Patio Level and DPC: How Far Below the Damp Course?

Your finished patio surface should sit at least 150 mm below the damp proof course (DPC) wherever it meets the house. That is the figure the government’s own guidance uses for extension walls, and it is the safe design target for a patio beside an existing wall too. You measure it in millimetres from the top of the paving to the DPC. Counting bricks is not a reliable shortcut.

This guide covers how to measure properly, what to do if you cannot reach 150 mm, how level thresholds and bifold doors change things, and how a threshold drainage channel fits in.

TL;DR

• Aim for a finished patio surface at least 150 mm below the DPC. Measure to the top of the paving, not the bedding or sub-base.
• “One brick below” or “two bricks below” is not a rule. Brick courses and mortar joints vary, so measure in millimetres.
• A patio does not need to be level. It needs a gentle fall away from the house, and the water needs somewhere real to go.
• For a level threshold at patio doors, damp protection and access rules must be designed together. A linear drainage channel is usually the safer detail, but only if it has a proper outfall.
• If you are planning a rear extension, settle the patio level before the slab is poured, not after.

Table of Contents

• What the DPC is and why paving height matters
• Where the 150 mm figure comes from
• How to measure it properly
• Is one brick or two bricks below the DPC acceptable?
• When you cannot reach 150 mm
• Does a patio have to be level?
• Level thresholds, bifold doors and the damp problem
• Threshold drainage channels: what to know
• Common mistakes and fixes
• Planning a patio alongside a rear extension
• Our view on patio levels
• Getting the levels right on your extension
• FAQ
• Sources

What the DPC is and why paving height matters

A DPC is a barrier built into the walls of a building, near ground level, to stop moisture rising from the ground into the masonry. Pavingexpert’s guide to damp proof courses explains that in most buildings less than 30 years old it takes the form of a damp proof membrane, an impermeable layer in the wall. Older houses may have slate, engineering brick or other materials instead.

The DPC works only while the wall above it stays dry. If soil, paving or standing water sits higher than the DPC, moisture can bypass it and get into the wall above. The usual result is damp patches on internal plaster, stained skirting and a musty smell by the rear door.

The 150 mm separation does two jobs, according to paving specialist Tony McCormack on the same Pavingexpert page. It limits rain splashback onto the wall, and it lifts the DPC above temporary standing water. That matters because a patio that looks perfect on a dry installation day can still cause trouble in a heavy storm if it is too high.

Where the 150 mm figure comes from

The government’s Approved Document C guidance says an extension needs a DPC in its walls at least 150 mm above ground level, made from impervious sheet material, engineering brick or slates in suitable mortar, or equivalent. You can read it in the Approved Document C FAQ.

Two points are worth understanding:

1. Approved Documents are statutory guidance on ways to meet the Building Regulations. The GOV.UK collection of Approved Documents describes them that way. They are not the only route, but they are the one building control officers know best.
2. The 150 mm figure is written for walls above ground level. For a patio against an existing wall, designers apply the same separation between finished paving and DPC. That is the sensible target, and it is what the paving trade works to.

This article covers England. Wales and Scotland have their own regulations, so check locally if you are elsewhere.

How to measure it properly

Most mistakes happen at the measuring stage, so do this before anyone digs.

1. Find the DPC. Look for a thin, continuous horizontal line in the brickwork, often a black strip or a layer of slate or dark engineering brick, usually a few courses above the soil. In many houses it sits roughly at internal floor level.
2. Decide the finished paving level. This is the top surface of the slab, not the bedding layer underneath.
3. Measure the vertical gap between the DPC and that finished surface, with a tape and a spirit level or laser level held plumb.
4. Work backwards for the dig. Pavingexpert’s typical specifications describe a build-up with a compacted crushed-stone sub-base of at least 100 mm and a bedding layer of 25 to 40 mm, plus the paving thickness itself. Add those up, then subtract from your finished level to find your excavation depth.

A worked example

Suppose the DPC is 300 mm above existing soil by the back wall. A 150 mm gap means the finished paving can be no higher than 150 mm above the soil. With 25 mm of paving, a 40 mm bed and a 100 mm sub-base, the build-up is 165 mm, so you dig down roughly 15 mm below the current soil level at the wall, and more where you want the surface to fall away. This is an illustration, not a specification: your paving thickness, sub-base and ground conditions will change the numbers.

The trap is building up from the sub-base without checking the finished height. Fifty millimetres of extra bedding mortar is enough to lose a third of your clearance.

Is one brick or two bricks below the DPC acceptable?

People ask this constantly, and the honest answer is that bricks are the wrong unit. A standard brick course, including its mortar joint, is typically in the region of 75 mm, but your wall may differ, and paving thickness and thresholds vary as well. Two courses might give you roughly 150 mm, or noticeably less.

The government guidance expresses the requirement in millimetres, not courses. So:

• Two bricks below is a rough visual guide at best. Measure to confirm it reaches 150 mm.
• One brick below is not automatically compliant. If the measured gap is under 150 mm, treat it as a constrained detail that needs a building designer or building control to look at it, with robust drainage.

When you cannot reach 150 mm

Sometimes the ground simply will not allow it. The most common cases are:

• A house on a sloping plot where the DPC is close to the garden level.
• An older property where the garden has been raised over the decades.
• A new extension with a level threshold that must be flush with a patio.

Practicing paving specialists describe a fallback for private domestic work. Where 150 mm cannot reasonably be achieved, the finished surface should still generally stay at least 75 mm below the DPC, with the owner’s informed agreement and enhanced drainage. That is explained on the Pavingexpert DPC page. Treat it as a compromise, not a licence. It does not allow paving to bridge the DPC, and it is not a casual shortcut.

If you are in this position, do these things:

• Measure and record the actual gap, with photographs.
• Talk to your building control body or approved inspector before work starts, especially if the patio is part of an extension.
• Lower the soil or hard surfacing next to the wall where you can, for example with a shallow gravel margin or a drainage channel set against the house.
• Make sure the paving falls away from the wall and that water has a defined route out.

Does a patio have to be level?

No. Despite the phrase “level patio”, the surface should be laid to a fall so rainwater drains away from the house. A common practical target is around 1:60 to 1:80, which is roughly 12 to 17 mm of drop per metre, chosen with your paving manufacturer and whoever designs the drainage. The official guidance does not set one universal gradient for every domestic situation, so site slope, surface type and where the water ends up all matter.

The real compliance concern is ponding. Water that sits against the wall, DPC or door threshold is what causes damp. Pooling after rain is the thing to check for on the first wet day after the paving goes in.

Also think about where the water goes. Falling away from the house is no good if it runs into a neighbour’s boundary or a low corner with no outlet. Decide whether the outfall is a soakaway, a gully, a drain connection or a planted border that can take it.

Level thresholds, bifold doors and the damp problem

Modern extensions often have large sliding or bifold doors, and owners want a flush step from the kitchen to the patio. That conflicts with the traditional 150 mm drop, so the two sets of rules must be handled together.

Approved Document M deals with access. It allows level or accessible approaches, but designers must take care that moisture resistance under Parts C and K is maintained. In other words, you cannot ignore one requirement to satisfy the other. Raising paving to the door threshold is not the answer.

Pavingexpert’s threshold drainage page notes that paving has traditionally been laid at least 150 mm below DPC level, usually about internal floor level, and then covers interceptor drainage solutions for level thresholds.

The usual approach combines:

• A door threshold or sill detail designed for flush access and weather resistance.
• A linear drainage channel immediately outside the door to intercept water.
• Paving falling away from that channel.
• Correct damp proofing continuity between the wall DPC, the door frame and the slab.

We cover the access side in more depth in our guide to level threshold patio doors, and in accessible extension design for M4 in England.

Threshold drainage channels: what to know

A threshold drain is a slim channel with a grating, set into the paving at the door. For a flush finish it is usually more dependable than a single point gully, because it catches water along the whole width of the opening.

Take ACO’s Threshold Drain as an example of the type. It is designed for residential thresholds and small patio edges, is rated for pedestrian loads (A15) and is about 60 mm wide. The maker states that a 10 m run can drain up to 50 m² in a 50 mm per hour rainfall event under its stated conditions. Do not carry that figure over to other products or layouts.

Installation points that decide whether it works

• Bed: ACO’s installation guidance calls for a concrete bed of at least C12/15 strength and 50 mm thick.
• Height: the channel is set 3 to 6 mm lower than the surrounding finish, so water flows into it.
• Outfall: a channel with nowhere to discharge is just a trough. It needs a connection to a suitable drain or soakaway.
• Capacity: a narrow channel has lower hydraulic capacity than larger systems. It suits a domestic doorway but not a large paved catchment draining onto it. Check the catchment area and local rainfall exposure rather than picking a grating for its looks.

If a big patio slopes towards the door, the channel is working too hard. Fall the paving away from the house and use the channel as a backstop.

Soakaways and surface water also interact with wider rules on extensions. Our note on SuDS for extensions explains the drainage side of planning a new build.

Common mistakes and fixes

If your patio is already too high, the fix depends on the cause. Sometimes removing a row of slabs beside the wall and installing a channel or gravel strip is enough. Sometimes the whole patio needs relaying. A builder or building designer can tell you after a site visit, and building control can advise where the work is regulated.

Planning a patio alongside a rear extension

If you are building a single-storey rear extension, the patio level should be fixed at design stage. The finished floor level of the new room, the DPC height, the door threshold and the patio all depend on each other. Changing one after the foundations are in is expensive.

A few points to settle early:

• Where the new DPC will sit relative to the garden, and whether the garden needs lowering.
• Whether you want a level threshold, and if so which drain and sill detail.
• Where surface water will go, including from the roof of the new extension.
• How the finished work will be signed off. Our guide to the completion certificate and building regulations for extensions explains what inspectors look for.

If you are also checking what size of extension you can build, see our pages on rear extension size limits under permitted development and extension depth rules.

Our view on patio levels

At The Extension Works we think the patio is the detail most often left until last and most often regretted. People spend months on kitchen layouts and bifold doors, then find the garden level has been left to chance.

Our approach follows from how we work. Design, planning, structural calculations and construction sit with one in-house team, so the DPC height, floor level, threshold and external levels are drawn up together in the live 3D model before anything is built. Seeing the door and the garden at the correct heights makes the step, the drain and the fall easy to talk about. It also means the compromise cases, like a sloping plot, are discussed at the start rather than discovered on site.

Our advice is simple. Treat 150 mm as the target. Measure in millimetres. If the site forces a compromise, get it reviewed and give the water a proper way out. A level-looking threshold that leaks costs far more to fix than a few extra weeks of design.

— Esskay

Getting the levels right on your extension

If you are planning a single-storey rear extension in our area, you can see a design and price online with our instant extension quote tool. Pricing is fixed and VAT-inclusive, and booking is online, with a booking fee of £499 one-off. Design, planning and construction are handled by the same team, with a 12-month workmanship warranty.

FAQ

Does a patio have to be 150 mm below the DPC?

That is the standard target, and it matches the 150 mm figure in government guidance for extension walls. Where it cannot be achieved, the finished surface should generally stay at least 75 mm below the DPC with enhanced drainage and your informed agreement. Speak to building control or a building designer before relying on that.

Is it acceptable to have a patio one brick below the DPC line?

Not automatically. Brick courses vary, so the only test is the measured distance from the finished paving to the DPC. If it is under 150 mm, treat it as a constrained detail and have it reviewed, with a drainage channel or similar protection.

Do slabs need to be two bricks below the damp course?

No, there is no dependable brick-count rule. The requirement is in millimetres, and two courses may or may not reach 150 mm. Measure the DPC and the top of the slab to check.

Does a patio need to be level?

No. It should fall gently away from the house, commonly around 1:60 to 1:80, so water does not pond against the wall or door. Pick the exact fall with your paving supplier and drainage designer, and make sure the water has an outfall.

Do I need a threshold drainage channel with level patio doors?

If the patio meets the door at a level or near-level threshold, a linear channel is usually the more reliable detail, because it intercepts water along the whole opening. It must have a real outfall and be sized for the area draining to it. A designer should agree the detail with the door sill and DPC.

Sources

• Approved Document C FAQ, GOV.UK
• Approved Document M, access to and use of buildings, GOV.UK
• Approved Documents collection, GOV.UK
• All About Damp Proof Courses, Pavingexpert
• Typical Specifications, Pavingexpert
• Threshold Drainage Solutions, Pavingexpert
• ACO Threshold Drain
• How to Install Threshold Drain, ACO

Recommended

• Level threshold patio doors: accessibility meets drainage
• 3m/4m and 6m/8m Limits: Extension Depth Rules for England Homeowners
• Foundation depth for extension: what you need to know