Roof terraces: sunlight and wind comfort together

A roof terrace sunlight wind assessment has to reach one answer about one surface, and the two halves of it pull in opposite directions. Sunlight wants the deck open, unshaded and unobstructed. Wind comfort wants it sheltered. Design the terrace for either criterion alone and the other assessment will show most of it unusable.

The area that counts is the part that passes both. That is the number to put in the design and access statement, and it is almost always smaller than the area drawn on the roof plan.

Policy expectations for communal amenity space

Requirements come from the local plan and, in London, from the London Plan's housing quality standards, which set private external space minimums per dwelling and leave communal provision to borough policy. Outside London the pattern varies, but the components are consistent:

• A quantity of communal amenity space, often expressed per dwelling or as a minimum area once a scheme passes a unit threshold.
• A usability requirement, which is where sunlight and wind come in.
• Accessibility, meaning step free access from every core rather than from one.
• Child play space where the child yield calculation requires it, which on a family sized scheme is a separate area with its own standards and cannot simply be the same deck counted twice.
• Long term management and maintenance.

The quantity is easy to satisfy on a roof. The usability is what officers question, and elevated space is exactly where the question is sharpest.

Sunlight availability on an elevated deck

Elevated space has one advantage: it clears most of the surrounding obstructions, so the sky view is better than a ground level courtyard in the same block.

The test is the same one used for gardens. Guidance published by BRE on site layout planning for daylight and sunlight suggests that at least half of an amenity area should receive at least two hours of sunlight on 21 March. For a roof terrace the obstructions are usually your own building: the taller wing of the same development, a lift overrun, a plant enclosure, a photovoltaic array on a raised frame, or the parapet itself where the deck is narrow.

Model the deck with everything on it. A sunlight study that omits the plant enclosure and the substation housing is the study that gets corrected at consultation stage.

Where the result is marginal, the moves that recover area are mostly about relocating equipment rather than reshaping the building. Plant on the north side of the deck, arrays on a separate roof, the lift overrun pushed towards the shaded corner. Those decisions belong at concept stage because they are almost impossible to change later. A full Daylight and Sunlight Assessment will produce the numbers, and an Overshadowing Study is often the more useful presentation for a deck, because it shows how the shade moves across the seating areas through the day.

Wind comfort on the same surface

Wind speed rises with height, and a deck at eighth floor level is in a different wind regime from a garden at ground level. Add the downdraught a tall element sheds onto a lower podium and you can produce speeds that make a terrace unpleasant to sit on.

The assessment framework in general use applies pedestrian comfort criteria that assign a threshold wind speed to each activity, from sitting through standing and strolling to walking, plus separate distress and safety thresholds. A Wind Microclimate Assessment tests the proposed geometry, usually by computational modelling and sometimes in a wind tunnel, and maps the deck by the activity each part supports.

What matters for a communal terrace is the sitting category. A deck that achieves standing or strolling comfort across its whole area has no usable seating, and the whole point of the space is sitting in it.

Two features to look for in the results:

• Seasonality. A terrace that is comfortable in summer and in the distress category in winter is not a year round amenity, and the assessment should report by season.
• Local acceleration. Corners, gaps between blocks and the edges of raised elements produce localised speeds much higher than the deck average. The average is not the answer.

Wind microclimate work needs an author who can defend the modelling. Councils expect a suitably qualified assessor for this, and professional review of any draft is strongly advised.

Where the two assessments conflict

The conflict is direct and predictable.

Deciduous planting is the one move that partly resolves the tension, since it shelters in summer and admits low winter sun. It also loses most of its wind performance exactly when winter shelter is needed, so it is a partial answer rather than a solution.

The workable approach is zoning. Accept that different parts of the deck do different jobs: a sheltered sitting zone on the shaded side, an open sunlit zone for standing and circulation, a planted buffer at the exposed edge. Then present the assessment by zone rather than trying to make the whole surface pass both tests.

Screens, planting and canopies as shared mitigation

Details that tend to work:

1. Porous screens of around half open area, which cut wind speed without producing the recirculation eddies a solid screen creates and without the shading a solid screen casts.
2. Staggered baffles rather than one continuous barrier, breaking up the flow along the exposed edge.
3. Planters with substantial evergreen and deciduous mixes at the windward corners, where local acceleration is worst.
4. Canopies over the sitting zone only, sited where the sunlight study shows the area is already shaded for much of the day.
5. Level changes and raised beds used as low windbreaks, which shelter seated users without casting shade at seated height.

Details that tend not to work: full height glass balustrades presented as wind mitigation, which do little for downdraught; single isolated trees in exposed positions, which will not survive; and retractable systems, because a condition cannot require residents to deploy them.

The interaction with enclosed balcony design is worth understanding too, since winter gardens change the daylight calculation for the flats behind them, as our note on winter gardens and enclosed balconies in daylight terms explains.

Usable area after mitigation

This is the figure to report, and reporting it honestly is what gets a scheme through.

Define usable area as the part of the deck that achieves the sitting comfort category and receives two hours of sun on 21 March, excluding circulation routes, maintenance access strips, plant enclosures and any area within the swept path of a door. Then compare that figure with the policy requirement.

If it falls short, the options in order of preference are to move plant, to enlarge the deck, to add a second smaller sheltered terrace at a lower level where wind speeds are lower, or to provide part of the requirement at ground level. What does not work is claiming the full deck area and hoping the assessments are not read together.

Where child play space is required, run the same exercise separately for it. A Play Space Assessment against the applicable standards has its own area, equipment, surfacing and overlooking requirements, and a windy strip along a parapet does not satisfy them.

Securing the space through conditions

The terrace has to still be a terrace in ten years, which means the conditions matter as much as the design.

What to expect, and what to offer before it is imposed:

• Details of screens, planting, surfacing and balustrades to be approved, with the wind mitigation identified as such so it cannot be value engineered out.
• A landscape and management plan, with a named responsible party and a maintenance schedule for the planting that performs the sheltering.
• Access arrangements secured for all residents from all cores, not just the core the terrace sits above.
• A restriction on future enclosure or subdivision of the communal space.
• Hours of use where the terrace adjoins habitable rooms in the same building or a neighbouring one, which is a noise question rather than a microclimate one.
• Plant noise limits at the terrace, since the same roof usually carries the plant.

That last point catches schemes out regularly. A deck that passes sunlight and wind and then sits two metres from an air source heat pump array is not a usable amenity space, and the acoustic assessment needs to treat the terrace as a receptor.

This is general guidance, not legal advice. Amenity space standards and assessment expectations vary by authority, so check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

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