What lies beneath?

Julian Thurbin of Wallbarn explores the key factors to consider when specifying the infrastructure beneath decking or paving that may be hidden, but is essential

A well-designed, specified and installed decking or paving area can form the crowning glory of an outdoor space. But what lies beneath these new-build or retrofitted spaces is key to ensuring their safety and long-term performance.

Many decked, paved or tiled areas are supported by a hidden substructure of pedestals or rails which must be robust enough to withstand loads placed upon them as well as being weatherproof, rot-proof, durable and long-lasting. This is especially important for public areas and if designers expect heavy traffic and/or planters, street art or heavy furniture on the suspended surface. Substructures must also offer flexibility to deal with deck falls, interruptions in the deck surface, and different surface finishes.

Therefore, understanding how the space is going to be used (domestic or commercial; public or private; fire-rated; multiple levels?) lays the groundwork for selecting the most appropriate and cost-effective substructure to support the decking or paving selected.

Paved hard landscaped areas supported by pedestals have slim gaps – millimetres wide – between pavers. These are formed when slabs are clicked into place on special lugs attached to pedestal headpieces. Lugs hold the floor elements in position and the gaps facilitate drainage, with rainfall directed into channels beneath the hard landscaping, either at street or roof level. This system also creates an opportunity to collect rainwater for recycling, on blue roofs for example. Decking will have a joist beneath the boards connected using mechanical fixings/clips. Often the joists will be laid onto adjustable plastic pedestals

Floating floor

Entry level fixed-height solutions to self-levelling, adjustable height, heavyweight and Class A fire-rated pedestals all provide a ‘floating floor’ with uniform flat surfaces separating paving/decking from the base structure, improving drainage and protecting the surface beneath.

Pedestals require no penetrations of roof membranes (there are no mechanical fixings), protect the roof finish, are lighter than bedding into mortar, and provide a quick and cost-effective way to transform areas into usable spaces. Paving can be lifted to allow access to the roof deck if required.

Specifiers and contractors have a wide choice of pedestals, substructure systems and paving/decking finishes. But there are pitfalls to be avoided and key decisions to be made to ensure the required finish and performance is achieved.

Class A?

Is the project at height, and within scope of Approved Document B, and BS 8579, BS 8579:2020 Guide to the Design of Balconies and Terraces? If so, only Class A non-combustible metal pedestals, substructures and fixings, and Class A paving can be specified in order to comply with relevant Building Regulations.

There are a number of solutions marketed as ‘fire-safe’ but it’s worth making sure that the system manufacturer/supplier can demonstrate compliance and that the pedestals do not have flammable elements such as rubber ‘gaskets.’ Expect pedestal weight tolerances of up to two tonnes and height ranges from 26 mm to 675 mm via threaded stems. Good Class A pedestal systems will be compatible with a range of height-adjustable profiled aluminium joists and rails to form a strong substructure frame to create Class A decking or terrace.

The retrospective ban on combustible materials in the external walls of high rise buildings above 18 metres (11 metres in Scotland) has compelled building owners to remove such materials from at-height balconies and replace them with Class A non-combustible products in order to certify buildings safe via the EWS1 Form. For these projects consider speaking with a supplier who can assist in creating a pedestal layout, as it can be difficult to ascertain exactly what lies beneath a wooden or PVC deck until strip-out begins. Having a supply partner that’s able to quickly and accurately estimate and lay out the height and position of pedestals is  extremely useful to specifiers.


Will the newly-created or refurbished space be open to the public, subject to high footfall, vehicular movements, and will it be supporting heavy furniture, planters etc? Understanding the usage is important to ensuring that the pedestal system can withstand the required weight tolerance long-term.

Failures can happen when an unsuitable product is installed. We were called in following the partial collapse of raised paving on a public roof area. Lifting slabs around the affected area revealed that the architect’s specification for quality pedestals with independent laboratory certification had been disregarded in favour of a cheaper alternative – injection moulded plastic pedestals with a large proportion of chemical fillers. The pedestals became brittle in cold temperatures and shattered. The original specification was not met, therefore the contractor was held liable for the claim.

What to look for? For standard plastic pedestals, a weight tolerance of 680 kg and temperature tolerance of -40°C to +75°C is more than achievable. Heavy duty plastic pedestals can offer a weight tolerance of over 1.5 tonnes each with a breaking point of two tonnes. Standard metal pedestals can withstand two tonnes.

Both plastic and metal pedestals are available in self-balancing systems, allowing installers to deliver super flat finishes even when deck surfaces are uneven (for example felt overlaps, blemishes, ridges etc) or when paving slab thickness varies.

Leading pedestal systems include all this plus self-levelling headpieces, positioning lugs, the ability to change pedestal height post-installation and a variety of heights (from 10 mm to 1 metre plus) to accommodate changes in deck heights, thresholds etc and work around deck obstructions. The substructure system of a platform of aluminium rails fitted to adjustable pedestals, then the tiles or decking fixed to them creates a laterally stable and ultra hard-wearing raft, improving the performance of the paving or decking system and allowing a greater degree of finishing and furnishing options as well as being able to accommodate more foot traffic.

Suppliers who offer a full and compatible substructure (a rail system connected to pedestals for tiles and decking) means that clients know all the individual elements connect effectively and they can source the entire system – pedestals, substructure and tiles/paving/decking – from one source.

Julian Thurbin is director of Wallbarn