Best Decking for Roof Terraces & Balconies

Choosing the best decking for a roof terrace or balcony involves much more than choosing the colour and style of boards.

At ground level, decking can often be designed around relatively straightforward foundations and drainage. On roof terrace or balcony, decking becomes part of much more complex construction. Before selecting Millboard, composite, hardwood or fire-rated system, you may need to consider fire performance, Building Regulations, structural loading, waterproofing, drainage, wind exposure, balustrades and guarding, finished floor levels, door thresholds, available construction height and maintenance access.

For some residential roof terraces, premium composite or Millboard may provide excellent solution. For certain balconies and buildings, however, non-combustible A1 or A2-rated decking may be required or more appropriate.

The correct material therefore depends on the building as much as the decking itself.

Roof Terrace vs Balcony — What's the Difference?

A roof terrace is generally accessible external area created over roof — above extension, over garage, on flat roof or apartment building. A balcony generally projects from, or forms part of, building providing external platform accessed from upper storey. From decking perspective, both can involve similar challenges — elevated construction, waterproofing, drainage, guarding, structural loading and fire safety — but legal and technical requirements may differ depending on exact building.

Start With the Roof — Not the Decking

This is most important principle.

A roof terrace is fundamentally a roof first and an outdoor living space second. Roof needs to continue performing primary function: keeping water out of building. Decking installation should therefore work with existing or proposed waterproofing system.

Before constructing deck, understand waterproof membrane, drainage falls, outlet positions, roof build-up, insulation, load capacity, upstands, thresholds and maintenance requirements. Simply covering flat roof with attractive decking without considering these details can create serious problems later.

A flat roof being physically strong enough to walk across during maintenance does not automatically mean suitable for occupied terrace. Terrace may introduce loads from decking, subframe, pedestals, occupants, furniture, planters, outdoor kitchens, balustrades, pergolas and snow. Supporting structure needs to be suitable for intended use. Where structural capacity isn't already established, appropriate professional assessment may be required.

Decking Weight Matters

Every component adds load — decking boards, framing, pedestals, balustrades, furniture, planters. Large planter can become particularly heavy once filled with soil and saturated. Outdoor kitchen can add significant permanent weight. Hot tub can add extremely large concentrated load and should never simply be placed on roof terrace without appropriate structural verification. This is why complete terrace build-up matters more than board weight alone.

Fire Safety for Balcony & Roof Terrace Decking

Fire performance is one of most important differences between ordinary garden decking and elevated decking on certain buildings. Requirements can depend on building height, building use, location of decking, relationship to external wall and fire classification of specific decking system.

You may see products marketed as A1 or A2 fire-rated decking, generally relating to reaction-to-fire classifications under BS EN 13501-1. A1 represents highest classification associated with non-combustible materials. A2 represents materials with very limited contribution to fire. However classification needs to apply to actual product or system being specified — don't assume board is automatically A1 or A2 because it looks metallic.

Does all balcony decking need to be A1 or A2? No — this should not be stated as a universal rule. In England, Building Regulations contain restrictions on combustible materials in external walls and specified attachments of certain buildings. Whether particular balcony or terrace falls within those requirements depends on circumstances. For some projects, materials achieving European Classification A2-s1,d0 or A1 may be necessary. For others same restriction may not apply in same way.

For readers researching A2-s1,d0, the classification relates to limited fire contribution plus smoke and flaming droplet performance. Check actual product documentation rather than relying on advertising language. See our dedicated A1 & A2 Fire-Rated Decking guide for detailed documentation.

Can Millboard, Composite or Hardwood Be Used?

Potentially, depending on the building and application. Millboard, conventional composite and hardwood all have their own fire-performance characteristics, so none should automatically be assumed suitable for every balcony or roof terrace.

For a private residential roof terrace, available material choices may be very different from those for balcony on multi-storey residential building. Correct process is simple: establish fire-performance requirements first, then choose decking system that satisfies them.

Waterproofing Under Roof Terrace Decking

One major advantages of raised decking system is visible walking surface can sit above waterproof roof. Water passes through board gaps and reaches waterproofing layer below. Roof then directs water towards drainage outlets. This means decking itself usually isn't primary waterproof layer — waterproofing system underneath remains critical.

Don't screw through the waterproof roof without a designed detail. This is one of biggest practical rules. Randomly drilling through waterproof membrane to secure supports can create leak paths. Where penetrations are required, they should form part of appropriate designed waterproofing detail. Many roof terrace systems therefore use adjustable pedestals, load-spreading supports, floating subframes and other non-penetrating arrangements.

Adjustable pedestals are widely used because they can support raised deck while allowing waterproof roof below to maintain drainage falls, create level finished walking surface, provide height adjustment and drainage beneath deck, reduce need for roof penetrations and allow service access. But pedestal spacing and load capacity need to suit structural system above — pedestal isn't automatically suitable simply because it can physically reach required height.

Supports shouldn't unnecessarily damage waterproofing. Depending on roof system, protection or load-spreading layers may be required beneath pedestals. Roofing manufacturer requirements should therefore be checked.

Roof Terrace Drainage and Access

Water management is essential. Roof terrace will receive rain through gaps between decking boards. That water needs to reach waterproof layer and then flow towards outlets, gutters or drainage channels. Decking subframe shouldn't block this route.

Never permanently hide important roof drain beneath fixed structure without considering future access. Leaves and debris can enter through decking gaps and outlets may need inspection or cleaning. We can incorporate removable access panels where required for roof outlets, drainage channels and inspection points. Little planning during installation can prevent large areas needing dismantling later.

What happens if roof drain blocks? Water can begin to pond. Remember: 1 litre of water weighs approximately 1 kg — even relatively shallow layer spread across large roof represents substantial additional load. Roof drainage therefore isn't cosmetic, it is part of performance of building.

Flat roofs generally aren't perfectly flat — normally constructed with falls to direct water towards outlets. Decking above, however, is usually intended to provide comfortable level walking surface. Pedestal or subframe system can allow these two requirements to coexist:

sloping waterproof roof underneath + level decking above.

Finished decking height needs to be planned carefully around doors. Simply raising decking until flush with internal floor can interfere with waterproofing upstands, drainage, thresholds and damp protection. Where low or flush threshold required, roof and door detail should be considered together.

Choosing the Roof Terrace Subframe

The supporting structure needs to suit available construction height, structural loading, waterproofing, drainage and decking system. There isn't one subframe automatically best for every roof terrace.

Recycled-plastic systems can be useful in moisture-exposed environments and low-level build-ups, provided manufacturer support centres and structural limits are followed.

Structural timber may be appropriate for some installations, but prolonged moisture exposure, ventilation and drainage need particular consideration because frame remains largely shaded beneath finished decking.

Low-profile engineered systems can be useful where very little construction height is available between waterproof membrane and door threshold.

For larger, raised or more structurally demanding terraces, we can design and construct bespoke galvanised-steel subframes, measured, cut, welded and assembled on site around required levels, decking layout and available support positions. Steel provides rigid, dimensionally stable structure and can be particularly useful where design includes substantial steps, balustrades or more complex geometry.

Whatever system is used, frame must not unnecessarily obstruct roof drainage or make important outlets inaccessible.

One challenge with existing roofs is often limited available height — only relatively small distance between waterproof membrane and door threshold. Within that space you need to accommodate support system, subframe, decking board, drainage and necessary clearances. Don't choose decking board before checking whether entire build-up physically fits.

Balustrades, Wind and Privacy

Roof terrace normally requires appropriate guarding around exposed edges — frameless glass, aluminium, post-and-glass or steel systems. Balustrade creates structural loads, its fixings need to transfer loads into suitable supporting structure — should not simply be screwed into decking boards.

Frameless glass can create exceptional terrace because preserves views and produces clean contemporary appearance, but base channel requires substantial structural support. Depending on system, significant loads can be transferred into roof-edge or terrace structure.

Visually minimal balustrade often requires considerable structure underneath.

Where Building Regulations guidance applies, external balconies and similar locations commonly use 1100mm guarding height under Approved Document K in England, however precise requirements depend on location and building.

Roof terraces can experience substantially greater wind exposure than sheltered gardens, affecting balustrades, privacy screens, pergolas, furniture, planters and lightweight components. Tall privacy screens can act like sails, large parasols and lightweight furniture can become hazardous. Wind loads should therefore be considered in design of permanent structures. Don't rely on heavy planters as structural fixings — simply making something heavy doesn't automatically make it structurally suitable for wind exposure.

Roof terraces can create excellent views but also overlooking, which may affect neighbours and planning. Privacy can be improved through slatted screens, planting, carefully positioned planters, pergolas and partial screening, but objective should be to create privacy without unnecessarily enclosing entire terrace and wind loading of screens must be considered.

Lighting, Slip Resistance & Board Gaps

Roof terraces are exposed environments, so wet slip performance, drainage and correct board spacing all matter. Look for recognised product testing rather than relying solely on marketing descriptions such as anti-slip, and remember that algae, grease, ice and other contamination can affect any external surface.

Board gaps should follow decking manufacturer's requirements rather than generic measurement. On roof terraces, these gaps also allow rainfall to reach waterproof drainage layer below.

If integrated lighting is planned, cable routes, drivers and maintenance access should be considered before decking installed. Avoid unnecessary penetrations through waterproofing and keep electrical components suitable for environment and accessible where required. See Is Composite Decking Slippery When Wet?, What Gap Should You Leave Between Decking Boards? and LED Decking Lights: What to Consider Before Installation.

Hot Tubs, Outdoor Kitchens and Planning

Can you put hot tub on roof terrace? Potentially — but never assume roof can support it. Consider 1,500-litre hot tub — water alone weighs approximately 1,500kg, then add tub, equipment and occupants — very substantial concentrated load. Structural engineer should assess suitability.

Outdoor kitchens are increasingly popular on premium roof terraces, but again weight matters, fire safety and appliance clearances matter, and kitchen should be included in terrace design from beginning rather than placed randomly on finished decking. Pergolas can provide shade and define seating area, but on exposed roof wind loads can be substantial — pergola shouldn't simply be screwed into boards and structural support needs to be properly considered.

Creating new roof terrace can have significant planning implications. Unlike simply replacing surface of existing lawful terrace, converting roof into new usable terrace can affect appearance, overlooking, privacy, guarding and access. Planning permission may therefore be required. Don't assume that because roof already exists, using it as occupied terrace is automatically permitted. Check planning position before investing in decking system.

Common Roof Terrace Decking Mistakes

One of biggest mistakes is choosing decking material before establishing building's technical requirements. Board may look perfect for design but still be unsuitable because of fire performance, structural loading or available construction height.

Waterproofing is another major consideration. Randomly drilling through roof membrane to secure decking supports can introduce leak paths, while poorly positioned frames or pedestals can restrict drainage or make roof outlets inaccessible.

Finished levels also need careful planning. Raising decking simply to create flush transition through doors can interfere with thresholds, drainage and waterproofing details.

Balustrades, privacy screens and pergolas need proper structural support rather than simply being fixed through decking boards. On exposed roof terraces, wind loading can be significant even when structure itself appears relatively lightweight.

Finally, don't permanently hide parts of roof that may need future attention. Drains, inspection points and important services should remain accessible without dismantling large sections of finished decking.

Which Decking Is Best for a Roof Terrace?

There isn't one winner.

Millboard can be excellent choice where premium timber appearance and low maintenance are priorities and its specification is suitable for building.

Composite decking provides broad choice and relatively low maintenance, provided specific product meets project's technical requirements.

Hardwood offers genuine natural timber character where its use is appropriate and ongoing weathering and maintenance are accepted.

A1 or A2 fire-rated decking systems become particularly important where building's fire-performance requirements determine specification.

Porcelain and other non-combustible finishes can also form part of terrace design, either alone or combined with decking.

Best material is therefore one that satisfies building requirements first, then provides appearance, maintenance level and performance you want.

Bespoke Roof Terrace & Balcony Decking

At Decking Gurus, we approach roof terraces from structure underneath rather than choosing finished board first. Waterproofing, drainage, available construction height, structural loading, fire requirements and future maintenance access all need to be understood before final decking system is selected.

Depending on project, finished surface may be Millboard, composite, hardwood or suitable A1/A2 fire-rated system. As Approved Millboard Installers, we can advise where Millboard is appropriate while recognising when requirements of building call for different solution.

For raised and more structurally demanding terraces, our bespoke galvanised-steel subframes can be measured, cut, welded and assembled on site around required levels and layout. Other suitable support systems can be specified where lower weight, reduced construction depth or particular roof conditions make them more appropriate.

Terrace can also be designed around glass or other suitable balustrades, integrated lighting, privacy screening, bespoke seating, planters, pergolas and outdoor kitchens, with their loads, fixing positions and interfaces considered before finished decking is installed.

The starting point for a roof terrace shouldn't be “Which decking colour do I like?” It should be “What does this building require?” Once that is established, design choices become much clearer.

Useful Guides

If you want to understand the structure underneath, Timber vs Steel Decking Frames compares timber, aluminium and bespoke galvanised-steel subframes for raised and roof-terrace projects, while Decking Drainage: How to Prevent Water Problems explains why drainage routes, falls and outlet access matter beneath a finished terrace, and What Gap Should You Leave Between Decking Boards? covers board spacing for Millboard, composite and hardwood.

For performance and safety, Is Composite Decking Slippery When Wet? looks at pendulum testing and why algae affects slip resistance on exposed terraces, Does Millboard Decking Get Hot in the Sun? is important for rooftop spaces receiving prolonged direct sunlight.

And for design details that make a roof terrace usable day and night, Which Balustrade Is Best for Your Decking?, Decking Lighting Ideas: Where Should You Position Your Lights? and LED Decking Lights: What to Consider Before Installation show how glass balustrades, step lighting and concealed lighting can be coordinated with waterproofing, thresholds and cable routes from the start rather than added as afterthoughts.