Real-time estimate of how much wind-driven rain a facade has to absorb. Adjust rainfall, wind speed, incidence angle, building height, terrain and cladding, and watch the WDR intensity, facade dynamic pressure and the margin against ASTM E331 / AAMA 501.1 test pressure update instantly — a quick check for rainscreen and seal-spec decisions.
Parameters
Rainfall R
mm/h
Hourly rainfall (heavy downpour: 20–50 mm/h)
Wind speed V
m/s
Free-stream wind speed at 10 m reference height
Wind direction
Incidence angle of wind on the wall
Building height h
m
Power-law height profile applied to wind speed
Facade material
Cladding type (reference)
Terrain exposure
Shape coefficient Cp varies (coastal is highest)
Relative humidity RH
%
Affects wall drying (evaporation) rate
Results
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WDR intensity (L/m²/h)
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Median drop D50 (mm)
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Terminal velocity V_t (m/s)
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Height-corrected V_h (m/s)
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Facade dynamic pressure p (Pa)
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Test pressure margin (Pa)
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Facade & wind-driven rain animation
Wind vectors, raindrop trajectories, incidence angle and wet film on the wall. Wall wetting intensity scales with WDR magnitude.
Marshall-Palmer (1948) median raindrop size and Gunn-Kinzer (1949) terminal velocity. Heavier rain means larger drops and higher terminal velocity.
$$V_h = V_{10} \cdot (h/10)^{0.143}$$
Hellmann power-law profile from the 10 m reference (exponent 0.143 for open terrain; higher over rough urban terrain).
Wind-Driven Rain — Facade Waterproofing & Test Pressure
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"Wind-driven rain"… is that anything more than rain hitting a wall? Why does it get a separate name?
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Good question. With no wind, rain falls almost vertically and barely hits the wall — most of it is caught by the roof and overhangs. The moment the wind blows, raindrops carry a horizontal velocity component and hit the facade head-on. Lacy (1965) captured this with the deceptively simple expression WDR = R·V/V_t·cosθ. At R=25 mm/h with V=15 m/s, roughly 100 litres per square metre per hour slam into the wall. That's why roof and wall waterproofing are designed as completely separate problems.
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100 litres — that's like buckets of water per hour per square metre… But facades are sealed with gaskets and silicone, right? They still leak?
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"Sealed properly" is defined by ASTM E331 and AAMA 501.1 — 137 Pa (5.5 psf) static differential for 15 minutes with zero water passage. But push wind speed in this tool to 25 m/s with a 100 m coastal building and you'll easily exceed 500–1000 Pa of facade dynamic pressure. At several times the test pressure, even tiny seal defects let water through. So commercial buildings spec a design pressure of 4–8× the test pressure, and if even that is not enough, a rainscreen is the next step.
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Rainscreen? I've heard the word — how is it different from just more sealant?
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It's called a pressure-equalised rainscreen: a vented cavity behind the outer cladding makes the pressure on the front and back faces of the cladding nearly equal. Once the driving pressure disappears, only capillary and inertial penetration remain, and field measurements typically show 90%+ reduction in water entry. Panel systems like EQUITONE Tonality and ALPOLIC use it as a standard. Instead of trying to seal harder, you let the structure cancel the driving pressure.
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I've heard stories about the Sears Tower in Chicago and the John Hancock Tower. Were those just design mistakes?
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The John Hancock Tower in Boston (1973) is the classic case — all 10,344 panes of glass on a 60-storey tower had to be replaced after combined thermal and wind-pressure failures. Sears Tower had high-level glass falls in 1981 too. Since the 1990s, wind tunnel testing combined with WDR analysis has been mandatory, supported by codes like Eurocode EN 1991-1-4 and ISO 15927-3. Hygrothermal solvers (WUFI Pro, Delphin, HAMcat) now solve 1D/2D moisture transport in the wall assembly so condensation and freeze-thaw are predicted up front. This tool sits at the front of that pipeline — a first-pass check of "WDR pressure vs test pressure" before the more detailed analysis.
Frequently asked questions
WDR is rain that has acquired a horizontal velocity component from the wind and so strikes the walls, windows and joints of a building instead of the roof. Lacy (1965) gave the simple relation WDR = R·V/V_t·cosθ, where R is rainfall (mm/h), V is wind speed (m/s), V_t is the raindrop terminal velocity (m/s) and θ is the angle between wind and the wall normal. For R=25 mm/h and V=15 m/s, 50–100 litres per square metre per hour can hit the wall, which is the dominant cause of seal and gasket water ingress. ISO 15927-3 and DIN 4108-3 define standard quantification procedures.
ASTM E331 spray-tests a wall at a static differential of 137 Pa (5.5 psf) with 3.4 L/m²/min for 15 minutes and requires zero water entry. AAMA 501.1 does the same dynamically with a propeller-generated wind. Commercial buildings typically specify a design pressure of 4–8× this value (548–1098 Pa), and this tool reports the facade dynamic pressure (Cp×0.5ρV²) and its margin against the test pressure in Pa. When facade pressure exceeds the test pressure the current seal spec is at risk and a rainscreen upgrade or higher test pressure is needed.
Friction with the ground reduces wind speed near the surface, so wind speed grows with height. A common simple form is V(h)=V₁₀·(h/10)^α, with α depending on terrain roughness (urban ~0.25–0.35, suburban 0.20, open 0.14, coastal 0.10). This tool uses Hellmann's exponent 0.143 (open terrain) for the height-corrected wind, then computes dynamic pressure q = 0.5·ρ·V_h² and facade pressure p = Cp·q. Codes such as Eurocode EN 1991-1-4 and Japan's Building Standard Law Article 87 provide more detailed profiles.
A pressure-equalised rainscreen places a ventilated cavity behind the outer cladding so that the pressure differential across the cladding is nearly zero. With no driving pressure, only capillary and inertial penetration remain and field measurements typically show 90%+ reduction in water entry. Drained-and-back-ventilated cavity walls such as EQUITONE Tonality are typical examples. When this simulator shows a chronically small facade-to-test-pressure margin, switching to a rainscreen reduces lifetime water-ingress risk far more reliably than thicker sealants.
Real-world applications
Spec review for tall glass curtain walls: for coastal-adjacent towers in Tokyo, Singapore, New York or similar locations, wind-tunnel testing is combined with a first-cut WDR estimate like this tool. The aim is to find zones — corners, top storey, atrium openings — where facade dynamic pressure is 5–10× the ASTM E331 test pressure. In those zones the response is triple sealants, water-deflector flashings and improved air seals; if margin is still too small, the system moves to a fully pressure-equalised unitised curtain wall such as Schüco USC 65.
Leak diagnosis on existing low-rise commercial: shopping malls and logistics warehouses 5–15 years old often see "leaks only during high winds from one direction". This tool can recompute directional WDR intensity and check whether the dynamic pressure on the leaking elevation exceeds the original test pressure. If yes, the root cause is design specification not sealant fatigue, so the remedy moves from re-sealing to rainscreen retrofit or cladding replacement.
Freeze-thaw risk for historic masonry walls: WUFI Pro and Delphin need WDR intensity as a boundary condition. This tool gives that from local weather (Tokyo: annual maximum ~35 mm/h, ~60 mm/h in typhoons, mean wind 8–15 m/s). Combined with absorption rate it gives annual water uptake, which times winter freeze-thaw cycles gives a quantitative spalling-risk metric — a routine step in preservation studies for Meiji/Taisho-era brick warehouses.
BIPV (building-integrated PV) waterproof design: when glass-substrate PV modules are integrated into the facade, perimeter seals need significantly higher water tightness than ordinary windows. This tool gives the WDR intensity and facade pressure for the target building, which combined with IEC 61215 Damp Heat 1000h testing supports a 25-year warranty risk assessment for water ingress and electrical failure.
Common misconceptions and pitfalls
The first trap is assuming "if it passes ASTM E331 at 137 Pa it will be fine in real wind and rain". 137 Pa is a minimum quality-check benchmark, not a design pressure for typhoons or storms. As this tool shows, the actual facade dynamic pressure on a coastal tower easily reaches 500–1500 Pa. Commercial-grade practice is to specify 4–8× the test pressure as the design pressure, and skipping that leads to costly seal re-work 5–10 years after handover. Specifications should call out a Design Pressure value, not the Test Pressure.
The second mistake is to use a single Cp value for the whole facade. In reality Cp varies strongly with face orientation, height and distance from corners, and wind-tunnel tests show that corner zones often see 1.5–2.5× the mean Cp used here. Eurocode EN 1991-1-4 splits the facade into zones A–E with separate Cp tables, and corner-zone width is defined as the smaller of 0.2× building width and 0.2× height. Use this tool's average Cp for early sizing, but always finalise with wind-tunnel testing or CFD (OpenFOAM, ANSYS Fluent) on the local Cp.
The third is to think that "same rainfall R means same WDR". Lacy's relation puts WDR proportional to R × V/V_t, so 25 mm/h with V=5 m/s and the same R with V=25 m/s give 5× different WDR. Annual maximum WDR is rarely on the maximum-rainfall day — it is typically the day with moderate rain plus a typhoon or strong cold front. The recommended ISO 15927-3 procedure is to take hourly R and V from the meteorological record, compute their joint distribution and use the 99th-percentile WDR as the design load.
How to Use
Enter rainfall intensity in mm/hr (typical range 5–50 mm/hr for moderate to heavy storms)
Set wind speed at facade height in m/s; simulator applies logarithmic boundary layer correction based on building height
Specify building height (m) and incidence angle (0–90°) to determine horizontal rainfall component striking the facade
Input relative humidity (%) to refine raindrop terminal velocity calculations
Read WDR intensity in L/m²/h, median drop diameter D50, corrected wind velocity, and resulting dynamic pressure on facade surface
Compare facade design pressure margin against simulated load to assess weather-tightness compliance
Worked Example
For a 25 m commercial facade during a 20 mm/hr rainfall with 15 m/s wind speed at reference height (10 m), incidence angle 45°, and 65% relative humidity: simulator calculates D50 ≈ 4.2 mm, terminal velocity V_t ≈ 8.1 m/s, height-corrected wind V_h ≈ 18.2 m/s (accounting for wind profile exponent 0.22), WDR intensity ≈ 185 L/m²/h, and facade dynamic pressure ≈ 198 Pa. Standard curtain wall systems rated to 300 Pa provide 102 Pa safety margin for this exposure.
Practical Notes
Coastal regions with salt spray require lower incidence angles (10–30°) due to wind channeling around building edges; tropical cyclone zones use 35–45° for perpendicular wind loads
Double-skin facades reduce effective WDR load by 40–60% depending on cavity ventilation; adjust test pressure margin accordingly
High-rise buildings (>50 m) experience 20–30% higher wind pressures at mid-height; use actual anemometer data rather than standard assumptions
Relative humidity below 40% accelerates drop evaporation; above 85% increases adhesion risk on horizontal ledges