How to build a deck on a sloped garden

Raised timber deck in garden surrounded by pine trees

If you have a sloped garden, building a deck can be a great way to create an area for dining or just a space to sit and relax. In fact, it may be the easiest way to achieve a level space.

A sloped garden can be a difficult space to use. However, one of the benefits of decking is that a deck can be built in almost any outdoor area. That means that, even if your garden is uneven or sloped, you can probably still install a deck. If you are considering decking for your sloped garden, there are a few issues to be aware of. Understanding the basic technical considerations and how you can overcome them will help you get your project off the ground.

We’ve asked our friends at the Timber Decking and Cladding Association (TDCA) to shed some light on the topic. Janet Sycamore, Director of Operations, answers our questions about building a deck on a sloped garden.

Can I build a deck in any sloped garden or are there limits?

A timber deck is a perfect structure to make the most of a sloping garden. No matter how steep the slope, timber decks are a cost-effective and practical way to create level areas from which to enjoy your outdoor space. Because wood is much easier to work with than other hard landscaping materials, features like steps and changes of level can be built in to add interest and create different zones for relaxing, entertaining or dining.

With steeper slopes, the deck platform can be cantilevered (only supported at one end) to extend the usable space beyond the support posts. This will create even more space and make the deck platform appear to float over your garden. You can even make a feature of an existing mature tree by incorporating it in your design and building the deck around it.

What do you need to do differently if you build a deck in a sloped garden, compared to a level ground?

Whether on a flat or sloping site, you will need to check which regulations you are required to meet. You will need to secure planning consent and building regulations approval for a timber deck that is more than 300mm from the ground at its highest point.

The key design consideration with sloping sites is to make sure that the posts used to support the deck frame have firm foundations. They should be made from timber with the correct strength grade to support heavy loads and the correct level of preservative impregnation to prevent decay and provide a long life.

The NHBC requires that all raised decks built on new homes conform to the TDCA Code of Practice: Raised timber deck structures on new homes to provide a desired service life of 60 years. This code represents the highest level of deck building good practices.

What common mistakes do people make when installing a deck in a sloped garden?

A common mistake is to assume that raised decks do not require planning or building regulations approval. As the property owner, you – and no one else – are responsible for obtaining all necessary permits. Failing to get planning approval or sign off from a building inspector can have serious consequences. That’s why you should check with your council planning department before you start your decking project.

When it comes to design and build, the most common mistakes are made in relation to the substructure. For example, using timber components with insufficient strength or which lack the durability (resistance to biodegradation) to provide a long and safe life. Timber deck substructures are commonly made from pressure treated, structurally graded softwood – C16 or the higher strength C24, which is best for decks on sloping sites.

You should only use softwood components factory pre-treated with an industrial preservative. The level of treatment is tailored to a component’s end use application or ‘use class’. Products in direct contact with the ground or any part of a substructure that would be difficult and costly to replace should always be impregnated to the use class 4 specification. The TDCA has some free guidance on how to make sure the treated wood you buy is fit for purpose.

Another common mistake is to use screws, nails and metal fixings that aren’t suitable for outdoor use. It may save money at the outset, but you will pay the price in the long run when problems start occurring. For example, rust can stain the wood or fixings may corrode, leading to premature failure of connections and safety issues. That’s why you should choose stainless steel or specialist coated fixings specifically designed for outdoor decking use.

If someone is looking for a professional to install their deck in a sloped garden, what advice would you give them in choosing the best person?

People generally enlist the services of a professional decking installer where a sloping site is involved. Before agreeing on a price and appointing an installer, always make sure they have the expertise to build raised, load-bearing decks.

This means you should thoroughly investigate the installer’s credentials by checking out reference projects, qualified testimonials, accreditations and confirmation of the standards they work to. It’s simply not good enough for an installer to say “I have 20 years plus experience and have always done it that way.”

Decks on sloping sites must be structurally sound and provide a long and useful life. Only put your trust in an installer who knows what they are doing. Take references and look at examples of their work. TDCA operates the DeckMark installer quality assurance and installer accreditation scheme. All members of the scheme are audited on an annual basis to ensure they are operating to TDCA good practice standards.

Are there additional maintenance tasks or considerations for a deck that is built on a slope?

Your decking will need to be swept clean every so often and washed down to keep it free from dirt and surface mildew. Some people like to give their decks a spring clean using a pressure washer and then apply a clear water repellent coating to freshen up the appearance.

The consequences of decking failing are potentially more serious with decks at height. That is why it’s wise to do more frequent safety checks, particularly as the deck gets older. Your installer may be able to offer you advice based on the specification they have worked to or even book in periodic inspections of your deck.

Would you advise using (or avoiding) any particular types of decking or fixtures when installing a deck on a slope?

The safety of those who will use the deck is the key factor. Building regulations require that where there is a danger of falling, a safety parapet should be installed. There are many different systems available, ranging from traditional timber balustrades to contemporary materials like stainless steel and sheet glass.

The important thing to remember is that the height of the parapet for decks on sloping sites must have a loadbearing capability and be a minimum of 1100 mm high. If the deck surface is up to 600mm above ground level then the minimum height of the parapet is 900mm.

Is there anything else you would add on this topic?

The TDCA is the UK advisory body for timber decking and timber cladding. They offer a technical enquiry service so, if in doubt, get in touch. The association was founded in 1998 to provide independent guidance on all aspects of deck design and construction and to set technical standards for the materials and installation practices used in their construction. The TDCA website contains inspirational images and practical guidance for anyone considering the benefits of living with a timber deck.

If you are considering installing a deck on a sloped surface, anti-slip timber decking could be the perfect choice. Opting for non-slip decking will not only give you the beautiful natural timber deck you desire, you will also have peace of mind that your decking will stay safe in wet or icy weather. Check out the Gripsure non-slip decking residential range for ideas and inspiration. If you are unsure which of our decking would work best for your sloped garden, get in touch to chat to a member of our team.

Get to know the TDCA

The Timber Decking and Cladding Association (TDCA) is a non-profit organisation based in the UK that represents the interests of the timber decking and cladding industry. The TDCA provides a range of services and resources to its members, including technical support, training and accreditation, industry news and updates, and guidance on best practices for the design, installation, and maintenance of timber decking and cladding products. The organisation also promotes the use of sustainable timber and advocates for the responsible and ethical sourcing of timber products. Overall, the TDCA plays an important role in promoting quality, safety, and sustainability in the timber decking and cladding industry in the UK.

Gripsure is proud to be a TDCA approved supplier.