hidden fix system is the best option. Joists with pre-mounted clips can secure compatible deck boards to eliminate any visible fixings.
A wood-composite decking solution with aluminium joists provides good durability and slip resistance. It uses a side-clip fixing with aluminium joists that can be installed directly onto pedestals or fixed to a timber substructure.
Fulham Pier is a great example of a subframe supporting two different surface finishes, using adjustable pedestals to carry both composite and bamboo decking boards. The high live-load capability of the pedestals was critical for the promenade, which needed to carry static and moving food trucks. It’s a reminder that anticipated use should always guide subframe selection.
More Than a Sum of its Parts
At The Outdoor Deck Company, we see the subframe as the structure that brings the whole system together. It drives stability, long-term performance and the finished look.
Our team can advise on the right combination of materials to meet your project’s design intent, technical needs and regulatory requirements. Get in touch to discuss your project.