Manhole Cover 315 or 400 – How to Choose the Right Size?
Manhole Cover 315 or 400 – How to Choose the Right Size?
At first glance, a trivial question that can cause quite a headache in practice. You arrive at a construction site, there is a manhole in front of you, and you realize you ordered the wrong size. Or, even worse, you order a cover for a manhole you haven't seen yet, knowing only that "there is a drainage manhole somewhere in the garden and it needs a new cover". In such a situation, it is important to know exactly what each dimension on the cover means, how to measure it, where each diameter is used, and what to pay attention to when choosing. This article will cover both common sizes – 315 and 400 mm – from the basics to the details visible only during actual installation.
What does the number on a manhole cover actually mean?
When we talk about Manhole Cover 315 or Manhole Cover 400, the number refers to the nominal outer diameter of the manhole to which the cover fits. More specifically, it refers to the outer diameter of the manhole pipe body (manhole ring or manhole pipe), not the diameter of the cover itself. The cover is always slightly larger in overall size because it must extend beyond the edge of the manhole and create a secure support.
In practice, this means: if you have a drainage or inspection manhole dug into the ground composed of a PE pipe with an outer diameter of 315 mm, you will need a 315 cover. If the outer diameter of the manhole pipe is 400 mm, you will need a 400 cover. This sounds simple – and in most cases it is – but complications arise when the manhole consists of multiple components from different manufacturers, when it is partially buried and difficult to measure, or when it is unclear whether it refers to the outer, inner, or clear diameter.
Typical use of 315 mm manhole – where you will encounter it
A manhole with an outer diameter of 315 mm belongs to the category of smaller inspection and drainage manholes. In practice, you will most often encounter it in the following situations:
- Garden and playground drainage – drainage manholes for collecting and draining rainwater from grassy areas, flower beds, or planting areas. These manholes are usually shallow, part of a drainage grid, and do not require large internal access.
- Inspection manholes on small connections – for example, on sewer connections DN 110 or DN 125, where it is not necessary to physically enter the manhole, but access is needed for flushing or camera inspection.
- Water supply manholes for private properties – manholes for water meter assemblies, check valves, or shut-off valves on private plots. In combination with, for example, sewage backflow valve DN 110 or sewage backflow valve DN 125, this is a very practical solution for protection against backflow.
- Manholes under terraces and roads with limited loading – a smaller cover diameter means a smaller area loaded by possible traffic, which is sufficient for light traffic or garden paths.
For a 315 mm manhole, the clear (inner) diameter typically ranges from 270–290 mm depending on the wall thickness of the specific product. This is the dimension that defines what actually passes through the manhole – for example, the hose diameter for flushing or the size of the fitting you need to insert from above.
Typical use of 400 mm manhole – more space, greater flexibility
A manhole with an outer diameter of 400 mm is significantly more versatile in practice. A larger clear diameter (usually around 340–360 mm) allows for easier access to components inside and also allows for the insertion of larger fittings or valves. Typical applications include:
- Inspection manholes on main sewer connections – including connections DN 150 and DN 200, where convenient access is needed for cleaning, inspection, or maintenance.
- Manholes with built-in fittings – if a larger diameter backflow valve is installed inside the manhole, for example, sewage backflow valve DN 200, you need enough space not only for the valve itself, but also for its maintenance and possible gasket replacement.
- Drainage manholes at collection points – at locations where multiple drainage grids converge, or where the manhole is a catch point for larger amounts of water. A larger diameter is welcome here because it allows for easier cleaning of the manhole bottom.
- Manholes near residential buildings with higher surface loading – a 400 mm manhole cover can be manufactured in a more robust version capable of withstanding greater mechanical loading, for example, from light garden equipment.
How to correctly measure an existing manhole
This is the most important step in practice and also the one where mistakes are most commonly made. If you need to replace a damaged, missing or unsuitable cover on an existing manhole, proceed as follows:
Measuring the outer diameter
The outer diameter of the manhole is measured with a caliper (if the manhole is accessible and relatively small) or with a measuring tape wrapped around the top edge of the manhole pipe. In the second case, you get the circumference, which you divide by the number π (3.1416) to get the diameter. For example: circumference 995 mm → 995 ÷ 3.1416 ≈ 317 mm → so the manhole is 315 mm.
Attention: ceramic or concrete manholes may have completely different dimensions than PE manholes. This article is exclusively about PE (polyethylene) manholes, where the dimensions 315 and 400 mm are standardized.
Measuring by visual comparison with the old cover
If you still have the original (even damaged) cover, look at its underside. Most manufacturers indicate the nominal size directly on the product – either cast in plastic or with an attached label. If not, measure the inner diameter of the flange of the cover – it should correspond to the outer diameter of the manhole.
What to do when the manhole is not accessible
In practice, it happens that the manhole is covered with soil, grass or paving. In this case, the simplest aid is the project documentation of the building, where the type and diameter of the manhole should be indicated. If the documentation is missing (which is not uncommon in older single-family homes), you can try to identify the manufacturer of the manhole according to the visible part of the pipe above ground and verify the standard dimensions with the manufacturer.
Material and construction of the cover – what to look at besides the dimensions
The dimensions themselves are only one of the properties of the cover. When choosing a cover, the material it is made of, its resistance to load, tightness and whether it is equipped with a suspension or a safety device against accidental opening also play a role.
Polyethylene (PE) vs. polypropylene (PP) vs. cast iron
For garden and light applications (drainage, inspection of a single-family house), PE or PP covers are common. They are light, non-corrosive, cheap and easy to replace. Their disadvantage is lower resistance to mechanical load – under heavy vehicles they could bend or crack. Cast iron covers of classes A15 to D400 are used on roads, but for common garden applications they are overdimensioned and financially inefficient.
Sealing and watertightness
Not all covers are sealed. Some are simply placed on the edge of the manhole without sealing – surface water can thus penetrate into the manhole. For drainage applications, this can be desirable (water should enter the manhole precisely from the top), but for inspection manholes, where we want to control what enters the manhole, it is better to have a cover with a rubber seal or a recess that minimizes the seepage area.
Lockability and safety device
In places where accidental opening is a risk (for example, in a garden with children, on publicly accessible areas), it is advisable to choose a cover with a safety device or lock. Most common PE covers have a simple mounted or snap-on connection without a lock – which is usually sufficient for a private property.
Manhole cover and backflow valves – how they are related
The manhole cover itself does nothing – it is just a cover for access. Important are the components inside the manhole. In practice, it often happens that a sewer backflow valve is placed in the inspection manhole, which protects the house from backflow of sewage water during floods or during clogging of the public sewer.
The size of the manhole is directly related to the size of the valve: if you have a sewer connection DN 110, the intermediate valve DN 110 will fit comfortably into a 315 manhole. If you have a connection DN 200, the intermediate valve DN 200 requires a manhole of at least 400 mm, so that manipulation with the valve during installation and service is even possible.
We will deal with this topic in more detail in the articles How to choose the right sewer backflow valve – diameter, type and location and Installation of a sewer backflow valve step by step. For those who are wondering what diameter of the valve they actually need, we recommend reading also What DN diameter do I need for a sewer backflow valve? and Backflow valve DN 110 vs. DN 160 vs. DN 200 – comparison of use.
Most common situations in practice – scenarios you will encounter
Scenario 1: New construction of a single-family house with garden drainage
The customer is building a single-family house, the designer has designed a garden drainage system with PE 315 garden manholes. At each inspection point (corners, bends, collection points), there is a manhole, each manhole needs a cover. A Manhole cover 315 is selected – light, green or black (depending on the manufacturer), easy to install. The garden looks neat, the covers are embedded in the lawn almost at ground level.
Scenario 2: Reconstruction of an old house with a manhole
The house has a DN 150 sewer connection and a 400 mm PE inspection manhole in the garden, which contains no flap – a direct pipe from the house to the public network. During heavy rain, the basement repeatedly floods due to backflow from the sewer. Solution: install a backflow flap into the existing 400 mm manhole. The lid is removed, the manhole is opened, the flap is inserted and connected to the connection, and the lid is returned to its place. Access through the 400 mm lid is still manageable for this task, although not very comfortable.
Scenario 3: Garden terrace with paving – lid replacement
While laying the terrace, the owner discovered that the lid of the old drainage manhole was cracked and protruded above the level of the paving. The new paving is to be smooth. Problem: the diameter of the manhole is unknown. Excavation reveals the top edge of the manhole, the circumference is measured = 995 mm → diameter 317 mm ≈ 315 mm. A new 315 mm lid with a lower profile is ordered (the telescopic extension is shortened to the correct height), and the terrace is completed.
Scenario 4: Garden pump and water supply manhole
The customer has a garden well with a submersible pump and wants to build a small manhole for control elements and a check valve. A 315 mm PE manhole is sufficient for this – a DN 32 or DN 50 valve will fit inside, and access for maintenance is adequate. The manhole is covered with a 315 mm lid, preferably lockable, to prevent heat loss in winter (to protect the equipment from frost).
Interchangeability of lids from different manufacturers – pay attention to details
Many customers assume that lids of different manufacturers with the same nominal size are interchangeable. In practice, this is only partially true. The nominal outer diameter of a 315 or 400 mm manhole is indeed standardized, but:
- The height of the lid flange (depth of seating on the manhole) may vary – a lid from one manufacturer may fit tightly, while a lid from another may be too loose.
- The shape of the top edge of the manhole (flat, slightly conical, with a groove) affects whether the lid fits tightly or with clearance.
- Some manholes have an integrated telescopic lifting device that only works with a matching lid from the same manufacturer.
Therefore, it is always better to buy a lid from the same manufacturer as the manhole, or at least to check compatibility with the seller before ordering.
Tabular comparison: 315 vs. 400 manhole lid – when to choose which
| Parameter | 315 manhole lid | 400 manhole lid |
|---|---|---|
| Outer diameter of the manhole | 315 mm | 400 mm |
| Typical clear diameter | 270–290 mm | 340–360 mm |
| Typical application | Drainage, small inspection manholes, manholes for DN 110/125 | Inspection manholes, flaps DN 150–200, larger connections |
| Possibility to insert a flap | DN 110, DN 125 | DN 110 to DN 200 |
| Physical handling inside | Limited (tools, pipe) | Better (greater hand access) |
| Lid weight (PE) | approx. 0.5–1.2 kg | approx. 1.0–2.5 kg |
| Typical load (PE version) | Pedestrians, garden equipment | Pedestrians, light garden equipment |
Installation of the lid – what to pay attention to during installation
The lid installation itself is simple, but a few details determine whether the lid will function properly for years:
- Clean contact surface – before placing the lid, clean the top edge of the manhole from soil, stones, and roots. Dirt under the lid can cause it to sit unevenly and may make it unstable or leaky.
- Height above ground – the lid should ideally be at ground level or 2–3 mm above it, to be accessible but not catch dirt and water from the outside. If it is set below ground level, it may become buried and the manhole could be lost.
- Telescopic extensions – many manholes allow the top edge to be extended with a telescopic sleeve. The height of the lid must match the height of the telescopic part. If the sleeve is too high, the lid will protrude; if it is too short, the lid will sit directly on the pipe, not on the sleeve.
- Intended use – embedding into paving or concrete – if the lid is to be embedded in paving, it is necessary to choose a lid with a special frame or to embed only the frame and place the lid after the paving is laid. Otherwise, the lid may be damaged during installation.
Lifespan and maintenance of manhole cover
PE covers are generally very durable – they are resistant to UV radiation (if produced with stabilizers), frost, and chemical resistance to common soil acids and alkaline substances. In practice, their lifespan in normal garden use exceeds 20–30 years, provided there is no mechanical damage (e.g., from heavy machinery passing over).
Maintenance of the cover itself is minimal: once a year, it is sufficient to check whether the cover is not overgrown with grass or buried in soil, and whether it fits properly on the manhole. If a crack or deformation appears, the cover is a simple and inexpensive part that can be replaced at any time without major intervention.
Much more important is the regular inspection and maintenance of components inside the manhole – especially backflow valves. We will elaborate on this topic in the articles Maintenance and cleaning of sewer backflow valve and Common backflow valve problems – why the valve leaks or gets stuck.
Economic considerations – is it worth going for a larger manhole from the start?
The price difference between a 315 and a 400 manhole cover is not dramatic. However, the price difference between a 315 and a 400 manhole (including the entire manhole body, moldings, telescopic parts) is noticeable. The question is therefore not only about the cover, but about the overall decision when designing the system.
From a long-term perspective, investing in a larger manhole (400 mm) pays off wherever:
- You expect to open the manhole and work inside at least once a year (e.g., for valve servicing).
- The connection has a diameter of DN 150 or DN 200, where greater comfort during valve installation saves time and reduces the risk of installation errors.
- You plan to install multiple fittings in the manhole (valve + valve + meter).
- The manhole is located in a place where access from above is complicated and each opening is time-consuming.
For purely drainage applications without fittings inside, a 315 mm manhole is fully sufficient and more economical.
Most frequently asked questions (FAQ)
Can I use a 400 manhole cover on a 315 manhole?
No, it cannot. A 400 manhole cover has a larger inner rim that will not fit on a 315 manhole – it will either be too loose and fall into the bottom or will not fit at all. Covers and manholes must have the same nominal size. If you need a cover, always choose it according to the outer diameter of your specific manhole.
How can I determine if I have a 315 or 400 manhole if nothing is visible above ground?
The most reliable solution is to dig up the top edge of the manhole (a few shovels of soil are enough) and measure directly. If you have the design documentation, the type of manhole should be indicated there. In the worst case, you can measure the circumference of any visible part of the manhole pipe (e.g., where it protrudes above the paving or ground) and calculate the diameter from the circumference.
Can a manhole cover withstand the weight of a passenger car?
Standard PE covers for garden use are not designed to withstand the weight of a passenger car. For use in driving lanes, driveways, or parking areas, it is necessary to choose a cover with a certified load capacity according to the EN 124 standard (class B125 for footpaths and garden paths, C250 for light traffic, D400 for regular roads). Garden PE covers typically correspond to class A15 (load 1.5 tons), which is sufficient for pedestrians and light garden equipment.
Is it necessary to seal or glue the manhole cover to the manhole?
Common garden PE covers are simply placed on the manhole – no glue or sealant is required. However, if you require absolute tightness (e.g., in a manhole where you do not want any surface water to penetrate), you can use silicone or butyl sealant on the contact surface. This will make disassembly more difficult during maintenance, so do it only where it is really necessary.
Can I embed a 315 or 400 manhole cover into a concrete paving or terrace?
Yes, but the correct procedure must be followed. The cover itself must not be embedded in concrete – only the frame (if the cover has one) must be embedded, or a proper expansion joint must be created around the manhole. The cover is placed only after the paving is completed. If an incorrect procedure is used, the concrete or paving may deform or embed the cover, making future replacement complicated.
Where can I find compatible fittings (valves, valves) for the manhole?
For common household sewer connections, various inline backflow valves are available at atria.sk – DN 110, DN 125, and DN 200. When selecting a valve, it is important to know the diameter of your sewer connection and also to check whether the valve fits into your manhole. The topic of selecting the right valve is covered in separate articles Inline vs. end-of-line sewer valve: what is the difference and where to use which and Protection of the house against reverse sewer flow – when is a valve required?
Conclusion – the decision is simpler than it seems
Choosing between a 315 and a 400 mm manhole cover is not technically complicated if you know the outer diameter of your manhole. Measuring takes five minutes, and calculating from the circumference takes another minute. Problems arise only when the measurement is skipped and the cover is ordered by estimation. Therefore: always measure, always verify the manufacturer, and if in doubt, consult with the seller.
From a long-term perspective, a simple rule applies: for pure drainage applications without fittings inside the manhole, a 315 manhole cover is sufficient. In all cases where a larger diameter valve, multiple fittings, or frequent service access is installed inside the manhole, a 400 manhole cover is a better long-term solution. Make the right decision once – and you will save unnecessary costs for future modifications.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.