- Buildings with a floor more than 18 m above fire service access level need fire mains; up to 50 m a dry riser is acceptable, but above 50 m BS 9990:2015 and Approved Document B require a wet riser because a pumping appliance cannot reliably push water that high.
- Landing valves are oblique-pattern globe valves in gunmetal to BS 5041-1 (wet risers, with built-in pressure regulation) or BS 5041-2 (dry risers), each terminating in a 65 mm instantaneous female outlet with a blank cap secured by a strap or chain.
- BS 9990 requires a full wet pressure test of every dry riser annually at 12 bar held for 15 minutes, plus a visual inspection at six-monthly intervals; wet risers get an annual flow test proving 1,500 l/min at the two highest outlets.
Rising fire mains are the arteries a firefighting operation depends on in any tall building, and the valves fitted to them decide whether water actually arrives at the fire floor. Dry riser valves in particular lead a hard life: they sit unused in stair cores and lobbies for years, get stolen for scrap, vandalised, painted over and forgotten, and are then expected to pass 12 bar without a whimper the night the fire service charges the main. This guide works through wet and dry riser systems from the inlet breeching at street level to the air valve on the roof.
It is written for the people who specify, install and test these systems: the landing valve patterns and materials, the non-return arrangements inside the breeching, pressure regulation on wet risers, the BS 9990 test regime, and the grubby practical realities of theft and vandalism that the standards only hint at. If you look after a building over 18 m, this is equipment you own whether you like it or not.
Wet vs Dry Risers: What They Are and Where Each Is Required
A dry riser is a normally empty vertical main, typically 100 mm nominal bore in galvanised steel, running up a firefighting stair or protected shaft. The fire service connects a pumping appliance to the inlet breeching at ground level and charges the main with water from a hydrant or the appliance tank. A wet riser is permanently charged, fed from a dedicated storage tank through duty and standby pumps, so water is available at every landing valve the moment the outlet is opened, at a regulated pressure.
The dividing lines come from Approved Document B and BS 9990:2015. Fire mains are required where any floor is more than 18 m above fire service vehicle access level, and in buildings with deep basements. Between 18 m and 50 m a dry main is acceptable. Above 50 m the main must be wet, for a simple hydraulic reason: a floor at 50 m already imposes roughly 5 bar of static head, and once you add hose friction losses and the 4 to 5 bar a branch needs to produce an effective jet, the required inlet pressure climbs beyond what a standard pumping appliance can be relied on to deliver through a breeching.
Riser sizing follows the outlet count. A 100 mm main is acceptable where only one landing valve is fed per floor and the building falls in the dry riser height band; 150 mm mains are required where two landing valves are served per storey, and for wet risers generally. Landing valves go in every firefighting lobby or protected stair at each storey, positioned so no point on the floor is beyond hose reach, with the valve handwheel and outlet set at a workable height, conventionally around 750 mm to 1,000 mm above finished floor level.
One point that regularly surprises building owners: the 2020s tightening of fire safety rules after Grenfell changed sprinkler thresholds to 11 m, but the 18 m fire main trigger stayed put. Plenty of 5 and 6 storey residential blocks now carry sprinklers but no riser, and firefighters still work those buildings off hose lines run up the stair. Where a riser does exist, though, it is firefighting infrastructure under the Regulatory Reform (Fire Safety) Order 2005, and the responsible person carries a statutory duty to maintain it.
Dry Riser Valves: The Oblique-Pattern Landing Valve in Detail
The landing valve is where the fire crew meets the system, and its design is more considered than its humble appearance suggests. Dry riser valves are made to BS 5041-2, wet riser landing valves to BS 5041-1, and both are oblique-pattern globe valves: the body turns the flow through roughly 45 degrees rather than the 90 degrees of a conventional angle globe. That oblique seat gives a straighter flow path, and it matters. At the 500 l/min a single branch can draw, a right-angle body would burn a bar or more of the pressure the pump operator is trying to put up the building.
The standard material is gunmetal, typically LG2 (CC491K to BS EN 1982), chosen because the valve spends decades doing nothing in a damp stair core and must then operate first time. Gunmetal does not rust solid, tolerates brackish flushing water, and survives the casual abuse of a lobby environment. The disc carries a renewable resilient facing onto a bronze seat; the stem is non-rising with a cast handwheel sized for a gloved hand. Working pressure rating is a minimum of 12 bar for dry riser patterns and 20 bar for wet riser valves, reflecting the static heads in taller buildings.
The outlet is the British Standard 65 mm instantaneous female coupling, the same connection carried on every fire service hose in the UK, fitted with a blank cap. BS 5041 requires the cap to be secured to the valve by a strap or chain, and the valve handwheel to be strapped and padlocked with a leather strap or equivalent so it cannot be casually opened. Frangible padlocks or straps are the norm: secure against mischief, breakable by a firefighter in seconds without a key. Specifying a heavy security padlock on a landing valve is a classic well-meaning mistake that costs a crew time on the fire floor.
Cabinets and Accessibility
Landing valves normally live inside a purpose-made cabinet or builders-work enclosure with appropriate signage. BS 9990 requires the door to be openable without a key, or glazed for break-in access, and the enclosure sized so a coupling can be swung onto the outlet with the door open. I have tested risers where a later stud wall left the outlet pointing into plasterboard 100 mm away: the riser passed its pressure test and was still operationally useless.
Inlet Breechings and Non-Return Arrangements
At street level the dry riser terminates in an inlet breeching mounted in an external wall box, faced with a wired-glass or louvre door marked with the appropriate signage. Locating rules are practical fire-ground ones: the inlet must be within 18 m of a hardstanding a pumping appliance can occupy, visible from it, and set with the inlets a workable distance above ground, conventionally between about 400 mm and 600 mm to centreline so a crew can couple hose quickly.
A 100 mm riser takes a two-way breeching with two 65 mm instantaneous male inlets; a 150 mm riser takes a four-way. The multiple inlets are not decoration. One 70 mm hose line flowing hard will pass around 800 l/min with sensible friction loss, so feeding a 150 mm main supplying two branches on the fire floor genuinely needs multiple lines, and the redundancy covers a burst length or a failed coupling without stripping all supply from the riser.
Inside each inlet sits a spring-loaded non-return valve, and this detail carries the whole operation. The non-returns let the pump operator charge the main through one line while a second is still being coupled, stop water blasting back out of an open inlet, and hold the main charged if a supply line bursts and must be swapped. They are also the components that corrode, jam and lose their springs after twenty years of sitting behind a vandalised door full of leaves and lager cans. On test, a non-return that will not seat shows up immediately: the test pump cannot hold pressure and water dumps back through the open inlet.
The breeching body also carries a 25 mm drain valve. After use or test, the riser is emptied through this drain, because a dry riser left full will freeze in the first hard winter and split a coupling, usually somewhere inaccessible. The drain discharges to a visible gulley or drain point; BS 9990 expects the drainage arrangement to empty the main completely, which is why the riser should have no untrapped low points. Every trapped section is a frost fracture waiting for January.
Pressure Regulation on Wet Risers
Wet risers introduce a problem dry mains never face: the pump set must generate enough pressure to serve the highest landing valve, which means the lowest landing valves see brutal static pressures. In a 100 m building the ground floor valve could stand at well over 12 bar. An unregulated 65 mm outlet at that pressure is dangerous; a charged hose line at much above 7 bar becomes very hard for a branch crew to control, and reaction forces climb with the square of nozzle pressure.
BS 9990 therefore sets the delivery window: a wet riser must provide a running pressure of 4 to 5 bar at the outlet of the two topmost landing valves with a total flow of 1,500 l/min, and the running pressure at any landing valve must not exceed 5 bar, with static capped at 12 bar. The regulation is done at the landing valve itself. BS 5041-1 wet riser valves incorporate a pressure-regulating element, either a field-set orifice or spring mechanism adjusted floor by floor during commissioning, or a self-acting pressure-reducing trim that throttles to hold outlet pressure regardless of demand.
Excess water from the regulating function has to go somewhere, so wet riser landing valves carry a small relief connection, typically 25 mm, piped back to the suction tank through a dedicated drain riser. Miss that drain connection during installation, and every operation of the valve floods the lobby. The supply end is a dedicated tank, 45,000 litres minimum effective capacity with an automatic infill capable of sustaining the design flow, and duty plus standby pumps on independent power so a single failure never strips the building of firefighting water. Commissioning must verify the pressure window at top and bottom of the building, not just at one convenient floor; I have seen risers where the ground floor valve was never set and would have delivered a hose-snapping 11 bar running.
BS 9990 Testing: The Annual Regime That Keeps Risers Honest
The test regime in BS 9990 is blunt and effective. Every dry riser gets an annual wet test: the main is charged from a test pump through the inlet breeching, raised to 12 bar and held for 15 minutes, during which pressure must not fall away and no joint, valve or coupling may leak. The test proves the thing a visual inspection never can, that the main is actually continuous and tight from breeching to roof. Alongside it sits a visual inspection at six-monthly intervals covering caps, straps, door glazing, signage, cabinet access and obvious damage.
The annual test is also the moment to exercise every landing valve through its full travel, check each outlet flows, confirm the highest outlet delivers, and verify the drain empties the main afterwards. Common failures, in rough order of frequency from my own test sheets: missing or stolen outlet caps, seized handwheels, inlet non-returns failing to seat, perished disc facings weeping at 12 bar, and couplings cracked by frost because last year’s tester never drained down properly. None of these are exotic. All of them stop water reaching a fire floor.
Wet risers carry the equivalent duty plus the pump set: annual flow testing to prove the 1,500 l/min and the 4 to 5 bar window at the top two valves, along with the routine pump servicing any sprinkler engineer would recognise, including weekly automatic start checks where the responsible person’s regime requires them. Records matter legally. Under the Fire Safety Order the maintenance records for fire mains are exactly the sort of evidence a fire authority asks for after an incident, and an untested riser is a straightforward enforcement notice, or worse, a prosecution exhibit.
Vandalism and Theft: The Ugly Operational Reality
Ask anyone who tests risers for a living what fails most and the answer is not corrosion, it is people. Gunmetal landing valves and brass blank caps have scrap value, and a stripped riser is depressingly common in unsecured stair cores. A missing landing valve does not just disable one floor; when the fire service charges the main, the water dumps out of the open branch connection on that floor and the outlets above it get nothing. A stolen inlet non-return or smashed breeching does the same job from the bottom.
The countermeasures are well established. Anti-theft landing valve designs use steel or coated components with minimal scrap value, or standard valves fitted with hardened locking caps and tamper-resistant fixings that still break away under fire service force. Replacing brass blank caps with plastic or zinc-alloy caps removes the incentive for the most common theft at near-zero cost. Cabinets in vulnerable buildings should be steel rather than glazed timber, monitored where the building has a security installation, and inspected far more often than the six-monthly minimum; a monthly caretaker walk-round costs nothing and catches a stripped riser eleven months before the annual test would.
Some estates go further and fit air pressure monitoring: the dry main is sealed and held at a small positive air pressure, a fraction of a bar, with a pressure switch signalling to the building management system. Cut the pipe or steal a valve and the pressure drops and raises an alert the same day. It is a cheap retrofit, and worth specifying by default on any residential tower whose stair core is not fully secure.
Air Release at Roof Level, Drainage and the Details That Get Missed
The top of a dry riser needs an automatic air valve, typically 25 mm, at its highest point. The reason is charging speed. A 100 mm main in an 8 storey building holds the better part of 250 litres of air, and if that air has no escape route it must back up through the appliance or blow out of the first opened landing valve in an unpleasant slug. The air valve vents the column as water rises, lets the main charge in seconds rather than minutes, then seals as water reaches it. On test, a riser that takes strangely long to pressurise usually has a failed or painted-shut air valve at the roof.
The same valve works in reverse on drain-down, admitting air at the top so the 25 mm drain at the breeching can actually empty the column instead of pulling vacuum. Testers should prove both directions. Other details worth an engineer’s eye: the riser must be electrically earthed and bonded, pipework should be galvanised to BS EN 10255 or equivalent with joints capable of the 12 bar test, and supports must take the water-filled weight, roughly an extra 8 kg per metre for a 100 mm main, which the structure never feels until test day.
Firefighting shafts in buildings over 18 m also put the riser inside a protected enclosure alongside the firefighting lift and stair, so any breach of that enclosure for the landing valve cabinet needs fire-stopping reinstated to the shaft rating. Plenty of risers pass hydraulically yet fail as compartmentation.
Specification Checklist and Closing Thoughts
Boil the whole subject down and the specification rules are short. Under 18 m, no fire main required, though deep basements have their own rules. From 18 m to 50 m, dry riser: 100 mm main for one landing valve per floor, 150 mm for two, BS 5041-2 landing valves, two or four-way breeching with spring-loaded non-returns and a 25 mm drain, automatic air valve at the head. Above 50 m, wet riser: 150 mm main, BS 5041-1 pressure-regulated landing valves holding 4 to 5 bar running, 45,000 litre tank, duty and standby pumps, relief drains back to tank.
Then maintain it: 12 bar for 15 minutes every year, eyes on it every six months, drain it after every wetting, and treat every missing cap as the operational failure it is. Rising mains are among the cheapest fire systems in a building and the most neglected, because they do nothing until the one night they must do everything. Keep the gunmetal on the wall, the non-returns seating and the test certificates current, and the riser will do its job when the pump operator opens the throttle.