A green roof is a permanent load pretending to be landscaping, and the honest way to carry it through a takedown is to weigh it wet, place it where it actually sits, and let StructLoads deliver the consequences to every beam, wall, and column below. The numbers span an order of magnitude: a thin extensive sedum build-up adds roughly 0.6 to 1.5 kPa saturated, while an intensive roof garden with deep soil, pavers, and trees can add 5 to 10 kPa or more, heavier than the floor loads of most occupied storeys. Which end of that range a project sits on decides whether the green roof is a line item or the structure’s governing story, and the deciding questions are the same every time: how deep is the growing medium, how wet do you design it, and who is allowed to walk on it.
Extensive, intensive, and the weight between them
The green roof industry splits systems into two families, and the split is really a structural one. Extensive systems are shallow: 60 to 150 mm of engineered lightweight substrate under sedums, mosses, and grasses, unirrigated, visited only for maintenance. Their saturated weight typically runs 60 to 150 kg/m², call it 0.6 to 1.5 kPa, comparable to a generous finishes allowance, and many existing roofs can accept one with modest checking. Intensive systems are gardens: 300 mm to a metre or more of substrate carrying shrubs, lawns, and trees, irrigated and occupied, with saturated weights from 300 kg/m² to 1,000 kg/m² and beyond before pavers, furniture, and people are counted. Between them sit semi-intensive build-ups in the 150 to 300 mm band.
The full build-up stacks more than soil: a root barrier, a drainage and water-retention layer (which by design holds water, so it weighs something wet), a filter fleece, the growing medium, and the vegetation itself. Each layer is light alone; the honest total is the sum at the worst credible moisture state, and that discipline of itemizing everything above structure is the same one that governs any superimposed dead load build-up. A tabulated dry weight from a supplier brochure is the beginning of the calculation, not the end of it.
Saturated is the design condition
Water is the swing factor. Growing media are engineered to hold moisture, and the difference between dry and saturated can double the load: soil densities run from roughly 1,200 kg/m³ dry for loose loam toward 1,700 to 2,000 kg/m³ wet, and lightweight green-roof substrates behave the same way proportionally, absorbing 30 to 50 percent of their volume in water. The retention layer is explicitly a reservoir. Design practice is therefore blunt: the dead load of a green roof is its saturated weight, because a roof that has just taken a week of rain is not an unusual condition but the ordinary one the structure must always be ready for.
Classification follows: the saturated build-up is dead load, permanent and always present, which matters beyond gravity totals. It enters every load combination at the dead-load factor, it counts fully in the seismic weight that drives lateral design, and, in the one direction where the weight helps, it resists wind uplift, though codes rightly make you use a reduced dead load, and arguably the dry weight, when weight is stabilizing, since the hot dry week is exactly when the storm may arrive. On top of the saturated dead load come the variable loads: maintenance live load on extensive roofs, full occupancy live load (terrace values, typically 3 to 5 kPa) on accessible intensive roofs, and snow, which does not politely subtract just because a garden is underneath, and which drifts against parapets and penthouse walls exactly as it would on gravel, stacking with the roof live and snow provisions rather than replacing them.
| Build-up | Depth | Saturated weight | Typical use |
|---|---|---|---|
| Extensive sedum | 60-150 mm | 0.6-1.5 kPa | Maintenance access only |
| Semi-intensive | 150-300 mm | 1.5-3 kPa | Light planting, limited access |
| Intensive garden | 300-1,000+ mm | 3-10+ kPa | Full occupancy, lawns, shrubs |
| Tree pits and berms | locally deeper | point and patch loads | Concentrated on chosen members |
The loads do not spread evenly, and that is the takedown’s problem
A landscape plan is a load map. Intensive roofs are rarely uniform: soil berms mound over some bays, tree pits concentrate a metre of saturated soil plus a growing root ball onto a few square metres, pavers and furniture gather on terraces, and drainage falls collect water toward chosen lines. The structural reading is patch loads and point loads, not one polite average, and the difference lands directly on individual members: a tree pit centred on a slab bay loads that bay’s beams at full intensity while the neighbouring bay sees sedum, and averaging the two flatters both. The right discipline is to place each zone at its own rate over its own area, the way material densities times real depths dictate, and then let the tributary geometry distribute it honestly.
That is the takedown’s job, and it is where a green roof stops being a roofing question and becomes a whole-building one. The roof beams see the patches directly; the columns under the heavy zones collect a premium that runs down every storey to the footings; and on an existing building, the question “can we add a green roof” is answered not at the roof but at the most stressed column splice or footing below it. Modeled in StructLoads, each landscape zone is drawn as its own area load at its saturated rate, and the multi-storey accumulation follows automatically, showing precisely which columns care about the garden and which never notice it.
A worked example: sedum field, terrace, and one tree
Take a 24 m by 18 m roof, illustrative round numbers throughout. Most of it carries an extensive sedum build-up at 1.2 kPa saturated. A 6 m by 8 m terrace zone carries a semi-intensive build-up with pavers at 3.0 kPa dead plus 4.0 kPa occupancy live load. One 3 m by 3 m tree pit holds 900 mm of saturated soil at about 17 kN/m³: 15.3 kPa over 9 m², roughly 138 kN, plus the tree.
The totals tell one story: sedum contributes 1.2 x (432 − 48 − 9) = 450 kN, the terrace 48 x 3.0 = 144 kN dead plus up to 192 kN live, the tree pit about 140 kN, a bit over 900 kN of new load on the roof. The distribution tells the real one: spread evenly, that would be about 2.1 kPa everywhere, but the beam under the tree pit actually receives a patch more than seven times the sedum rate, and the terrace columns carry roughly triple the sedum bays’ premium. The column at the terrace corner, taking a quarter of the terrace patch plus its ordinary tributary, gains around 84 kN at this level alone, and that gain rides down the stack to its footing. An average would have told that column it gained half as much, which is exactly the kind of error that surfaces years later as an overstressed pad.
Existing buildings and the construction stage
Retrofitting a green roof onto an existing building inverts the workflow, the same logic as any vertical extension: the capacity is fixed, and the question is how much garden it buys. The reserves hide in specific places. A roof originally designed for gravel ballast that is being removed trades roughly 1 kPa of old dead load for the new build-up, often covering an extensive system entirely, the same reserve hunt that decides solar retrofits. A roof where snow governs by a wide margin may absorb a light sedum layer inside the snow case’s slack, because the saturated build-up and the design snow event stack in the same combination but the members were sized for the peak. And a roof whose live load reduction was never taken in the original design carries hidden margin in the columns below. None of these reserves can be claimed by assertion; each is demonstrated by rerunning the takedown with the new loads in place and comparing member by member, which is a few minutes of remodeling rather than a guess.
The construction stage deserves its own check, because the heaviest day in a green roof’s life is often before it is finished. Substrate arrives in bulk bags craned to the roof, and a pallet of saturated bags stockpiled on one bay can exceed the finished design load several times over on a patch the size of a car. Staging plans belong in the load documentation: where bags may land, how many at once, which bays are off limits. The permanent design being correct does not protect a beam from a temporary pile placed where nobody checked.
Key takeaways: green roof loads
Weigh it wet: saturated substrate plus every layer of the build-up is the dead load, entering combinations, seismic weight, and settlement checks as a permanent presence, while snow and occupancy stack on top rather than substituting. Respect the range: 0.6 to 1.5 kPa for extensive sedum, 3 to 10+ kPa for intensive gardens, with tree pits and berms as concentrated patches on specific members. And distribute honestly: place each landscape zone at its own rate in the takedown, because the columns under the heavy zones, not the roof average, decide whether the structure can carry the garden.
Quick answers
How heavy is a green roof?
Saturated, an extensive sedum build-up adds roughly 0.6 to 1.5 kPa (60 to 150 kg/m²), a semi-intensive system 1.5 to 3 kPa, and an intensive roof garden 3 to 10 kPa or more before occupancy live load, with tree pits locally far heavier. The design number is always the saturated weight of the full build-up, substrate, retention layer, drainage, and vegetation together, treated as dead load; supplier dry weights understate the real condition the structure lives with.
Is a green roof dead load or live load?
The saturated build-up is dead load: permanent, always present, factored as dead in every combination and counted fully in seismic weight. Variable loads stack on top: maintenance live load on extensive roofs, terrace-level occupancy load (typically 3 to 5 kPa) on accessible gardens, and the site’s full snow load including drift against parapets. Where the weight helps, resisting wind uplift, use the reduced or dry value, since the stabilizing weight may be absent in a dry spell.
Why must green roofs be designed at saturated weight?
Because the wet state is ordinary, not exceptional: growing media are engineered to retain water, retention layers are reservoirs by design, and a week of rain routinely doubles a dry substrate’s weight, with wet soils reaching 1,700 to 2,000 kg/m³. A structure checked only at dry weight is understrength every wet month of its life. Saturated dead load is the honest permanent condition; dry weight matters only where lightness governs, as in uplift resistance.
Do snow loads still apply on a green roof?
Yes, in full: snow accumulates on vegetation exactly as it does on gravel, drifts against parapets and penthouse walls the same way, and adds to the saturated build-up rather than replacing any of it. Some jurisdictions allow no reduction whatsoever for the garden underneath. The governing winter case is saturated dead load plus drifted snow, and on accessible terraces the code’s roof live versus snow rules decide which variable case governs each zone.
When should you not average a green roof load across the whole roof?
Whenever the landscape is not uniform, which is nearly always on intensive roofs: berms, tree pits, terraces, and paver zones concentrate weight on specific bays, and an average flatters the loaded members while padding the light ones. A tree pit can load its bay at several times the field rate. Draw each zone at its own saturated rate, in StructLoads or by hand, and let tributary geometry deliver the real premiums to the real columns.