Patios will be covered in algae with 1 common mistake

When it comes to patios, all sorts of things can plague slabs – from moss to algae and even persistent puddles that just won’t go away. According to one expert, there’s a hidden mistake that’s costing homeowners thousands – and most people don’t even know they’re making it.

The problem shows up months after installation, turning what should be a source of pride into an expensive headache. Water pooling where it shouldn’t, slabs that shift and crack, algae and discolouring, and worst of all, damp problems that creep into the house itself. According to home improvement expert Dale Smith, Founder of Fence Guru, this scenario plays out in gardens everywhere, all because of one fundamental oversight.

The mistake? Failing to create a proper drainage slope when laying a patio. It sounds simple, but this oversight causes more long-term damage than any other patio planning error.

“Most people think a flat patio looks better, but that’s exactly the problem,” explains Dale. “Without the right slope, water has nowhere to go. It just sits there, and that’s when your troubles begin.”

He added: “The reason so many people get this wrong is that proper drainage planning isn’t obvious, and it’s not something you can see in photos or YouTube tutorials.

“DIYers often focus on the visual aspects like pattern and materials, while contractors sometimes rush the foundation work to save time.”

A properly installed patio needs a minimum fall of 1:80 away from your property – that’s roughly 12mm drop for every meter of patio width. It might sound tiny, but this gentle gradient is what keeps water flowing away from your home instead of pooling on the surface.

“The slope should always direct water away from your house and towards a suitable drainage point,” says Dale. “It needs to be consistent across the entire surface, not just in patches.”

In addition to poor drainage creating immediate problems, it can also cause expensive damage over time. Water that pools and freezes creates freeze-thaw cracks that split slabs apart. Water can also seep into your home’s foundations, leading to damp problems indoors.

“I’ve seen cases where water damage from a poorly drained patio has cost homeowners thousands in structural repairs,” Dale warns. “The patio itself might only be worth a few hundred pounds, but the damage it causes can be devastating.”

Warning signs your patio has poor drainage

1. Persistent puddles: Water that sits on your patio for hours after rain is the clearest warning sign. “If you can still see standing water the next morning, your drainage is wrong,” Dale notes.

2. Shifting or sinking slabs: When water pools underneath paving, it gradually washes away the foundation material. This leads to uneven surfaces and wobbly slabs that rock when you step on them.

3. Moisture around door frames: Damp patches appearing near your back door or patio doors often indicate water is flowing toward your property instead of away from it.

4. Algae and moss growth: Constant moisture creates perfect conditions for green growth. “If your patio looks like a pond after every shower, you’ve got a drainage issue,” says Dale.

How to fix patio drainage

Already got a patio with drainage problems? Don’t panic – it can be fixed, though it requires more work than getting it right the first time.

“Sometimes you can improve things by adding drainage channels or creating additional outlet points,” Dale explains. “In severe cases, you might need to lift sections and re-lay them with proper falls.”

However, the best time to address drainage is during the planning stage, before you’ve laid a single slab. Dale recommends mapping out where water will flow and ensuring you have adequate drainage points.

“Start by working out the highest and lowest points of your space,” he suggests. “Your patio should slope from high to low, with the low point directing water to a drain, lawn area, or gravel bed.”