Is laying decking on grass a good idea? Experts reveal the problems and solutions

Laying decking on grass might seem like a quick and easy option. It involves little or no preparation, and you can have a new deck laid in just a few hours. But, there are a few downsides that will become obvious a lot quicker than if you laid a deck the right way.

You don’t want to spoil your garden decking ideas, you want them to last and look good for as long as possible. And laying decking directly on grass isn’t the answer. Here, the experts reveal the problems that can happen and the steps you need to take to make your deck last longer.

Mark Irving

Mark has over 20 years of experience in the building sector. He led Build & Plumb’s expansion into the online marketplace. He has a wealth of knowledge surrounding building and renovating that he enjoys sharing with others.

Can I lay decking directly onto grass?

It’s not whether you can lay decking directly onto grass; it’s more of a question of whether you should or not. Decking expert Mark Irving from Build & Plumb says, “Whilst it’s possible to lay decking onto grass, it’s not recommended as it will make the decking highly susceptible to water damage, which can result in the development of mould and wood rot."

But, it's not just water ingress that can cause problems as Irving adds, “Furthermore, grass can attract a variety of insects under your decking which can cause damage by burrowing into the wood.” He adds, “It also has the potential to create a visual eyesore as over time grass and weeds may grow through the slats in between the decking.”

Thomas Oldham, owner of UK Construction Blog says, “The grass needs to be removed, and the ground levelled first.” Alongside the issues Irving mentions, Oldham adds, “ If you don’t, the decking could shift or sink.”

A sinking deck will ultimately need to be replaced, typically much sooner than a properly installed deck.

Try these out for laying decking

Thomas Oldham

Thomas Oldham has spent the past four years using his expertise of working in the engineering and construction industry for over a decade to develop the UK Construction Blog - a leading online platform dedicated to empowering individuals and businesses in the construction industry.

Should I remove the grass under decking?

The simple answer is yes. Irving says, “The best solution is to completely remove the grass prior to installation and use a grass killer to prevent the likelihood of weeds and grass growing in between the decking boards.”

Oldham adds, “Yes, you should remove the grass. Leaving it there could cause unevenness, create a perfect spot for weeds to grow, or lead to moisture build-up.”

But removing the grass might not be as obvious as it seems. Irving adds a point to consider: “However, this must be thoroughly considered, as in the long term, it can cause issues if you choose to remove the decking and grow the grass back in this area.”

If you recently removed decking and want to cover it with grass, check out our How to Lay Turf guide for expert insight.

What do I need to put down before decking?

Before any decking is installed, you must prepare the ground to ensure that the area is in optimal condition for the longevity of the decking. Oldham advises, “To establish a solid foundation, remove the grass and level the ground.”

Once the decking area is prepared and ready, Irving suggests, “You should lay down a weed protection membrane to prevent weeds from growing under the decking.” He adds, “Make sure to cover the entire area beneath the decking completely, as weeds can find their way through even the smallest gaps in the membrane.”

Once a weed membrane has been added to the ground, the next step is to add a layer of gravel. As Irving says, “It’s important to place a layer of gravel on top of the membrane. Not only will this keep it in place, but it will improve drainage below the decking and prevent the accumulation of moisture.”

He adds, “Stagnant water can significantly damage the structure of the decking and lead to mould or wood rot."

What is the minimum gap under decking?

While laying decking directly on grass is not recommended, leaving too small a gap between the ground and deck will also cause issues. Oldham says, “Leave a gap of at least 150mm (6 inches) between the decking and the ground.”

He adds, “This allows for airflow, which reduces moisture build-up and stops the timber from rotting.” You can lay decking closer to the ground if needed, but it's not recommended. If you do, make sure that a weed membrane is well-fitted, and that there is good drainage.

FAQs

Should I use C16 or C24 timber for decking?

When laying a timber deck, it is essential to select the best wood for decking, budget permitting. A common choice is C16 or C24 timber, as this rating defines specific characteristics of the two.

Irving states, “Both C16 and C24 timber can be utilised for decking, but typically, C16 timber is preferred as it is more economical and suitable for outdoor applications."

He adds, “However, C24 timber is deemed superior since it is a stronger grade of timber, making it more durable and capable of supporting heavier loads.”

Regardless of the timber you select, both must be treated. As Irving points out, “Both C16 and C24 timber should always be pressure-treated prior to outdoor use to prevent wood rot, weather or insect damage, and warping.” If left untreated, they will deteriorate much faster than treated timber.

How far apart should decking boards be?

Decking needs to have a gap between the boards; otherwise, it will lead to unwanted issues. Irving suggests, “For the majority of decking, there should be a 3mm to 6mm gap (though this can be as wide as 10mm) between decking boards, as this allows for expansion and contraction of the boards when the temperature or moisture changes.”

If the gap is too small, Oldham says, “You risk the boards warping or collecting water, which could lead to damage or make your deck slippery.” Check out our How to make decking non-slip guide to ensure your decking is safe to walk on.

If you’re thinking of building a new deck (and not laying it on grass), check out our small decking ideas and decking lighting ideas guides for inspiration. Once you have your new deck in place, it can become slippery, especially if it gets wet frequently. Anti-slip decking strips are a cost-effective solution to keep your deck safe.