How to read greenhouse dimensions and technical drawings

This guide explains how to assess greenhouse dimension drawings, plans, elevations and assembly documents. Use it to check the outer footprint, foundation, door area, heights, interior and connections before ordering or installation.

A greenhouse drawing is a condensed technical description, not a decorative product image. It shows the structure’s footprint, where components meet, and clearances required for foundations, assembly and use. Errors usually come from confusing nominal, external, foundation and clear internal dimensions rather than from complex geometry.

Never rely on a single dimension. Treat the plan, elevations, details and assembly instructions as one document set. Have any unclear measurement confirmed in writing before constructing the foundation, especially for large orangeries, T-shaped layouts and models joined to an existing wall.

Identify the drawing type first

The type of drawing determines what you can establish. A plan view shows the greenhouse from above. It is useful for checking width, length, door position, building shape and foundation outline. A front, side or rear elevation describes heights, roof pitches, eaves, ridges and the division of wall and roof panels. A section looks conceptually through the building and may show clear internal heights or profile connections.

Exploded drawings and assembly steps show component order, fixing points and orientation. Details enlarge connections such as the base, threshold or wall profile. Perspective views aid spatial understanding but are normally unreliable for setting out.

Record each document’s model, variant, revision and view. On apparently symmetrical structures, mark north or label the garden side to avoid confusing left and right.

Understanding the main greenhouse dimensions

Overall width and overall length

Width and length may use different reference points: a profile’s outer edge, the base edge, a structural centreline or the foundation edge. Dimension arrows and extension lines identify them. Lines reaching the outermost profiles usually indicate an external dimension; lines ending at an inset base may indicate the base footprint.

For rectangular models, multiplying two values is not enough for complete site planning. Roof overhangs, downpipes, door movement and assembly access may extend beyond the stated footprint. Information on choosing different designs is available in Greenhouses and conservatories. The guide to orangeries, sizes and designs also covers the design background to more complex installations.

Eaves height, ridge height and total height

Eaves height is taken where the side wall and roof meet; ridge height is the highest point of a conventional gable roof. A stated total height may include other components. Always check the drawing’s lower datum: finished foundation top, ground, base underside or another defined level.

Height affects more than headroom: check branches, gutters, wall projections and other site features. Open roof vents can extend beyond the closed envelope, so do not plan only from the closed elevation.

Door dimensions

For doors, distinguish the structural or frame opening, clear passage width and door unit’s external size. For wheelbarrows, benches or equipment, the clear opening matters; the frame’s outside measurement does not show the usable passage.

Also check how the door opens and the space it needs. A hinged door requires swing clearance; a sliding door needs space along the façade. Threshold height and final path construction may also matter. Assess the door with the external path, internal path and foundation, not in isolation.

Bay grid and glazing divisions

Many aluminium greenhouses have repeated bays. Dimension chains may show a centreline grid, individual panel widths or total length. Add partial measurements only when they clearly use the same reference points. Profile widths or end bays can prevent assumed panel widths from adding up exactly to the outside measurement.

The division is useful for accessories, ventilation openings, shelving and planting plans. However, it must not be treated as a freely selectable building grid. The article on polycarbonate in a greenhouse explains the characteristics of multiwall sheets. The model’s technical documents remain authoritative.

Reading the language of dimension lines

Technical dimensioning usually combines a value, dimension line, arrows or end marks, and extension lines leading to the measured points. An overall dimension covers the full distance; a chain divides it into sections. Depending on the document, bracketed or specially marked values may be informative only. Avoid assumptions and consult that drawing’s legend.

Symbols for diameter, radius, angle, fall or level each have a distinct function. Centre lines and hidden edges must not be confused with real external edges. If no legend is provided, clarify every interpretation that is important for construction.

Never measure from a screen or printout

Do not measure a PDF on screen with a ruler. Zoom, screen size and page fitting alter its appearance; printing can also reduce, enlarge or distort it. Even when a scale is stated, the printed dimension figure prevails unless the document says otherwise.

If the drawing says “not to scale” or uses an equivalent abbreviation, use only the stated dimensions. A missing value must not be derived by measuring the graphic.

Check units on every document

Check whether measurements use millimetres, centimetres or metres. Product drawings often use millimetres; site plans commonly use metres. Mixed documents invite errors. Keep units with every calculation and convert only at the end.

Decimal separators also deserve attention. Do not transfer values unchecked into planning software or spreadsheets whose language settings treat commas and full stops differently.

Nominal size, actual footprint and usable internal space

A size label often organises models within a range and need not match the outer footprint exactly. The nominal size is therefore not automatically a foundation dimension. The actual footprint describes the outline needed for the plinth or base. The usable internal space is further reduced by profiles, braces, doors and internal fittings.

Dimension type	Typical use	Check before construction
Nominal size	Model comparison and sorting	Do not use directly for foundation work
Maximum external size	Space requirement and clearances	Include projections and opening components
Base or bearing dimension	Planning the load-bearing base	Check reference edges and squareness
Clear internal dimension	Paths, benches and use	Allow for braces and door areas

Similar terms occur with other outdoor structures. The pergola size guide, the guide to pergola foundations and the product page for the Elma S outdoor sauna show why product size, external outline and installation area should always be considered separately.

Break complex plans into separate zones

T-shaped designs, orangeries and structures with projections are easier to check when divided into rectangles and connection zones. Mark the main body first, then each cross wing, entrance and recess. Record every zone’s width, length and position relative to the common axis.

1. Mark the outermost overall outline.
2. Add main axes and symmetry lines.
3. Dimension each partial area separately.
4. Count and mark internal and external corners.
5. Check dimension chains against the overall dimension.
6. Add the door and path zone.

A plan can contain mathematically plausible partial dimensions yet still be orientated incorrectly on the site. Define one unambiguous reference side, such as “house side” or “garden gate”. Historically inspired design ideas in the article on Victorian greenhouses should be considered only after this technical groundwork.

Transfer the drawing to the installation site

Begin from one fixed reference line, not four separately placed points. Use a building line, string axis or accurately surveyed baseline, then set the other points out square to it. Mark external and foundation outlines in different colours.

Local requirements must also be checked before work begins. Whether building consent, resource consent or specific conditions apply depends on the structure, its use, foundations, services, site and council district plan. The article Building consent for a greenhouse or pergola offers initial guidance; confirm the current requirements for your individual project with the relevant council.

Check more than four corners

Four corners are insufficient for long sides or articulated plans. Check intermediate axes, the door opening, recesses and connections, then measure both diagonals and side lengths. Equal diagonals establish a rectangle only under suitable geometric conditions and do not replace level or height checks.

Also check falls, high points and level changes. A correct horizontal outline can still cause installation difficulties on uneven ground.

Set out a full-size model where space is tight

Strings, marking spray or flat battens make the required space visible. Place a ladder or cardboard marker at ridge height or near the eaves, open garden gates and simulate a wheelbarrow route. This exposes conflicts that a small plan may hide.

Drawings for lean-to and wall-mounted greenhouses

For a lean-to greenhouse, the existing wall has an important geometric and structural role. You need more than width and depth: check connection height, roof pitch, wall-profile positions and clearances to windows, pipes, roof overhangs and drainage.

Inspect the wall for the full connection length. Base projections, uneven render or slopes may stop a profile sitting flush. Assess the wall construction’s project-specific suitability and load capacity. A similar distinction appears in the comparison wall-mounted or freestanding pergola. Further construction differences are covered under bioclimatic or classic pergola.

Plan the interior using clear dimensions

Clear widths are decisive for use. Do not simply subtract an assumed profile thickness from the outside dimension. Use stated internal dimensions or have critical clearances confirmed. Plan the main path, secondary paths, work surfaces, plant containers and door area separately.

Tall crops do not need ridge height everywhere. Under a pitched roof, available height decreases towards the side wall. Vent windows, braces and gutters must not be blocked by shelving. Articles on lettuce in the greenhouse and temperature and humidity can help with use-focused planning. The structure must nevertheless remain accessible in accordance with assembly and maintenance instructions.

Use documents in the right order

Check the locally displayed library of assembly instructions early. A sensible working sequence is:

1. Identify the product variant and order scope clearly.
2. Read the technical overview drawing for external outline and heights.
3. Identify the foundation or base drawing.
4. Check the assembly instructions for base, door orientation and connection details.
5. Compare the current document revision with the supplied documents.
6. Clarify open dimensions before making permanent components.

If two documents differ, do not choose the value that merely looks more plausible. Record the model, document title, revision status and affected dimension, then contact us. General help is available under Useful information.

Checks on delivery and before assembly

Before assembly, compare the model designation, package count on delivery documents and visible component labels. Open and store packages only as instructed. The site plan may be correct even if another variant or door configuration was selected or delivered.

• Do the model and variant match the order?
• Does the door position match the approved plan?
• Is the foundation complete, level and dimensionally checked?
• Are all necessary access areas available?
• Have connection details and drainage routes been clarified?
• Is the correct assembly guide available?

Organisational guidance is available under Purchasing and delivery and How to order. For delivery to New Zealand, including islands and territories, check the product-specific delivery terms; free delivery applies to most products. Greenhouse delivery to New Zealand provides relevant guidance.

Common mistakes when reading drawings

• Using the model name as a foundation dimension: A rounded size description is not a construction dimension.
• Reading only one view: The plan and side elevation answer different questions.
• Confusing outside and inside dimensions: This can make paths or work areas too narrow.
• Ignoring door movement: The opening later conflicts with a bed, fence or step.
• Planning a mirror image: A perspective image is wrongly read as a binding door orientation.
• Measuring the PDF: Zoom and print scaling create incorrect values.
• Mixing units: Millimetre values are treated as centimetres or metres.
• Checking diagonals only: Levels, side lengths and intermediate points remain unchecked.
• Forgetting maintenance space: External surfaces and drainage later become inaccessible.

Guidance on long-term access is available on the page Caring for an aluminium greenhouse. Experiences shared by other buyers in greenhouse reviews may offer additional practical perspectives, but they do not replace a model-specific drawing.

Questions before approving the greenhouse layout

• Is the exact model and equipment variant specified?
• Which line identifies the maximum external size, and which identifies the base?
• From which height level are eaves and ridge measured?
• Where is the clear door opening and how does the door move?
• Is the structure correctly orientated on the site plan?
• Are there external clearances for assembly, cleaning and drainage?
• Have wall projections, trees, services and site slopes been recorded?
• Do partial dimensions, overall dimensions and diagonals agree?
• Is the foundation solution confirmed for the model and site?
• Have local requirements been checked before construction?

Frequently asked questions

Are stated greenhouse dimensions internal or external?

Do not make assumptions without an explicit label. Product overviews often state nominal or external sizes, while technical drawings may also include base and internal dimensions. Dimension arrows, reference lines and the legend of the particular document are decisive.

Can I build the foundation from the dimensions on the product page?

Not without checking the model-specific foundation, base or assembly drawing. A product page is primarily for selection. Have critical dimensions confirmed before pouring concrete, building masonry or making other components that are difficult to alter.

Why does the nominal size differ from the technical drawing?

Nominal sizes make it easier to classify a model range and may be rounded. A technical drawing, by contrast, describes specific reference points. Profiles, bases and construction divisions can lead to different actual dimensions.

What does “not to scale” mean?

The drawn length must not be determined with a ruler or screen tool. Use only the inserted dimension figures. If a required value is missing, ask for it.

How can I tell which side the door is on?

Use the plan and look for the door symbol, opening direction or component label. Also define from which side the building is viewed. If the variant can be mirrored or the drawing is unclear, confirmation is required.

Which dimensions matter for a lean-to greenhouse?

Besides width and depth, check wall connection height, eaves position, highest roof edge, roof pitch, profile position and clearances to existing elements. Also assess wall flatness and the structural suitability of the supporting construction.

How much extra space should I leave around it?

There is no single value for every model and property. The required space depends on the assembly process, cleaning, open windows, drainage, paths and local clearance rules. Plan a genuinely usable working and maintenance area and check project-specific council requirements.

Do equal diagonals prove the foundation is correct?

Not completely. Equal diagonals are an important check for a rectangular plan, but they must be assessed together with side lengths, intermediate dimensions, levelness and height. For complex shapes, check every separate zone.

Which document applies if two dimensions differ?

This cannot be decided generally by document type. Check the model allocation and revision status, and have the governing value confirmed by the supplier. Do not alter the foundation until the matter is clarified.

Where can I find guarantee and care information?

Terms are available in the Guarantee section. Care guidance must be considered separately because dimension drawings, assembly instructions and maintenance information have different purposes.

Final drawing check

Make the final check at the set-out site, not only at a desk. Compare the model, document revision, outer outline, base dimension, heights, door, diagonals and connection zones. Then test use: can the door operate fully, paths remain passable, and roof vents, gutters and outer surfaces stay accessible?

A dependable plan emerges only when the same reference points are used in product documents, foundation drawing, assembly instructions and site set-out.

Record approved dimensions in a simple checklist and photograph the set-out. This makes the planned variant traceable. The Bloomcabin Blog offers further ideas for use, but technical decisions should always be based on the documents for the selected model.