How to Measure Sliding, Patio and Bi-Fold Doors Correctly
Planning guide for patio access
A large glazed opening onto a patio can look straightforward on a drawing: choose a width, height and door. On site, however, the structural opening, finished floor level, wall build-up, travel direction and adjoining patio must work together before the door can be sensibly planned. One measurement is not enough, especially for sliding and bi-fold doors. Ordering a preferred size too early can create clashes with flooring, shading, furniture or drainage. This guide clearly distinguishes a site survey, planning dimensions and the final order size. It helps you document the opening consistently and ask the right questions of designers, installers and suppliers.
Check before making an enquiry 1
First establish the door type at the opening
Your measuring approach starts with the movement the opening needs to provide. A single-leaf patio door requires a swing area; a sliding door moves one panel parallel to the wall; a bi-fold system folds several leaves into a stack. This affects more than overall width: it also affects usable passage width, furniture placement and the operating side. Do not compare total dimensions alone. Draw a plan showing the patio, seating, kitchen island or radiators, and mark where people will actually walk. Wide glazing may bring in generous light while a narrower moving panel is enough for daily access outside. Conversely, a frequently used route for carrying items outdoors may call for a different arrangement from an occasional balcony entrance.
For an initial comparison, you can consider a Patio Life lift-and-slide door as an example of a large sliding solution alongside a conventional hinged door. Its name alone does not confirm dimensions, performance or equipment; request these separately. The drawing is what matters: which panel is fixed, which moves, and on which side should it sit when open? Also note whether floor-to-ceiling glazing should visually continue an adjoining window. For alterations, establish whether the existing wall opening will remain unchanged or be redesigned. Measurements only have a reliable purpose once the use, views and movement sequence are clear.
Record structural and clear openings separately
The structural opening is the building aperture before installation; the clear opening is the unobstructed passage or visible opening within an existing construction. Both are useful, but they are not interchangeable. In new construction, measure width at the bottom, centre and top between the reveals, and height at the left, centre and right from the intended reference level. In an existing building, linings, old frames, plaster edges and thresholds can hide the true building opening. Always state exactly which surfaces each measurement runs between. A photograph with marked measuring points can prevent misunderstandings later.
Use a rigid tape measure or suitable laser distance meter, and check notable differences manually. Record every individual measurement in millimetres, not only the largest value. The smallest dimension may govern planning, but the cause of variation matters just as much: an uneven reveal, sloping floor or uncertain plaster build-up requires a different response from a uniformly sized opening. Measure the diagonals too. Different diagonal dimensions show that the opening is not square. This record does not replace a survey by a qualified professional, but it provides a precise basis for discussion and comparing options.
Check before making an enquiry 2
Set the level at the finished floor
For patio doors, height means little without a clear reference point. Use the top of the finished floor as the reference and mark it on the wall and drawing. In a new build, include every layer still to be installed, such as build-up, screed and floor finish. In an existing property, establish whether the internal floor will remain, be renewed or be replaced with a finish of different thickness. Measure from this planned level to the underside of the lintel. If the floor build-up is still undecided, also record the current height to the structural slab. This shows whether a figure represents an interim condition or the eventual usable height.
Outside, record a second level sequence: from the same reference level, across the threshold zone, to the top of the future patio finish. Slope, finish thickness and the facade connection belong on the drawing, even if they are not finalised. A door may work internally while the external junction remains unresolved. A sliding door should therefore be assessed against the planned floor and patio arrangement, not solely against the structural edge visible today. Ask the designer or installer which information is needed for the intended installation and who will coordinate the levels. During renovations, that person should also know whether adjoining rooms are intended to finish at the same level.
Record width, depth and position within the wall
As well as opening width, wall depth affects how the frame, reveal and internal junctions come together. Measure wall depth in several places and note its build-up where known: structural layer, insulation, lining, plaster or service wall. A photograph of the visible reveal alone is not enough. In a multi-layer external wall, the installation position must be coordinated with the complete wall construction. Draw the proposed door position within the wall depth if it has already been decided. Without this information, an external view can only be coordinated to a limited extent with internal reveals, shutters or window boards. Mark unknown layers clearly as unverified. Also check whether visible wall depth matches at the top, bottom and both sides. Differences can indicate changing linings or concealed junctions. A small section drawing showing inside and outside is often more informative than several unlabelled detail photographs.
Measure the wall sections available to the left and right, up to corners, downpipes, columns, sockets, radiators or other openings. In a side-sliding system, a fixed panel may shape the elevation while the moving panel needs no swing space when opening. A side-sliding Siegenia door cannot therefore be assessed from total width alone: record particularly which side is intended for the moving leaf and which view is being considered inside and outside. If curtains, solar shading or fitted furniture are planned, mark their required space too. This reveals early whether the controls, handle and access for cleaning will remain reachable.
Check before making an enquiry 3
Always mark the opening direction from the correct view
The terms left and right quickly cause errors if the viewpoint is not recorded. Document every door with two simple views: an internal view and an external view. For hinged doors, draw the hinges on the relevant side and show the swing arc. State explicitly whether the door opens inward or outward. For sliding elements, add arrows for travel and identify fixed and moving panels. For bi-fold systems, show the fold points and stacking side. A photograph of the existing situation helps, but it cannot replace arrows because the desired new movement direction cannot be reliably derived from a photograph.
An outward-opening leaf can save space indoors, but it needs a clear area outside. A single-leaf outward-opening patio door should therefore be checked particularly against planting, balustrades, steps and furniture. An inward-opening leaf, by contrast, takes up room space, for example in front of a dining table or bench. Do not ask only “Which side is left?” Ask: from which viewpoint is the side named, where does the leaf move, and can the door open fully? These three details make an order much clearer. For future use with a pram, tray or garden furniture, also establish whether the intended route works without people having to move aside.
Measure a bi-fold system as a movement sequence
With a bi-fold sliding door, the number and arrangement of leaves determines how wide the opening is in everyday use. First measure the full opening, then allow space for the folded leaf stack. Where will it park: inside or outside, left or right? Can it stand there without blocking a corner, light, screen or route to the patio? Also identify which leaf should serve as the everyday access door, where this is intended. This avoids opening the entire system for a brief trip outside. For wide openings, a scale paper sketch is worthwhile for checking the movement area clearly.
An outward-opening bi-fold door is an example of a configuration where external space and stacking direction need especially careful planning. Before making the final choice, confirm the leaf division, operation and required clear spaces using product documentation and the proposed installation. Where a patio already exists, measure distances to balustrades, level changes and canopy supports. A leaf stack that fits on a plan can still narrow the main route in real use. During renovation, also note whether old fittings, linings or an existing canopy affect the desired opening. Check the movement first, before making the visual decision about equally sized panels or glazing bars.
Check before making an enquiry 4
Assess the passage, not total width
The overall width of a system does not automatically describe the clear route outside. With hinged doors, the frame and open leaf affect the space differently from sliding or bi-fold systems. In multi-part systems, one panel may remain fixed, or the parked leaf stack may take up part of the width. Set an everyday-use test for your plan: should one person pass comfortably carrying a tray, will garden furniture be moved regularly, does the route lead to a dining area, or is the door primarily for ventilation and views? This description is more useful than the general request for “as large as possible”. It guides the selection towards a suitable opening type and sensible leaf arrangement.
A single-leaf inward-opening patio door can be the straightforward solution where wall width is limited, provided the room has clear swing space. A larger system may extend the view and access, but needs more coordination with the structure, wall junction and outdoor area. Draw furniture at its actual depth rather than idealising it; include curtains and the space needed to open a leaf. Test the route with two people moving in opposite directions if the patio and kitchen are often connected. Record the resulting passage requirement as a usage goal, not as a self-determined order size. The clear dimensions ultimately achievable depend on the chosen construction and confirmed survey.
Document lintels, reveals and the existing building carefully
The top of the opening deserves the same attention as the floor. Measure to the lintel at both sides and in the centre, photograph visible cracks, services, roller-shutter boxes or unusual junctions, and note their positions. During replacement, the old frame may conceal parts of the reveal. Do not remove anything simply to measure if this could damage the building fabric or waterproofing; record visible dimensions and uncertainties instead. If the opening is to become wider or higher, ask the relevant municipality whether plans or approval are required, and have suitably qualified professionals assess changes to the wall and effects on adjoining components.
Also check the flatness of side surfaces with a long straightedge, recording bulges or recessed areas. This matters because a neat measurement alone says nothing about the junction surface. Note whether a window board, radiator, lining or electrical installation immediately adjoins internally. Outside, brickwork, plaster, cladding or existing waterproofing can affect detailed planning. Dividing the survey into structural opening, internal junction and external junction gives the professional a better basis for decisions. The final solution should not be based on the chance that an old door leaf happens to fit the current opening.
Check before making an enquiry 5
Draw the patio, water route and outdoor area
A door does not end at its frame. Draw the first metres outside the facade: patio finish, slope, steps, drainage elements, canopy, balustrades, planters and paths. If the patio is being renewed, add the proposed layout rather than only the existing one. In rainy weather, outdoor planning must not be treated as an afterthought; junction details need to suit the door project. This guide does not replace technical waterproofing or drainage design. It does, however, help make the essential questions visible in time: where will the finished patio surface sit, where will water run, and who will coordinate the door, facade and patio?
For a double-leaf outward-opening patio door both swing areas should be drawn on the external view. Check whether open leaves obstruct paths, seating or a step. The answer need not always be a different door model: furniture, a barbecue parking position or route layout can sometimes change instead. The important point is to make the choice consciously. Also note whether an awning, external solar shading or insect screen is planned, so its installation space and operation can be discussed early. In a townhouse garden, even a narrow side passage may be decisive; on a generous patio, furniture in front of the glazing may matter more.
Turn the measuring sheet, enquiry and order into a process
A useful measuring sheet includes the project address, room name, date, contact person and an unambiguous reference number for every opening. Add internal and external photographs, a floor plan, an elevation and every individual dimension in millimetres. State what exists and what is proposed: new floor finish, altered patio, internal fit-out, shading or lining. Do not place dimensions from different construction stages in one column. A field marked “open” is better than an assumption. When enquiring, provide more than width by height: include the desired opening type, viewing side, leaf movement and whether the dimension describes the structural opening or finished construction. This allows the supplier to ask focused questions. Give every revision a clear version and visibly strike out outdated sheets. Before approval, establish which drawing applies, who recorded changes, and whether internal finishes, facade planning and patio planning use the same level reference. This prevents correct individual details from being accidentally combined from different design stages.
A double-leaf inward-opening patio door is only clearly specified when hinge sides, active leaf, room arrangement and reference level are known alongside the overall opening. Before ordering, ask for every position to be checked against your drawing and establish who is responsible for the final site survey and junction coordination. Bloomcabin states a five-year guarantee for windows and doors; delivery takes four to six weeks, subject to order confirmation. These details replace neither technical approval nor review of all project-specific matters. Keep confirmed drawings, dimensions and revision records together. Where there are several facade openings, a clear position list helps prevent a correctly measured door from being planned in the wrong place.
Frequently asked questions
Should I state the smallest or largest opening measurement?
Record every measured value with its measuring point rather than selecting only one. The smallest available dimension is often relevant to further planning, but variations must be understood. An irregular reveal, slope or incomplete floor build-up will prompt different follow-up questions. A professional should confirm the final order size.
From where do I determine left and right on a patio door?
State the viewpoint explicitly, for example “viewed from inside”. For hinged leaves, add the hinge side and a swing arc. For sliding doors, mark moving and fixed panels and the travel direction. Labelled internal and external views are far more reliable than an unsupported “left-hand” designation.
Can I use the dimensions of an existing door for replacement?
They are a useful starting point, but not a safe order size. The old frame can conceal the true building opening, while new floor finishes, wall junctions or a different opening type can change the plan. Record the visible situation, possible removal limits and planned finishes. A qualified professional should check the final installation arrangement.
Must my patio already be fully planned?
Not completely, but levels and key elements should be known or marked as open. Important details include the proposed top of the finish, slope, steps, paths and nearby obstacles. This information allows junction details to be coordinated. If external planning is still missing, the door decision should not rely on an assumed patio finish level.
What should an enquiry include besides width and height?
State the opening type, internal and external view, leaf movement, structural or finished dimension, wall depth and relationship to the finished floor. Add photographs, a sketch and information about the patio, shading and adjoining furniture or components. In existing properties, notes about uneven reveals, linings and planned alterations are helpful.
Next steps
Accurate measuring for patio, sliding and bi-fold doors means considering how the space will work later: who walks through and when, where the leaf moves, at what height the floor meets the patio, and which components adjoin it? Record the opening, wall, internal room and outdoor area as one connected situation. Note every dimension with its reference point instead of making an order size from one figure. A clear sketch, photographs and a list of open questions make it easier to compare the preferred door type properly. The final configuration and installation should then form part of coordinated planning with the appropriate qualified professionals.
