For outdoor seating areas, wind comfort class A or B applies under the NEN 8100 standard used in the Netherlands. Class A means wind speeds above 5 m/s occur less than 2.5% of the time, while class B allows up to 5% exceedance. Whether a formal wind study is required depends on the project type, location, and local permit conditions. Below, we answer the most common questions about wind comfort for terraces and outdoor seating.
Which wind comfort class applies to outdoor seating areas?
Under the NEN 8100 standard, outdoor seating areas should meet wind comfort class A as a minimum target. Class A means that wind speeds above 5 m/s are exceeded less than 2.5% of the time, which translates to fewer than 219 hours per year. Class B (2.5 to 5% exceedance, up to 438 hours per year) can be considered acceptable in some contexts, but class C and below are generally regarded as unsuitable for sitting.
The logic behind this is straightforward. When you sit still on a terrace, you generate less body heat than when you walk, which makes you more sensitive to wind. A gust that feels mild while you are moving can feel uncomfortable the moment you sit down with a coffee. NEN 8100 recognises this by applying stricter thresholds to “sitting” activities than to “strolling” or “through-walking.”
The table below summarises how NEN 8100 rates wind conditions for different activity types:
| Class | Exceedance of 5 m/s | Sitting | Strolling | Through-walking |
|---|---|---|---|---|
| A | < 2.5% (< 219 hrs/yr) | Good | Good | Good |
| B | 2.5–5% (219–438 hrs/yr) | Good | Good | Moderate |
| C | 5–10% (438–876 hrs/yr) | Good | Moderate | Poor |
| D | 10–20% (876–1,752 hrs/yr) | Moderate | Poor | Poor |
| E | > 20% (> 1,752 hrs/yr) | Poor | Poor | Poor |
For projects outside the Netherlands, the Lawson criteria serve a similar purpose and are widely used in the UK and internationally. The thresholds and activity labels differ slightly, but the underlying principle is the same: sitting areas require the calmest wind conditions of all pedestrian activities.
What wind speeds are considered acceptable for sitting outside?
A wind speed of 5 m/s at eye height (1.75 m above ground) is the reference threshold used in NEN 8100 to classify wind comfort. For outdoor seating, the exceedance of this threshold should remain below 5% of the time annually to achieve at least class B, and below 2.5% for class A. In practical terms, this means the terrace should be calm on most days throughout the year.
It is worth noting that these thresholds apply to statistical averages across all wind directions and seasons, not to individual gusts. A terrace that occasionally experiences strong winds during a storm is not automatically non-compliant. What matters is the long-term probability, calculated from meteorological data typically spanning several decades.
Wind safety is a separate concern. Under NEN 8100, locations where wind speeds above 15 m/s are exceeded more than 0.3% of the time are classified as dangerous and are considered unacceptable regardless of the intended use. Even exceedance between 0.05% and 0.30% is flagged as a limited risk that may require assessment depending on the context. For a terrace, this threshold should never be approached.
Does outdoor seating always require a formal wind study?
No, not always. Whether a formal wind engineering assessment is required depends on the type of project, the permit conditions set by the municipality, and the building context. For a small ground-level terrace in a low-rise neighbourhood with no significant obstructions nearby, a formal study may not be mandatory. For terraces near high-rise buildings, in exposed urban locations, or as part of a permit application, a study is often required or strongly advisable.
Municipalities increasingly include wind comfort requirements in their spatial planning policies, particularly for new developments. If a terrace is part of a larger building project, the wind study for that project will typically cover the terrace area as well. In those cases, the question is not whether to do a study, but whether the terrace location has been included in the assessment scope.
Even when a formal study is not legally required, it can still be a smart investment. Discovering that a newly built terrace is too windy to use comfortably is an expensive problem to fix after construction. A wind assessment during the design phase lets you identify and address issues before they become costly revisions.
How does building geometry affect wind comfort at a terrace?
Building geometry is one of the most significant factors determining wind conditions at a terrace. Tall buildings redirect wind downward toward street level, creating downwash flows that can make adjacent outdoor areas uncomfortable. Corner effects, where wind accelerates around the edges of a building, are another common cause of poor conditions at ground level.
Several geometric factors directly influence terrace comfort:
- Building height relative to surroundings: A building that is more than twice the height of adjacent structures significantly increases the risk of wind problems at ground level.
- Facade orientation: Placing the widest facade perpendicular to the prevailing wind direction channels more wind toward the base of the building. Aligning the facade parallel to the dominant wind reduces this effect.
- Setbacks and podium levels: Stepped facades and setbacks can redirect downwash flows upward, away from terrace areas, though they need to be deep enough to be effective. A rule of thumb is at least 5 metres of depth for a building around 100 metres tall.
- Openings and passages: Gaps or archways oriented toward the prevailing wind direction act as pressure shortcuts, accelerating airflow through and creating uncomfortable spots on the other side.
- Square or open plazas: When more than 25% of a plaza’s windward side is open, the risk of wind discomfort increases significantly.
Understanding these relationships early in the design process is what makes wind studies genuinely useful. A CFD simulation can map airflow patterns around a proposed building before a single brick is laid, giving designers the information they need to position a terrace where conditions will actually be comfortable.
What can be done when a terrace fails wind comfort requirements?
When a wind assessment shows that a terrace location does not meet the required comfort class, there are several practical options. The right solution depends on how severe the wind problem is, what the building geometry looks like, and how much flexibility remains in the design.
Mitigation measures are typically grouped by the level at which they intervene:
- Relocate the terrace: Sometimes the simplest fix is moving the terrace to a different position on the site, away from corner flows or downwash zones. Even a small shift can make a meaningful difference.
- Wind screens and barriers: Physical screens redirect local airflow and can improve comfort in a specific area. They are effective but have limitations: wind accelerates around the edges of a screen, which can create new problem spots nearby. Screens also affect sightlines and can raise concerns about social safety.
- Canopies and overhangs: Canopies provide some protection but are less effective than recesses in the building facade. They tend to redirect downward airflow to the edge of the canopy rather than eliminating it.
- Planting: Trees, hedges, and shrubs can improve comfort in summer, but deciduous trees lose their leaves in winter, exactly when wind speeds are highest. Planting alone is generally not sufficient as a primary wind mitigation measure.
- Aerodynamic building design: Rounded or tapered facades guide wind along the building surface rather than pushing it downward. This is most effective when incorporated early in the design process.
- Recessed or sunken seating areas: Lowering the terrace slightly below the surrounding pavement level reduces exposure to ground-level wind flows and can be architecturally attractive.
The earlier in the design process these options are considered, the more effective and cost-efficient they are. Solving a wind problem at the urban planning stage is far easier than retrofitting a building that has already been constructed.
How we help with wind comfort for outdoor seating
With over 21 years of experience in pedestrian wind comfort assessments, we help architects, developers, and project managers understand and improve wind conditions at terraces and outdoor spaces. We work across the Netherlands, Belgium, the UK, and internationally, applying both NEN 8100 and Lawson criteria depending on the project location.
Here is what working with us looks like in practice:
- We assess wind comfort at terrace and seating locations using advanced CFD simulations, producing colour-coded maps that you can present directly to clients, planners, or permit authorities.
- We advise on which comfort class applies to your specific use case and whether a formal study is required for your permit application.
- We identify problem areas early in the design phase and propose practical mitigation measures, ranked by effectiveness and feasibility.
- We deliver reports that are technically defensible and clearly structured, so they hold up under scrutiny from municipalities and review committees.
- For regular clients, we can start the next day if needed. Our internal automation means results are delivered faster than traditional workflows allow.
You can find a full overview of our services on the Actiflow homepage. Want to know more about who we are and how we work? Visit our about us page.
Curious how we can help with wind comfort for your outdoor seating project? Feel free to contact us. We would be happy to discuss your project and help you find the right engineering solution.
