Open beams versus closed ceilings, deflector-to-beam geometry, the every-third-bay rule, roof truss flange scenarios and the "one row under the plasterboard" trap.
On a final physical check before client handover at a spare parts warehouse, we found half the sprinkler heads sitting directly beneath the lattice trusses. The contractor said "I put one head per 12 m² for OH-2, it was checked." The standard is looking somewhere else: EN 12845 defines separately how a sprinkler relates to the ceiling or roof and to the beams, trusses and suspended ceiling elements there. Even with the horizontal density correct, a head shadowed by a truss flange leaves a dry patch beneath it, and a fire there escapes into the area of a neighbouring head that opens late. This article covers deflectors under beams, narrow bays and roof trusses in field language, and links the most common error under plasterboard ceilings to the suspended ceiling clauses.
Open beams versus closed ceilings
EN 12845 treats sprinkler layout under a roof in two settings. The first is an open roof: lattice trusses hung directly under profiled decking, with the sprinkler run dropped from them. The second is where a plasterboard or similar closed ceiling is fitted beneath. The wall distance differs accordingly: 2.0 m for standard spacing, 2.3 m for staggered, but only 1.5 m where the ceiling or roof is open beam or the truss is visible from below. That short line adds real cost to a project; in an open truss zone the racking and sprinkler pipework interfere, the wall distance is tight, and the head count rises by roughly 15–25 %.
Deflector height is tied to the same framework. Below a combustible ceiling the clearance is at most 0.3 m, and below an A1/A2 non-combustible roof at most 0.45 m. The preferred range is 0.075–0.15 m, and unless flush or recessed heads are used you are not expected to leave that window. In site terms, the standard says: keep as close to the ceiling as you can, drop a little where you must, and keep the dropped area as small as possible. A single deflector at 0.3 m beside a lift shaft is not an issue; running a whole hall at 0.3 m is a breach.
Deflector geometry under beams
Beams are the most graphically dependent part of the standard. The figures define the horizontal distance from the deflector to the beam (a) and the vertical distance from the underside of the beam to the deflector (b). A positive b means the deflector is below the beam, a negative one above it. A curve for each sprinkler type — spray pendent, conventional upright, spray upright, flat spray, conventional pendent — shows the permitted a-b pairs. The key logic: the closer to the beam, the further the deflector must drop below it; the further away, the higher it may stay. That is the minimum angle the horizontal spray cone needs to reach behind the beam.
Three solutions are offered: follow the figure coordinates; apply the narrow bay rules; or place sprinklers on both sides of the beam as if it were a wall. In addition, where a sprinkler is placed directly over a beam or duct no wider than 0.2 m, the deflector must sit at least 0.15 m above it — the classic arrangement for throwing water past a narrow profile on both sides. Where none of the three options works, the standard suggests closing the underside of the roof with plasterboard to form a flat ceiling and dropping the sprinklers below it.
The most common site error: a sprinkler placed 0.4 m horizontally from a 0.3 m deep beam with the deflector level with the roof (negative b). That combination falls in the prohibited zone for a conventional pendent, and water cannot reach behind the beam. The fix is usually to move the sprinkler 1.2 m past the beam, or to add an extra dropped head at that point.
Narrow bays and the every-third-bay rule
Where beams are closer than 1.5 m apart and the beam depth exceeds 450 mm in non-combustible (A1/A2) construction or 300 mm in combustible construction, ordinary point layout is not enough. The rule then is a row of sprinklers in the centre of every third bay, plus a further row on the centre line of the beam separating the two unprotected bays between. Two rows of heads protect three bays: the sprinkler within its own bay covers it and protects the two neighbours from the edge. Spacing along the bay is limited by the hazard class tables; wall distances are at most 1 m from a wall parallel to the beams and 1.5 m from a wall perpendicular to them. Deflectors within the bay stay in the 0.075–0.15 m band. Where the beam depth exceeds 0.7 m, the standard refers the matter to the authority having jurisdiction.
| Structural condition | EN 12845 solution | Typical error |
|---|---|---|
| Beam spacing ≤ 1.5 m, depth > 450 mm (A1/A2) | Every-third-bay layout: two rows per three bays | A head in every bay, faking the horizontal density |
| Beam depth > 0.7 m | Referred to the authority having jurisdiction | Waved through on a standard drawing |
| Truss flange ≤ 0.2 m | Sprinkler directly above or below | Treating the truss as an obstruction and creating a shadow |
| Truss flange 0.1–0.2 m | At least 0.3 m horizontally | Sprinkler right under the flange, cone shadowed |
| Truss flange > 0.1 m | At least 0.6 m horizontally | 0.3 m treated as sufficient, cone cut |
| Roof pitch > 30° | A row within 0.75 m of the ridge | Ridge row omitted, no early operation |
| Suspended ceiling with combustible void above | Two levels of sprinklers | One row below only, void unprotected |
Roof trusses and flange width
A roof truss is examined in three scenarios. First: where the flange is narrower than 0.2 m, the sprinkler may sit directly above or below the truss, since a thin flange is assumed not to shadow the spray cone significantly. Second: where the flange does not exceed 0.1 m, the sprinkler must be at least 0.3 m clear of the truss members. Third: where the flange is wider than 0.1 m, the minimum clearance rises to 0.6 m. Behind this lies the standard obstruction logic: the wider the obstruction, the greater the horizontal distance needed for the spray to reach past it. NFPA 13 expresses the same idea more aggressively in numbers with its three-times-the-obstruction-width rule; EN 12845 short-cuts it with fixed 0.3 m and 0.6 m thresholds. In practical geometry the two arrive at similar answers: to clear an obstruction, increase the horizontal distance in proportion to its size.
Roof pitch above 30° and profiled ceilings
Deflectors are mounted parallel to the roof pitch, and on roofs steeper than 30° a row of sprinklers must sit within 0.75 m radially of the ridge. The trapezoidal decking common in industrial buildings stays in the 5–10° range, where the rule causes no difficulty; but conference halls, hangars and historic timber roofs easily exceed 30°. Omit the ridge row and smoke and heat collect at the hottest point; without a ridge sprinkler, the surrounding heads open late and the flame spreads lengthwise.
Profiled (ribbed or cellular) ceilings change the flow characteristic surprisingly. A two-way cellular ceiling channels water along the direction of the ribs; spread in the perpendicular direction can drop 20–30 % against a flat ceiling. The standard catches this through the beam and narrow bay clauses combined: where the ribs are closer than 1.5 m and exceed the depth thresholds, the narrow bay rule applies. One case we met was a composite panel roof with 0.3 m ribs in one direction only; the supplier's brochure said "may be treated as a flat ceiling", but the measurement fell inside the narrow bay threshold, and the project added a row along the bay direction.
The plasterboard ceiling and the void above it
The most common geometric trap in office and retail projects is fitting pendent sprinklers below a plasterboard ceiling and leaving the void above unprotected on the assumption that it does not need sprinklers. EN 12845 states plainly that sprinklers may not be fitted below a suspended ceiling unless it is demonstrated that the ceiling material does not impair sprinkler protection — meaning a supplier's fire test report is required. For open cell ceilings there are further criteria such as 70 % open area and a 25 mm minimum aperture, along with a maximum 3 m sprinkler spacing and at least 0.8 m between the deflector and the true ceiling for LH and OH.
The real danger is combustible surfaces in the void between suspended ceiling and roof — composite insulation panels rather than mineral wool, PVC duct casing, cable containment. In that case the intermediate-level sprinkler philosophy applies: the void must be protected too, meaning two levels of sprinklers. The vertical separation and hydraulic demand are calculated separately; the void level usually runs off a smaller main, but the design density in mm/min is the same as the main space. If that line is forgotten in a refurbishment and the fire starts in the void, the sprinklers below stay shut until the plasterboard is breached and falls — by which time the roof has burned.
Comparison with NFPA 13 obstruction rules
NFPA 13 states its obstruction approach numerically through the three-times rule and beam rule tables: for a standard spray pendent, a sprinkler must be at least three times the obstruction width clear of it, or the deflector must drop below a given threshold. EN 12845 expresses the same idea through curves; the numerical results overlap but the method differs. NFPA additionally tabulates every obstruction to discharge below sprinklers — suspended ducts, beams, top of racking — while EN 12845 simplifies this into thresholds such as "wider than 0.8 m and closer than 0.15 m to a wall". The practical outcome: NFPA gives finer granularity, EN 12845 sharper thresholds. A contractor working between the two needs both sets of tables, because in the same space an NFPA calculation may pass while EN 12845 demands extra sprinklers.
Turkish context
BYKHY requires sprinkler design to TS EN 12845 or an equivalent international standard. Its text does not describe beam and ceiling geometry in detail; it delegates that to EN 12845. Because the fire brigade or municipal official reviewing an approval does not have the standard's commentary to hand, structural inconsistencies usually come back after installation, as "why is this head here". Adding a summary table on the drawing showing the beam geometry against the figure references for each sprinkler head eases the approval process considerably. The authority having jurisdiction — the fire brigade, in the referral case — wants to see what it is being asked on one page.
Frequently asked questions
How far below a beam must the deflector sit?
It depends on the horizontal distance to the beam: the closer the sprinkler, the further the deflector must drop below the beam; further away it may stay at the higher level. Where a sprinkler is placed directly over a beam or duct no wider than 0.2 m, the deflector must be at least 0.15 m above it. Where none of the three permitted solutions works, closing the roof underside to form a flat ceiling is recommended.
What if the roof pitch exceeds 30 degrees?
The deflector is mounted parallel to the pitch, and a row of sprinklers must sit within 0.75 m radially of the ridge. Omitting that row delays the operation of surrounding sprinklers while heat and smoke collect at the apex.
What exactly is the every-third-bay rule?
Where beams are 1.5 m or closer and their depth exceeds 450 mm in non-combustible or 300 mm in combustible construction, a row of sprinklers goes in the centre of every third bay, plus a further row on the centre line of the beam between the two unprotected bays. Two rows protect three bays. Beam depths above 0.7 m are referred to the authority.
Why does truss flange width matter?
A truss is assessed in three scenarios by flange width: up to 0.2 m the sprinkler may sit directly above or below; up to 0.1 m it must be at least 0.3 m clear horizontally; above 0.1 m at least 0.6 m. A wide flange shadows the spray cone like a beam and leaves a dry patch beneath.
I have sprinklers under a plasterboard ceiling with an empty void above. Is one level enough?
No. Sprinklers may not be fitted below a suspended ceiling unless the material is demonstrated not to impair protection. Where the void contains combustible surfaces — composite insulation, PVC casing, cable containment — two levels of sprinklers are required, with the demand calculated separately.
Frequently Asked Questions
How close to a beam can a sprinkler be?
It depends on the deflector height: the closer to the beam, the further the deflector must drop below it. Over a beam or duct up to 0.2 m wide, the deflector sits at least 0.15 m above.
When does the every-third-bay rule apply?
Where beams are 1.5 m or closer and deeper than 450 mm in non-combustible or 300 mm in combustible construction. Two rows of sprinklers then protect three bays.
How far from a roof truss must a sprinkler sit?
Directly above or below where the flange is under 0.2 m; at least 0.3 m clear up to a 0.1 m flange; at least 0.6 m clear above that.
Does the void above a suspended ceiling need sprinklers?
Yes, where it contains combustible surfaces. Two levels of sprinklers are then required, with the void demand calculated separately.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.