Dry risers, wet risers and the height that separates them
Approved Document B puts dry rising mains above 18 metres and wet mains above 50 metres. The switch brings a water store, pumps and a plant room that a core drawn for a dry main cannot absorb.
The dry riser wet riser height question has one answer that matters at planning stage and one that matters later. A dry rising main is acceptable in buildings with a floor more than 18 metres above fire service vehicle access level. Above 50 metres a wet rising main is expected instead, because a pumping appliance at street level cannot reliably charge a dry main to that height. Approved Document B sets both figures, and BS 9990 covers the detail of how each system is designed and tested.
That 50 metre line is a planning issue even though risers are a Building Regulations matter. A wet system needs a water store, duty and standby pumps and a plant room with maintenance access, and none of those fit into a core drawn on the assumption of a dry main.
What each system does
A dry riser is an empty steel pipe running the height of the building inside a protected shaft, with a breeching inlet at ground level and a landing valve on each storey. It holds no water until a fire appliance connects to the inlet and pumps into it.
A wet riser is permanently charged. Water is held in a tank within the building, pumped to the landing valves and maintained at a working pressure so that a hose can be connected and used immediately. The pumps are duplicated so that a single failure does not leave the main dry, and the whole installation is monitored.
The practical difference for a designer is that the dry system is a pipe and a box on the elevation, while the wet system is a pipe, a tank, a pump room, an electrical supply with standby arrangements, an overflow and a drain.
The building height at which a wet riser is expected
Height is measured to the level of the topmost floor above fire service vehicle access level, not to the ridge or the parapet. That distinction changes the answer on schemes that sit close to a threshold, so establish the access level and the top floor level on a section before the storey count is fixed.
Two other thresholds sit near this one and are frequently mixed up with it. Firefighting shafts with firefighting lifts are expected where a floor is more than 18 metres above access level, or more than 10 metres below it. And the Building Safety Act 2022 regime treats buildings of at least 18 metres or seven storeys as higher-risk, which brings the Building Safety Regulator in as a statutory consultee at Planning Gateway One.
| Provision | Trigger in Approved Document B |
|---|---|
| Dry rising main | Floor more than 18 metres above fire service vehicle access level |
| Wet rising main | Floor more than 50 metres above fire service vehicle access level |
| Firefighting shaft with firefighting lift | Floor more than 18 metres above, or more than 10 metres below, access level |
Where the building is a relevant building under Article 9A of the Development Management Procedure Order, a Fire Statement has to accompany the planning application. That document is required at Planning Gateway One under the Building Safety Act 2022 framework and must be prepared by someone with the necessary fire engineering competence. An AI-assisted draft is a starting point for that professional, not a substitute for them.
Inlet location and appliance access
The inlet has to be reachable by a pumping appliance and visible from where that appliance stands. Approved Document B expects the inlet to be within 18 metres of, and within sight of, the appliance parking position, which makes the inlet position a site planning decision rather than a detail.
Three things follow on the drawings you submit:
- Mark the appliance parking position on the site plan, with the hardstanding dimensions and the load capacity noted.
- Dimension the distance from that position to each riser inlet.
- Show the inlet box on the elevation at the correct height, because a box that appears late usually lands in the middle of a carefully composed entrance bay.
Landscape design is where this most often goes wrong. A raised planter, a cycle store or a substation enclosure placed between the appliance position and the inlet breaks the line of sight, and the fire engineer discovers it after the landscape drawings are approved.
Outlet positions in lobbies
Landing valves sit in the protected lobby of the firefighting shaft, or in the protected stairway where there is no lobby, at each storey served. They are positioned so that a hose run from the valve reaches every part of the floor it serves, which is what limits floorplate depth and the distance from the core to the furthest flat entrance door.
On residential floors that produces a familiar pattern: a compact core with the valve cabinet in the lobby, and corridor lengths set by hose reach rather than by the arrangement of flats. Long single-stair corridors are where the geometry fails, and moving a riser after the plan is fixed usually means losing a unit.
Pump and tank space for wet systems
A wet riser installation needs a dedicated water store, and BS 9990 sets the capacity and the pressure requirements. The store is measured in tens of thousands of litres, so treat it as a room rather than a cupboard, and check the structural implications of its weight and its position.
Alongside the tank the design needs duty and standby pumps, a pump control panel, an electrical supply arranged so that a fire does not disable it, a break tank arrangement acceptable to the water undertaker, an overflow and a floor drain. Plant rooms are often placed at basement level, where the waterproofing design has to accommodate the tank, the drain and the maintenance access, which brings in the issues set out in basement waterproofing, BS 8102 and groundwater.
Pump noise is the amenity point to settle early. Test running is periodic rather than constant, and a pump room sharing a wall with a habitable room will still generate complaints. Move the room or line it before the plans are submitted.
Coordination with the core design
The core is where fire strategy, structure, lifts, services and saleable area all compete, and the riser is a late arrival in most projects. Three coordination habits reduce the damage.
Fix the access level and the top floor level early, because they decide which regime applies. Draw the shaft at the size the fire engineer asks for, including the lobby, rather than at the size the plan can spare. And decide the wet or dry question before the basement layout is fixed, since retrofitting a tank and a pump room into a basement full of parking and substation is the change nobody has budgeted for.
A Fire Safety Strategy to BS 9991 or BS 9999 is the document that records those decisions. It is a fire engineering document and needs a competent author, and where the basement is substantial it sits alongside a Basement Impact Assessment dealing with the ground and groundwater questions.
Showing the arrangement on planning drawings
Planning does not approve a riser design. It does fix the things that make the riser buildable: the storey heights, the core footprint, the appliance access, the elevation composition and the extent of the basement. So the drawings that accompany the application should show the firefighting shaft and lobby at their real size, the inlet position on the elevation, the appliance standing on the site plan with dimensions, and the plant room in the basement or ground floor plan with an access route.
Officers rarely comment on any of it. The Building Safety Regulator, consulted at Gateway One, does. And the cost of moving a core between permission and technical design is the reason to draw it properly the first time.
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This article is general guidance, not legal advice. Fire safety design is a matter for a competent fire engineer and for the Building Regulations, so check the current standards and the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
At what height is a wet riser required instead of a dry riser?
Approved Document B expects a wet rising main where a floor is more than 50 metres above fire service vehicle access level. Below that height a dry main charged by a pumping appliance is acceptable, provided a floor is more than 18 metres above access level so that a main is needed at all. Height is measured to the topmost floor, not the parapet.
How close must the riser inlet be to the fire appliance?
Approved Document B expects the breeching inlet to be within 18 metres of, and within sight of, the appliance parking position. That makes the inlet position a site planning decision, because a planter, cycle store or substation enclosure placed between the two breaks the line of sight and invalidates the arrangement.
Does a riser need to be shown on planning drawings?
Planning does not approve the riser design, which is a Building Regulations matter. The drawings should still show the firefighting shaft and lobby at real size, the inlet on the elevation, the appliance standing with dimensions on the site plan and the plant room in the basement or ground floor plan, because permission fixes all of those.
What space does a wet riser installation need?
A dedicated water store sized and pressurised to BS 9990, duty and standby pumps, a control panel, an electrical supply arranged to survive a fire, a break tank acceptable to the water undertaker, an overflow and a floor drain. The store runs to tens of thousands of litres, so it is a room with structural and access implications, not a cupboard.
Is a Fire Statement needed for a building with a dry riser?
A Fire Statement is required with the planning application where the building is a relevant building under Article 9A of the Development Management Procedure Order, which turns on height and use rather than on the riser type. It must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.