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Construction management in depth

Quality management in depth

Quality in the construction process and tolerances in building construction after DIN 18202: tolerance types, checking deviations and planning tolerances.

1Quality in the construction process

Quality in construction means achieving the contractually and technically required properties economically. This works when quality is planned, controlled, assured and improved, and checks are built into the construction sequence instead of looking for defects only at acceptance. Dimensional accuracy is a central, measurable part of it.

  1. Quality planning

    Set requirements: what is required, where it applies and how it is measured.

  2. Quality control

    Design workflows and responsibilities so that the requirements are met.

  3. Quality assurance

    Check against an inspection plan: what, where, with which instrument, how often and by whom.

  4. Quality improvement

    Evaluate results and carry the lessons into standards and follow-up projects.

2Tolerances in building construction after DIN 18202

No component is built exactly to plan. Inaccuracies in setting out, manufacture and assembly are unavoidable. DIN 18202 sets out which deviations in size, shape and position are permissible in building construction, so that shell and fit-out components fit together without adjustment or rework. The tolerances apply to buildings and components regardless of material and describe the accuracy that normal care achieves.

Not covered are time- and load-dependent deformations, including temperature, and height offsets between adjacent components. Component and execution standards, for example for concrete structures, refer to DIN 18202 for dimensional accuracy unless something else is specified.

Nominal sizeActual sizeMinimum sizeMaximum sizeToleranceDeviation
Fig. 1 Terms on a component: the actual size must lie between the minimum and maximum size.Drawing: rheonda, schematic
outer envelope(maximum size)nominal size, positioninner envelope(minimum size)actual surface
Fig. 2 Box principle: every point of the component surface lies between the inner and outer envelope.Drawing: rheonda, schematic

Box principle. All tolerances of the standard follow the box principle: the component surface must nowhere pierce the outer or inner envelope formed by the nominal size and the permissible deviations. Requirements for dimensions, angles, flatness and alignment must each be met on their own; they are not added up.

Shape and position. The standard distinguishes requirements for the shape of a component, i.e. dimensions, angles and plane surfaces, from requirements for its position in space, i.e. distances to axes and levels, angles to the vertical or horizontal and alignments. Both are checked separately.

3The tolerance types of the standard

DIN 18202 sets out four types of tolerances. Which one applies depends on what is checked: a dimension, an angle, a surface or a row of columns. The limits are graded in the tables of the standard by nominal size or measuring point distance.

  • DIN 18202, table 1

    Limit deviations for dimensions

    Limit how far an actual size may deviate from the nominal size. The limits are graded by nominal size and distinguish dimensions in plan and elevation, clear dimensions and openings, the latter also with finished reveals.

    Applies to: lengths, widths, heights, axis and grid dimensions, cross-sections, openings for windows, doors and installed elements

  • DIN 18202, table 2

    Limits for angular deviations

    Limit the deviation from the nominal angle, expressed as an offset related to a nominal length. Dimensional and angular tolerances apply side by side: using up one must not exceed the other.

    Applies to: vertical, horizontal and inclined surfaces, including openings

  • DIN 18202, table 3

    Limits for flatness deviations

    Limit the deviation of a surface from a plane, as an offset related to the distance between measuring points. The standard distinguishes unfinished and finished surfaces and normal and increased requirements; increased requirements must be agreed separately.

    Applies to: top and underside of slabs, screeds, floor finishes and walls, in any position

  • DIN 18202, table 4

    Limits for alignment deviations of columns

    Limit how far an intermediate column deviates from the line connecting the end columns of a row. The reference length is two column spacings.

    Applies to: rows of three or more columns

Joints at interfaces. Where components or trades meet, their tolerances are absorbed by varying the joint width. If a joint pattern must be fixed for design reasons, it has to be settled before construction in which adjacent components the tolerances are taken up instead.

jointjointwallwallinstalled elementcommon reference (axis)tolerance zone
Fig. 3 Fit of an installed element: the joint takes up the tolerances of opening and element.Drawing: rheonda, schematic

4Checking deviations

Whether tolerances are met is decided by measurement. We plan and assess checks so that they are transparent and robust: with a defined reference, measuring points according to the standard and a stated measurement uncertainty.

  1. Set the reference

    Agree reference points and reference type before construction: boundary, axis, centre or edge reference. Design, construction and checks use the same reference.

  2. Place measuring points

    At corners and edges about 10 cm from the edge, at the centre of the component or room and on axes and their intersections, depending on the check.

  3. Measure

    The inspector chooses the method, e.g. straightedge and wedge, surface levelling or total station. Method and measurement uncertainty are documented.

  4. Assess

    Evaluate shape and position and dimensional, angular, flatness and alignment deviations separately and judge them by the box principle.

Checking flatness. For a single measurement the straightedge rests on two high points of the surface; the offset is measured at the lowest point, related to the distance between the high points. The limit must be met for every combination of two high points. Larger areas are levelled on a surveyed grid and evaluated.

Timing. Checks are made when needed and, because of later deformations, as early as possible, at the latest when the following trade takes over or immediately after completion.

offsetmeasuring point distancestraightedgeactual surfacehigh pointhigh point
Fig. 4 Flatness check with a straightedge: offset and the related measuring point distance.Drawing: rheonda, schematic

5Planning tolerances

The tolerances of the standard describe normal accuracy. Where function or design require more, designers must set the requirements before construction starts. We help choose the right tolerances and substantiate them.

  1. Clarify requirements

    Where do the standard tolerances suffice, and where are increased requirements needed, e.g. for exposed surfaces or floor finishes? Increased accuracy is agreed on economic grounds.

  2. Calculate fits

    Combine the tolerances of adjacent components, deformations from time, load and temperature and functional limits such as the permissible strain of a joint sealant.

  3. Settle interfaces

    Plan joints as compensation and specify where tolerances are taken up, especially between shell, façade, windows and fit-out.

  4. Provide for checks

    Set reference points, measuring points and inspection dates in time and carry them into the tender and the inspection plan.
Our serviceQuality managementWe make the geometric quality of a building measurable: we assess tolerances after DIN 18202, check deviations on the building and help designers specify tolerances and fits correctly from the start.