Planning a Patio Layout: How to Handle Irregular Shapes, Drains, and Mixed Slabs

Indoor tiling is fiddly. Patio laying is fiddly and unpredictable.

You're not working with a nice rectangular room that has clean walls and a flat subfloor. You're working with a garden. And gardens come with curves, slopes, drain covers in awkward places, manhole frames you can't move, fence posts that don't line up with anything, and ground that was "levelled" by someone who apparently eyeballed it.

Oh, and the slabs are heavier, the joints are wider, and you might be mixing two or three different slab sizes in the same layout.

Most indoor tiling guides don't prepare you for any of this. And the standard advice, "sketch it out on paper first," works fine right up until you try to draw a curved flowerbed boundary on graph paper and realise you've been erasing and re-drawing the same section for twenty minutes.

There's a simpler way to do this. A growing number of DIYers now plan their patio layout on screen before ordering, mapping the area, placing the obstacles, and seeing how the slabs actually fit before a single one gets laid. It doesn't solve every outdoor challenge, but it solves the planning ones. And the planning ones are where most of the expensive mistakes happen.

Let's walk through the specific problems patios throw at you, and how to handle each one without wasting slabs, time, or patience.

Why Patios Are Harder to Plan Than Indoor Floors

If you've tiled a bathroom, you might think a patio is just a bigger version of the same job. It isn't. The planning challenges are fundamentally different, and most of them come down to one thing: nothing outside is regular.

The shape is rarely a rectangle. Indoor rooms have walls that (mostly) meet at right angles. Patios have whatever shape you or a previous owner decided on, and that might include curves along a lawn edge, angles where the patio meets a side return, or a tapered section where the garden narrows.

The obstacles are fixed and non-negotiable. A drain cover in the middle of your patio isn't going anywhere. Neither is the manhole frame, the downpipe at the corner of the house, the gas meter on the wall, or the fence posts along the boundary. Every one of these creates cuts, and unlike indoor obstacles, they're often positioned in the worst possible spot for your layout.

The joints are much wider. Indoor tile might use a 2mm to 3mm (1/16″ to ⅛″) spacer. Patio slabs typically have 10mm to 15mm (⅜″ to ⅝″) joints, sometimes more. That wider gap adds up quickly across a large area and shifts where the cuts fall by a significant amount. A layout planned without accounting for joint width will be noticeably off by the time you reach the far edge.

You might be using mixed slab sizes. Many popular patio ranges come as project packs: a mix of two, three, or four different slab sizes designed to be laid in a specific pattern. Planning a single-size grid is one thing. Planning a multi-size layout where each slab size needs to appear in the right proportion and the right position? That's a different level of complexity entirely.

The Irregular Shape Problem (And Why Graph Paper Gives Up)

Let's start with the most common patio planning headache: the shape of the area itself.

If your patio is a simple rectangle, say a 4m by 6m (13 ft × 20 ft) slab area against the back of the house, a hand-drawn sketch works fine. You can grid it out, count the slabs, estimate the cuts, and be reasonably confident in your numbers.

But how many patios are actually simple rectangles?

Most have at least one irregular edge: a curve where the patio meets the lawn, an angled section following a fence line, a jog around a bay window or a conservatory footing, or a step down to a lower level. Some have all of the above.

Curved edges are the real killer for hand-drawn plans. You can't draw a smooth curve on graph paper and then accurately count how many slabs it intersects. You end up approximating, and approximations at the edge of a patio are exactly where waste comes from. Every slab that crosses that curve needs cutting, and the offcut is an irregular shape that almost never fits anywhere else.

On screen, a patio layout planner lets you define that curve as part of the boundary. The tool then calculates which slabs cross it, what shape each cut piece will be, and how much of each slab gets used versus wasted. It's not magic; it's just geometry that would take you an hour by hand and the computer does in seconds.

Angled edges are nearly as bad. If one side of your patio runs along a fence that sits at 85 degrees instead of 90, every slab along that edge gets a slightly tapered cut. On paper, you'd need to calculate each one individually using trigonometry you haven't thought about since school. On screen, you draw the boundary at the correct angle and the cuts appear automatically.

The point isn't that you can't plan an irregular patio by hand. You can, if you're patient, careful, and comfortable with geometry. The point is that a digital planner does the same thing faster, more accurately, and without the erasing.

Dealing With Drains, Manholes, and Other Immovable Obstacles

This is the part of patio planning that catches people off guard, especially first-timers who've never had to lay slabs around a drain cover.

Indoor obstacles like vanity cabinets and kitchen islands are inconvenient, but they're usually rectangular and they sit against a wall (a tile layout planner handles those well). Patio obstacles are different. They're in the middle of the paving area, they're odd sizes, and they can't be moved.

Here's what you're typically working around:

Drain covers and gullies. These sit at a fixed position determined by the drainage layout. You can't shift them to suit your slab pattern. Instead, the slabs around the drain need to be cut to frame it, and those cuts are often irregular shapes that waste a significant portion of each slab.

Manhole covers. Same problem, larger scale. A standard manhole frame might be 600mm by 450mm (24″ × 18″). If it doesn't align with your slab grid (and it almost never does), you'll need to cut four or more slabs to fit around it. Some DIYers install a recessed manhole cover that sits flush with the paving, but even then, the slab layout needs to account for the frame dimensions.

Downpipes, meter boxes, and posts. These are smaller obstacles, but they still create cuts. A downpipe at the corner of the house means the adjacent slab gets a notch cut out of it. A fence post in the wrong spot means the slab next to it needs trimming. Each one is manageable on its own. Add four or five of them and the cut count starts climbing.

The Wide Joint Factor (It Adds Up Faster Than You Think)

Here's something that trips up almost every DIYer who's used to indoor tiling: outdoor joints are wide, and they eat into your layout more than you expect.

Indoor tile spacers are typically 2mm to 3mm (1/16″ to ⅛″). Over a 3-metre (10 ft) bathroom wall, the total grout width across 10 joints might add up to 20mm or 30mm (¾″ or 1¼″). That's noticeable but not dramatic.

Patio joints are 10mm to 15mm (⅜″ to ⅝″). Over a 6-metre (20 ft) patio length with 600mm (24″) slabs, you'll have 9 joints. At 12mm (½″) each, that's 108mm (4¼″). Nearly 11 centimetres of joint space.

What this means in practice:

• The slabs along the far edge of your patio will land in a different position than you expect if you planned the layout without including joint width. A slab you thought would fit neatly against the boundary might overhang it, or fall short, leaving a gap that needs a thin, ugly cut piece.
• Multi-size layouts are especially sensitive to this. In a project pack pattern, each slab size has to land at a specific position to maintain the repeating design. If the joints shift the positions even slightly, the pattern can drift and eventually misalign. Over a large patio, this becomes visible.
• Pointing compound (mortar) quantities also depend on joint width. If you've estimated based on tight indoor joints, you'll run short on pointing.

A patio layout planner that includes joint width in its calculations avoids all of this. You set the joint size once, and every slab position, every cut, and every material estimate reflects the actual spacing, not a joints-excluded approximation.

Mixed Slab Sizes: The Project Pack Puzzle

If you're using a single slab size, say 600mm by 600mm across the whole patio, the layout is relatively straightforward. But many of the most popular patio products are sold as project packs: a mix of sizes (often four) designed to create a specific laying pattern.

A typical project pack might include:

• 600 x 600mm (24 × 24″)
• 600 x 300mm (24 × 12″)
• 300 x 300mm (12 × 12″)
• 600 x 900mm (24 × 36″) (in some ranges)

The manufacturer provides a laying pattern that uses each size in a set proportion. Follow the pattern correctly and you use the pack evenly. Deviate from the pattern, or have an irregular patio shape that cuts through it unpredictably, and you can end up with leftover slabs of one size and not enough of another.

A planner that handles mixed sizes lets you lay out the full pattern in your actual patio shape and see exactly how many of each size get used, cut, or wasted. That's the only way to know whether the standard pack ratio works for your area, or whether you need to order an extra pack to cover the shortfall in one size.

Hand-Drawn Sketch vs. Digital Planner: What's the Real Difference?

Let's be fair to the pencil-and-paper method. For a simple rectangular patio with one slab size and no obstacles, a hand sketch is perfectly adequate. You can grid it out, count the slabs, and estimate cuts with reasonable accuracy.

But once your project involves any of the following (irregular edges, curves, multiple obstacles, wide joints, or mixed slab sizes) the sketch method gets slow, error-prone, and frustrating.

The sketch isn't wrong. It's just limited. And the more complex your patio, the more those limitations cost you: miscounted slabs, unexpected waste, and Sunday afternoons spent driving back to the builders' merchant for one more pack.

What to Look for in a Patio Layout Planner

If you're going to plan on screen, here's what the tool should actually do for you:

Accept irregular boundaries. Not just rectangles: curves, angles, L-shapes, tapered areas. If your patio has a straight edge along the house and a curved edge along the lawn, the planner should handle both.

Let you place obstacles at exact positions. Drains, manholes, posts, steps, with real dimensions. The tool should show which slabs are affected and what cuts are needed around each one.

Include joint width in every calculation. You set it once and the planner adjusts all slab positions accordingly. Non-negotiable for outdoor layouts.

Support mixed slab sizes and project packs. You should be able to select a multi-size pattern and see how the proportions play out across your specific shape. If you'll run short on one size, you should know before you order.

Show edge cuts clearly. Every slab that crosses the boundary should be visible, with its cut shape shown. This is how you catch thin slivers and assess total waste.

Give you a material count per slab size. Not just total area, but a breakdown by size, including waste, so your order matches what the layout actually needs.

The Bottom Line: Patios Have More Variables, So the Plan Needs to Be Better

Indoor floors are tricky. Patios are tricky with extra dimensions: irregular shapes, fixed obstacles, wider joints, mixed slab sizes, and ground conditions that don't forgive mistakes the way a flat subfloor does.

None of that means you need professional help to plan one. It means you need a plan that accounts for the real complexity of your project, not a sketch that rounds every edge and forgets the drain cover.

A patio layout planner that handles your actual shape, your actual obstacles, and your actual slab sizes gives you something a hand sketch can't: confidence that the slabs you order are the slabs you need, and a clear picture of how they all fit together before you mix the first batch of mortar.

You've already committed to the project. Spending a few minutes mapping it on screen before you start means fewer surprises, fewer trips to the merchant, and a finished patio that looks like it was planned. Because it was.

If you're getting ready for a patio project, try entering your area and slab details into a layout planner before you order. You'll see the full layout (obstacles, joints, cuts, and all) and know exactly what you need before you start.