Overshadowing of a communal amenity deck

Communal amenity space on a podium or a roof is tested for sunlight the same way a garden is: the guidance looks at whether at least half the area receives at least two hours of direct sun on 21 March. On a deck surrounded by the building that created it, that test is harder to pass than it looks, because the obstructions are your own blocks rather than somebody else's.

The design response has to happen early. Once the massing is fixed, the only variables left on a shaded deck are where the seating goes and which plants will survive, and neither of those rescues a space that gets forty minutes of sun at the equinox.

How communal space is tested for sunlight

The assessment is a sun-on-ground study: model the proposed massing, plot the shadow across the day on 21 March, and measure the proportion of the usable amenity area that receives two hours or more. The equinox is used because it sits between the extremes, and because it is the date on which the guidance's own figures are set.

Two refinements matter in practice. The usable area is not the same as the deck area, so exclude planters, plant enclosures, circulation that is too narrow to sit in, and any zone with a maintenance restriction. And the test should be run for the summer and winter solstices as well, because a deck that performs acceptably in March and sits in permanent shadow in December will be criticised by residents even where it passes.

Where an existing adjoining amenity space is affected by the proposal, the comparison is the before and after sunlit area, and a reduction below roughly four-fifths of the former sunlit area is treated as noticeable. An overshadowing study should report both the absolute result for the new space and the relative result for existing spaces.

Podium and roof terrace geometry

Three geometric facts set the outcome, and all three are decided at massing stage.

The first is the aspect ratio between the deck's width and the height of the walls around it. A courtyard deck twelve metres across between eight-storey blocks will be shaded for most of the day whatever is done to the surfaces. Widening the gap by three metres does more than any landscape intervention.

The second is where the gap in the enclosure faces. A southern opening lets sun into the deck at midday, which is when the two-hour test is most easily satisfied. A northern opening is close to useless for sunlight, though it may be the right answer for wind.

The third is level. A roof terrace above the surrounding parapet behaves like open ground. A podium deck a storey above grade and surrounded by taller blocks behaves like a courtyard. Moving the amenity space from a podium to a roof is often the single change that turns a failing sunlight result into a comfortable one, at the cost of accessibility from the dwellings.

Wind and shade acting together

The two constraints work against each other, which is why they should be assessed together rather than in sequence. The open corner that lets in sun also lets in wind, and a deck sheltered by solid screens on three sides will be calm and dark.

Comfort criteria for pedestrian areas distinguish between sitting, standing, strolling and walking, and a seating terrace needs the calmest category. On a tall building that usually requires some combination of porous screens, planting with a mature height that works year round, and a plan arrangement that avoids the downdraught zone at the base of the tallest element. A Wind Microclimate Assessment run on the same massing model as the sunlight study saves a round of coordination.

The conflict is real and it has to be resolved by the architecture rather than by the landscape drawing. A screen added late to fix a wind result will reappear as a shadow on the sunlight study.

Planting that depends on sun hours

Landscape architects specify by light condition, and a planting schedule that assumes full sun on a deck receiving two hours in March will fail within two seasons. The honest sequence is to produce the shade study first, divide the deck into light zones, and specify planting to each zone.

Substrate depth compounds the problem. A podium build-up with 300 millimetres of soil in deep shade supports a narrow palette, and the species that tolerate both conditions are limited. Trees need considerably more depth, irrigation and wind stability, so a tree shown on a deck plan needs a confirmed build-up depth and a root anchorage detail before it means anything.

Where play space is provided on the deck, the sun requirement is explicit in policy terms in some areas and implicit everywhere: a shaded play area gets used less. Position play space in the sunniest zone identified by the study, and show that reasoning in the Play Space Assessment rather than placing it where the plan happens to have room.

Locating seating and play by sun

Once the zones are mapped, the layout decisions are straightforward and they are worth recording.

1. Put fixed seating in the longest-sun zone, and provide one shaded seating group for hot days.
2. Place toddler play in sun with partial afternoon shade, and older children's play where ball games will not overlook ground-floor windows.
3. Keep growing beds and any community growing provision in the best-lit zone, because yields depend on it directly.
4. Put bin stores, cycle stores and plant enclosures in the permanently shaded zone, which is the only good use for it.
5. Keep circulation out of the two-hour zone wherever the plan allows.

That last point is the one most often missed. A desire line across the sunniest part of the deck turns the usable amenity area into a path, and the assessment measures usable area rather than deck area.

Evidence for the design and access statement

The package that answers an officer's questions is short: shadow plots at 21 March, 21 June and 21 December at hourly intervals, a two-hour contour plan showing the usable area and the compliant proportion, the light zone diagram, and the planting and layout response keyed to the zones.

Put the numerical result in the text of the Design and Access Statement rather than leaving it in an appendix, and state the usable area and how it was derived. Where the result falls short of the guidance figure, say so and explain the compensating provision: private balconies with their own sunlight performance, a second roof terrace, or access to nearby public open space with its distance stated. Privacy between a deck and surrounding flats is a separate question that often reshapes the layout, covered in our note on roof terraces and neighbour privacy.

Conditions on landscape and use

Expect conditions requiring the landscape scheme, including planting species and sizes, substrate depths, irrigation, and a management and maintenance plan for a defined establishment period. Expect a condition securing the amenity space for the use of residents and preventing its loss to plant or storage.

Two drafting points worth raising at the officer stage. Ask for the landscape condition to allow substitution of species within the same light tolerance class, because nursery availability changes over a three-year build. And check that any condition on hours of use of the deck matches what the acoustic assessment assumed, since a condition closing a terrace at 21:00 when the noise model assumed 23:00 creates a contradiction somebody will have to resolve later.

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