How to assess wind and snow loads for greenhouses

Wind and snow-load figures are meaningful only when the reference basis, test method, greenhouse configuration, foundation and site all match. This guide explains how to compare technical information reliably and what to clarify before ordering and installation.

Wind and snow-load information is intended to show which climatic actions a greenhouse in a defined configuration can withstand. One number rarely answers the question fully. The calculation method, reference basis, fitted accessories, anchoring and actual property location are all important.

This guide helps you read technical documents systematically. It does not replace a local assessment or structural advice where required. For an initial overview of designs, see our greenhouses and conservatories. Other buyers’ experiences in greenhouse reviews can be useful, but they do not replace documented load values.

What a load-capacity figure really means

A structural load figure describes the behaviour of a specified construction under stated assumptions. It is not a general assurance that every model will perform identically at every site, with every installation method and in every weather condition.

For a reliable interpretation, at least the following points should be known:

• Load type: wind pressure, wind suction, snow or another action.
• Reference basis: for example, speed, area load or weight per area.
• Design condition: The figure may refer to serviceability, calculated structural capacity or a test situation.
• Tested configuration: dimensions, glazing, number of doors and windows, braces and other components.
• Boundary conditions: foundation, fixings, closed openings and defined ground conditions.

If any of these points is missing, do not use the number alone to make a purchasing decision. Request the related explanation and check that the documented version matches your order.

Wind figures: establish the meaning before comparing

Wind can press against a greenhouse, create suction on its leeward side and produce uplift at the roof. These forces are not evenly distributed. Corners, roof edges, ridges, doors and roof vents can experience higher local stresses.

A stated wind speed cannot therefore be judged by its magnitude alone. Check whether it refers to a gust or an averaged wind, the height at which it was applied and the terrain profile assumed. An open field, coastal location, slope and densely built-up setting produce different wind flows.

The building envelope also matters. A closed door creates a different pressure distribution from an open or incompletely locked door. The same applies to roof vents. When choosing between attached and freestanding structures, consider their different load paths; our guide wall-mounted or freestanding pergola explains this principle for pergolas.

Terms that can change a wind figure

Term	Why it matters
Gust speed	A short peak cannot be compared directly with a longer averaged value.
Reference height	Wind speed changes with height and with the surface roughness of the terrain.
Terrain category	Open ground and dense development create different wind profiles.
Basic or design wind	Terms can represent different calculation stages in different documentation systems.
Wind pressure	A pressure value already includes assumptions that need checking for a simple speed figure.
Closed building envelope	Open doors or windows can alter internal pressure and component loading.

Do not simply compare two speed values. Compare the complete definitions. The same applies to other outdoor structures. For winter weather effects on pergolas, see pergolas in winter: snow, ice and wind.

Snow loads: ground snow is not roof snow

Ground snow load describes a climatic action at a site. It is not automatically the load that a particular greenhouse roof may carry. Roof form, pitch, drifting, nearby taller structures and local snow accumulation all influence roof loading.

Snow can slide from a sloping glass or panel roof, but this must not be assumed without qualification. Wet snow can adhere, ice can obstruct drainage, and uneven loading can arise at eaves, ridges or junctions. As well as evenly distributed snow, asymmetric loading is relevant.

Ask specifically whether a manufacturer figure identifies an allowable roof load, an assumed ground snow load or only a test load. If documentation simply says “snow load” without stating the reference area and underlying configuration, the information remains incomplete.

Why snow depth alone is unreliable

Ten centimetres of loose fresh snow weighs considerably less than the same depth of wet, compacted snow. An ice layer can be substantially heavier again. Ageing, temperature changes and rain increase density without a matching increase in visible depth.

A blanket conversion from centimetres of snow to an area load is therefore unsuitable for safety decisions. Mass per area, rather than layer depth alone, is what matters. Also observe whether snow gathers on one roof side, behind an obstacle or at a connection.

Comparing greenhouse specifications correctly

Create the same comparison matrix for every model under consideration. Product size and construction must match, because a figure for a smaller version cannot be applied to a longer or wider variant without confirmation. Our pergola size guide shows, for a related product group, why dimensions and intended use must be considered together.

1. Record the model, external dimensions and roof shape.
2. Record the type and thickness of glazing or panels.
3. Check the precise definition of wind and snow load.
4. Record every required brace, reinforcement and anchor.
5. Check whether doors, windows and accessories form part of the assessed configuration.
6. Compare identical units and the same design reference.
7. Keep exclusions and installation conditions in writing.

The roof covering is also part of the system. Background information on multiwall sheets is available under polycarbonate for greenhouses. Material thickness alone does not prove whole-system capacity; profiles, connections and fixings must transfer forces together.

The site can determine the decision

A structure that may suit a sheltered urban location is not automatically suitable for an exposed property. Relevant factors include local wind and snow conditions, topography, elevation and obstacles close by. The criteria that apply at the particular site must be checked locally.

Pay particular attention to escarpments, level changes, open paddocks, valleys with channelled airflow and spaces between buildings. Taller neighbouring buildings can redirect wind and drift snow onto a lower roof. Trees may partly break the wind but can also cause turbulence, falling branches or falling snow.

Your preferred garden style must not be confused with structural suitability. Inspiration such as Victorian greenhouses or an orangerie greenhouse can help define the look, but load requirements still need separate confirmation.

Recording site exposure systematically

• Is the property densely built-up, loosely developed or largely open?
• Is the installation site on a rise, slope or close to an edge?
• Are there higher buildings, walls or roofs from which snow can slide or drift?
• Could trees, loose objects or roof tiles strike the greenhouse during a storm?
• Is there enough clearance to inspect the roof and fixings safely?
• Can water be directed away from the foundation?
• Will doors remain reliably closed and locked in a storm?

Document the site with measurements and photographs from several directions. If you also plan a sauna, assess its location separately. This applies whether you choose an outdoor sauna Elma S or a barrel sauna HC150 ; do not mix product documentation for different construction types.

Consents, local criteria and professional assessment

Whether building consent, resource consent or another approval is needed in New Zealand depends on the actual project, use, dimensions, site and relevant council rules. A building-consent exemption does not remove Building Code or district-plan duties. Ask your council about the greenhouse, including foundations, services, boundary fire separation and stormwater, before ordering.

For an introduction, see our guide building consent for a greenhouse or pergola. However, only the relevant council or appropriately qualified professionals can provide advice specific to your property.

Professional assessment is particularly useful where local requirements are close to the documented limit, the site is exposed, a special foundation is planned or the proposed configuration differs from the documentation. For unresolved technical questions, you can also contact Bloomcabin.

Foundation and anchors belong to the load path

Wind and snow forces do not end at the aluminium frame. They pass through profiles, corner connections and base points into anchors, the foundation and ground. Any break in this load path can limit the overall performance.

A level foundation makes accurate, plumb installation easier. It must also suit the specified fixings and the ground conditions. Existing paving, a thin slab or an edging stone must not automatically be treated as a load-bearing anchoring base. Frost, drainage and possible settlement must also be considered.

Do not transfer foundation details from one product group to another. Our pergola foundation guide explains general planning principles, but does not replace the greenhouse installation instructions. The requirements of an outdoor sauna Elma L also differ from those of a light glazed structure.

Basic rule: High frame strength does not compensate for inadequate anchoring. Use every specified fixing point and do not alter the load path without technical approval.

Before ordering: technical-document checklist

• Is the wind figure supplied with definition, unit and boundary conditions?
• Is it clear whether the snow load concerns ground or roof?
• Does the figure apply exactly to the required size and glazing?
• Are necessary reinforcements and anchors included or purchased separately?
• Is the required foundation design described?
• Are restrictions for doors, vents and accessories stated?
• Have local consent and load criteria been checked?
• Is access suitable for delivery and safe temporary storage?
• Are current installation documents available before construction starts?

Find guidance on the order process under how to order. Information about logistics and receiving goods is available in purchasing and delivery. For delivery throughout New Zealand, including its islands and territories, see greenhouse delivery across New Zealand .

Receiving goods and installation

On arrival, check the number and visible condition of packages. Compare labels and parts lists before disposing of packaging. Profiles, panels and fittings should be stored dry and safely in accordance with product-specific instructions. A full check before installation reduces the risk of a missing part during a critical stage.

Use only the documentation for the supplied version. Instructions for a similar size or visually related model may specify different holes, fixings or braces. The same principle applies to other kits such as the barrel sauna HC 180. Do not mix parts or instructions from different models.

Quality checks during installation

1. Check foundation dimensions, level and diagonals before assembly.
2. Align the frame first according to the instructions and secure connections in the specified order.
3. Fit all braces in the stated positions.
4. Install panels or glazing without stress and in the correct orientation.
5. Check doors and windows for free movement, complete closing and secure locking.
6. Install every specified anchor; do not omit fixing points.
7. After completion, inspect frame geometry, fixings and drainage routes.

Improvised drilling, omitted braces or independently substituted screws can change the assessed configuration. If anything is unclear, stop installation and resolve the technical issue before proceeding.

Maintenance preserves the installed condition

A load figure normally assumes a correctly installed and maintained condition. Regularly inspect anchors, screw connections, seals, glazing retainers, doors and roof vents. An additional visual inspection is sensible after a severe storm or unusual snow loading.

Clear leaves and debris from drainage paths. Repair loose parts early and use suitable replacement parts. More detailed guidance is available in caring for your aluminium greenhouse. Information about possible guarantee terms is available separately under guarantee.

The internal climate affects use, but must not be relied upon as snow-load protection. Guidance is available in greenhouse temperature and humidity.

Common mistakes when reading load figures

• A gust speed is compared directly with an averaged wind speed.
• A ground snow load is interpreted as an allowable roof load.
• A figure for a small model is applied to a larger version.
• Material thickness is treated as whole-system capacity.
• A sheltered garden is assumed wind-free without assessing the site.
• Anchoring is treated as an accessory rather than part of the structure.
• Open roof vents are ignored in the wind assessment.
• Photographs, reviews or long use are treated as structural evidence.
• A visually similar structure is assumed technically equivalent.
• Later-fitted accessories are not checked for structural effects.

The distinction between a bioclimatic and classic pergola also shows that outwardly similar garden structures can function differently in technical terms. Always read load values as product- and configuration-specific.

Frequently asked questions

Is a higher wind speed always better?

Only if both figures use the same wind definition, reference height, averaging period, terrain category, safety basis and product configuration. Without this alignment, the higher number may simply result from a different method of presentation.

Can I convert wind pressure into wind speed?

A rough physical conversion is possible, but usually inadequate for comparing products. Pressure coefficients, gust assumptions, height, terrain, roof shape and internal pressure all affect actual component loading. Do not use a conversion instead of the full specification.

Is ground snow load the weight my greenhouse roof can carry?

No. Ground snow load describes a site value. Roof loading is derived from it while considering roof and site conditions. Ask for the specific design basis for the greenhouse you want.

How many centimetres of snow equal a stated snow load?

This cannot be answered reliably without knowing snow density. Dry fresh snow, compacted older snow, wet snow and ice have very different weights. Mass per area, not visible depth alone, is decisive.

Does an aluminium frame eliminate wind and snow risks?

No. Aluminium enables light, corrosion-resistant structures, but does not remove the need for correct design. Profile geometry, connections, glazing, braces, anchors and foundation together determine performance.

Is an attached greenhouse automatically more stable?

Not automatically. The connection can transfer forces, but it must be suitably designed and executed. The house wall, roof edge and wind flow also change loading. Fixing to facade layers without a verified load path is not a reliable reinforcement.

Do trees or fences provide sufficient wind protection?

They can influence airflow, but do not guarantee adequate protection. Turbulence occurs behind obstacles; trees change with the seasons and can lose branches. The structure must be suitable for the governing local action.

Should roof vents remain open during a storm?

As a rule, openings should be securely closed and locked in accordance with product and operating instructions. An open vent can raise internal pressure and may itself be damaged. Check automatic openers for proper operation and closing behaviour.

Will heating melt enough snow to protect the roof?

Safety should not depend on this. Heat distribution, outside temperature, power loss, insulation and snow type are not reliably constant. Partial melting can also create heavier wet snow or ice.

Can I fit extra braces or anchors to increase the load rating?

A modification may improve or worsen force transfer, or shift loads into unsuitable components. A higher load rating does not result merely from adding material. Changes need a technically sound assessment and should be discussed with the manufacturer or suitable professionals.

What if the local requirement is higher than the product figure?

Do not assume the product is adequate in that documented configuration. Choose a demonstrably suitable configuration or have an appropriate solution professionally assessed. A lower manufacturer figure must not be replaced by experience or assumptions.

Are customer photographs proof of load capacity?

No. Photographs can illustrate design, installation surroundings and use. They show neither hidden anchors nor actual wind and snow loads or possible later damage. Technical documentation and site criteria are required for assessment.

A final decision rule

Order a greenhouse only when three elements align: the locally relevant wind and snow criteria, the clearly documented product configuration, and the foundation and anchoring at the specific site. If any one of these elements is unresolved, check it before ordering or installation.

Further planning and product information is available in useful information. Never assess figures in isolation: a sound decision comes from definitions, site conditions, the complete load path and careful execution.