Knee brace

Knee wall (also known as a knee wall or knee) refers to the vertical wall surface between the top edge of the exterior wall of the top full story and the bottom edge of the roof rafter. It is the wall that rises below the roof slope in the attic and significantly determines the usable ceiling height and living space under the roof. The higher the knee wall, the more vertical floor space is created in the attic, and the more comfortable the use of the space becomes. The knee wall height plays an important role in the building permit application and in the calculation of living space.

Knee Wall Height and Its Significance for Living Space

According to the Living Space Ordinance (WoFlV), areas with a clear room height of at least 2 m are fully counted; areas between 1 m and 2 m in height are counted only as half; those below that are not counted at all. A low knee wall (e.g., 60 cm) means that a large portion of the attic space counts only as half the living area. A high knee wall (e.g., 1.20-1.50 m), on the other hand, significantly increases the proportion of the area that is fully counted. When converting an attic, it is therefore worth planning for the knee wall to be as high as possible.

A concrete example illustrates this: An attic with an 80 m² floor area, a knee wall height of 60 cm, and a gable roof (45° pitch) may have only 55 m² of full room height and an additional 10 m² counted as half-resulting in approximately 60 m² of countable living space. If the knee wall is raised to 1.20 m, the full-height areas increase to approx. 70 m² and the total living area to approx. 75 m². This means: 15 m² more countable living area-with the same floor plan.

Raising the knee wall - technical options

For existing attics that are to be converted, the knee wall can be raised by adding a roof extension (raising the entire roof) or installing a roof terrace. This is a significant structural intervention that generally requires a permit and must meet structural, design, and planning regulations. Alternatively, the attic space can be better utilized through dormers or skylights without raising the knee wall.

Modern knee wall constructions in new buildings consist of wood-frame or reinforced concrete and are specifically dimensioned during planning to achieve the desired ceiling height. Knee wall heights of 1.0 to 1.80 m are common, depending on zoning plan requirements. Some zoning plans specify a maximum eave height-a higher knee wall shifts the eave upward and may thus exceed the permitted eave height.

Knee Walls in Energy Consulting and Insulation

The knee wall is one of the critical thermal bridge points in the attic. In many older buildings, the knee wall is uninsulated or poorly insulated, leading to increased heat loss. As part of an energy-efficient attic renovation, the knee wall should be insulated from the inside with at least 10-14 cm of insulation material (mineral wool, cellulose, or PIR boards).

Typically, the uninsulated knee wall lies outside the exterior wall insulation plane: The exterior wall of the main building is insulated, but the knee wall ends at the attic level and is often uninsulated. This creates a linear thermal bridge along the entire eave line-particularly problematic in winter. In the case of very low knee walls, the space is often used as a crawl space-this area should also be insulated and protected against moisture ingress.

Knee Wall as a Value Factor in Real Estate Purchases

The knee wall has a direct impact on property value, especially if the attic has been converted into living space or can be converted. When purchasing a house with an attic, the knee wall height should be specified in the floor plan or building plans-if this information is missing, it must be measured on-site. The living area in the attic listed in the property description can vary significantly depending on the knee wall height and roof pitch.

Practical Tip for Homeowners in Nuremberg and Franconia

In Nuremberg and Franconia, knee walls ranging from 60 to 90 cm in height are commonly found in older buildings from the Wilhelminian era and the 1950s through the 1970s. Anyone wishing to convert the attic should check early on whether the knee wall height is sufficient for a comfortable room height or whether adding a story is feasible and approvable. In popular Nuremberg neighborhoods such as Gostenhof, St. Johannis, or Schweinau, attic conversions can significantly increase a property’s value-provided the knee wall height and roof geometry allow for it.

At my-home.de, we can help you assess whether an attic conversion will economically increase the value of a property in a specific location-and how much usable living space can realistically be created. Before purchasing a house with an unused attic, it’s worth measuring the actual knee wall height and discussing the conversion potential with an architect.

Frequently Asked Questions

At what knee wall height does an attic conversion make sense?

A knee wall height of at least 1.0 m is considered the minimum for a comfortable conversion; from 1.20-1.50 m, the use of space is significantly more pleasant. With knee walls under 60 cm, a full-scale residential conversion is hardly possible and rarely economically viable.

Does raising the knee wall require a permit?

Yes, generally speaking. Raising the knee wall by adding masonry or an additional story changes the eave height and the enclosed volume-which requires a permit. The eave height and roof shape are usually specified in the zoning plan. The responsible building authority (in Nuremberg: Bauratsamt) must be consulted before any project begins.

Does the knee wall count toward living space?

The knee wall itself is a wall surface, not usable floor space. The living space of the attic is determined by the clear room height, which is determined by the knee wall height, the roof pitch, and the ridge height. A higher knee wall increases the proportion of areas with full room height (≥ 2 m) and thus the countable living space.

What materials are used for the knee wall, and how long do they last?

Historical knee walls in Nuremberg’s old buildings are often made of timber-framed or brick masonry. Wooden structures have a lifespan of 80-120 years with proper maintenance; masonry lasts even longer. Problems arise when the knee wall is damaged by moisture ingress (leaky roof, missing vapor barrier) or pest infestation (dry rot, woodworms). Before converting the attic, the condition of the knee wall should be assessed by a building inspector.

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