Planting Over a Structure Questions Checklist

Write down what sits directly beneath the proposed planted area, including any room, garage, store or space belonging to somebody else.

The consequences of anything going wrong land in that space, which is what makes this different from a border.

Establish who owns the structure, who owns the layer keeping water out, and who owns the surface you would plant on.

These are frequently three different parties, and finding that out early changes the whole shape of the project.

Check whether any lease, agreement or management arrangement covers what may be placed on that surface, and who would need to agree.

Permission questions here are contractual as often as they are technical, and they take longer than the design does.

What may be placed on a shared or leased structure is determined by the relevant agreements and the parties to them, which vary by property.

Note when the structure was built or last worked on, and gather whatever drawings, warranties or specifications exist for it.

Whether anything is known about the structure decides whether a designer starts from information or from investigation.

Ask whether the structure was designed to carry planting at all, or whether it was built as a roof, deck or slab only.

A structure never intended to carry this is not necessarily ruled out, but it is a different conversation from one that was.

What a structure can carry is a determination for a qualified structural engineer for that structure, and cannot be inferred from its appearance or use.

Note anything visible from the space below, such as staining, damp or previous repairs, and record it without concluding what it means.

The space underneath reports the condition of the layers above it, and it is the one place an owner can look safely.

Note what the surface itself is used for, including whether it forms part of a route people would use to leave the building.

A surface with a role in how people get in or out is a different proposition from one that is only looked at.

Whether planting may occupy any part of a route used to reach or leave a building is a matter for qualified professionals and the local authority.

Establish who is responsible for the layer keeping water out, and whether any warranty or guarantee applies to it now.

Placing anything on top of it can affect who stands behind it, and that question is answered before rather than after.

Ask what may and may not sit on that layer, and get the answer from whoever is responsible for it rather than from a general source.

What is acceptable varies by the particular arrangement, and a general answer is not one anybody will stand behind.

What may be placed over a waterproofing layer is confirmed by the party responsible for it and by a qualified professional for that installation.

Ask what would have to be removed to reach that layer if it ever needed attention, and how long that would take.

Whether the planting can be taken off again is the question that decides how much of a commitment this is.

Ask how roots are dealt with in relation to that layer, who specifies whatever is used, and who stands behind it afterwards.

This is a specialist matter rather than a planting preference, and the answer belongs to the waterproofing rather than the garden.

Note anything that already passes through the surface, such as a vent, an outlet, a light or a guarding fixing, and mark each on a plan.

Penetrations are where two requirements meet at one hole, and planting around them is a detail somebody has to own.

Ask what the arrangement would be if water appeared below, including who investigates and what has to be moved to find it.

Agreed in advance this is a procedure; unagreed it is an argument held while somebody's ceiling is wet.

Ask whether any warranty, guarantee or insurance arrangement covering the structure or the space below would be affected by planting placed on it.

Placing anything on a waterproofed structure can change who stands behind it, and that is answered before rather than after.

What any warranty, guarantee or insurance arrangement covers is a matter for the parties to it and their own advisers, and varies by property.

Ask a qualified structural engineer what the structure can carry before any planting scheme is drawn rather than afterwards.

A scheme drawn first has to be redrawn, and the redrawing usually removes the part the owner most wanted.

What loads a structure can carry, and where, is determined by a qualified structural engineer from the structure's own records and condition.

Ask whether weight may sit anywhere on the surface or only in particular positions, and get that marked on a plan.

Where load may go is frequently more restrictive than how much, and it decides where beds can be at all.

Ask whether the answer accounts for the medium being saturated, and record what you are told rather than assuming it does.

Dry and saturated are different conditions, and the difference is exactly what a wet week delivers.

List everything else the surface would carry, including containers, furniture, a store, people gathering on it and anything brought out seasonally.

Planting is rarely the only thing placed up there, and the engineer needs the whole list rather than the part you are designing now.

Flag any tree or large plant separately, including what it would be growing in and what would hold it against wind.

A large plant is a different question from a bed, and it brings anchorage and exposure with it.

Keep the structural answer in writing with the property records, since every future change up there will need it again.

The next person to put something on that surface starts by asking the same question, unless the answer is findable.

Ask where water leaves this surface, how many outlets there are, and mark each on a plan of the space.

Everything planted up here sits above a route to an outlet, and knowing where those are shapes the layout.

Ask how each outlet is reached and kept clear once planting is in place, and who would do that.

An outlet that planting has grown over is the commonest way a surface like this starts holding water.

Ask what happens if an outlet blocks, whether there is a secondary route, and where surplus water would go.

The secondary route operates only when the first has stopped, so nobody ever sees it working or notices it missing.

Ask what depth is available for growing, and what decides that, recording the answer rather than working towards a number you wanted.

Depth is set by the build-up and the structure rather than by the planting ambition, and it constrains what can be grown.

Available build-up depth and the layers within it are specified by qualified professionals for that structure and its waterproofing.

Ask a horticultural professional what the available depth reasonably supports, and take that as the starting point for the planting.

Asking in this order avoids designing a scheme the structure cannot host and then negotiating with it.

Note how exposed the space is to wind and sun, and ask what that means for how often it would need water.

Raised spaces dry differently from ground, and that is the part that sets the recurring commitment.

Walk the route materials would take to the space, note every stair, door, corner and threshold, and photograph it.

The route is the constraint that quietly removes options, and it is rarely looked at before a scheme is drawn.

Ask roughly how much material would have to be brought up, and by what means, and where it would stand while work happens.

Material has to wait somewhere, and up here there is usually nowhere for it to wait.

Ask what protection the route and the surface itself need during the work, and who provides and removes it.

The surface being worked on is also the layer keeping water out of the space below, which makes protection a different matter here.

Ask whether any lifting or mechanical access is needed, where it would stand, and what agreement that would require.

Access arrangements of this kind involve other parties and lead times, and they are easier raised early.

Access arrangements, lifting operations and working at height are planned and carried out by qualified professionals under their own arrangements.

Ask how material, arisings and spent growing medium would come back down again afterwards, and who arranges and pays attention to that.

Everything that goes up eventually comes down, and the downward journey is planned by nobody.

Note what would need bringing up routinely afterwards, such as compost or replacement planting, and who would carry it.

A recurring haul up a stair is the reason planting up here is often quietly abandoned rather than removed.

Establish who is permitted to go onto the surface and under what arrangements, and whether that includes a gardener or only particular contractors.

The answer is occasionally nobody without particular arrangements, which changes the entire upkeep picture.

Working at height and at edges is subject to arrangements made by qualified professionals; nothing here establishes who may safely work on any surface.

Note where any edge, guarding or parapet sits, mark it on the plan, and ask what that means for working near it.

Where somebody can and cannot stand shapes where planting should be, and that is a layout decision.

Ask what tools and methods may be used on this surface, and whether anything is ruled out because of what is beneath.

Ordinary garden tools meet a layer here that they do not meet in a border.

Ask where prunings, spent compost and debris go, and how they leave, rather than assuming a bin will be near.

Arisings pile up on a surface with drainage outlets, which is a quiet route to a blocked one.

Ask what watering arrangement is proposed, where the water comes from, and what happens if it fails while nobody is up there.

An unnoticed failure up here is both a planting loss and a question about where the water went instead.

Keep the structural answer, the waterproofing advice and the upkeep arrangements together in one record that stays with the property rather than with you.

Somebody will want to change this space again, and what they find determines whether they start well or badly.