Basement Construction Details – Type A

Loft Conversions

In this post I am going to share with you some information about basement construction and detailing. We will look a the three types of basement construction, along with a focus on Type A – tanked protection.

Basement construction provides valuable space for both commercial and residential use. They can provide sustainable benefits of improved energy efficiency, making good space heating savings compared to similar buildings above ground.

Basements require careful design in order to achieve the correct level of waterproofing required for the internal environment.

There are three different types of basement construction. Type A, B and C.

Type A Basement Construction

Type A is a tanked protection. This construction consists of a water resistant layer in the form of a membrane, usually installed to the external face of the structure. There are many variations of type A constructions. This form of basement construction does not have integral protection against water penetration and therefore completely relies on the waterproofing membrane.

Type A Basement Construction

Type B Basement Construction

Type B is a structurally integral protection. This is usually a form of prestressed or reinforced concrete that is water resistant. The concrete is not as effective in resistance to water vapour movement so is often incorporated into a Type A or Type C construction.


Type B Basement Construction

Type C Basement Construction

Type C basement construction incorporates a drainage cavity within the structure which collects water and removes it to a pump drainage system.

Type C Basement Construction

Type A – in focus:

These structures rely on a waterproofing membrane to keep water out. This membrane can be installed in four ways:

On the exterior face of walls and floor – External

On an external source of support – Reversed

Within the construction – Sandwiched

On the interior face of walls – Internal

If the tanking system is installed in a location that has a high water table, any defects in the tanking will allow water to enter the structure as free water. If this water is not removed, the basement will fill to the level of the water table.


External tanking is the preferred option of a Type A construction, providing site conditions permit. The external water pressure forces the membrane against the structure thus creating a strong resistance. Although a reliable system, access for repairs and maintenance can be difficult and it can be expensive to install.

TYPE A1_Basement detail


This method applies the tanking membrane to a surface (often piling support or temporary/enabling works) prior to construction of the main structural elements against it.

For example, in constructing a oor level tanking, the concrete blinding layer will be positioned, followed by the membrane, which is then protected by a layer of screed, before the concrete slab is cast on top. The membrane will be taken well beyond the edge of the slab to allow for a suitable lap joint to be formed with the wall.

TYPE A2_Basement detail


Should external tanking be unsuitable, it is suggested sandwiched tanking to be used. This method places the membrane to a structural masonry wall (or floor), and fully supported by a loading coat (concrete slab for floors and blockwork for walls) which will prevent it from being detached and pushed away from the surface by external pressures.

TYPE A3_Basement detail


Internal tanking is applied to the inside of a structural wall, however, it is susceptible to hydrostatic pressure, and as such must be  xed using mechanical anchorage or in some cases a non structural inner skin.

TYPE A4_Basement detail

A couple of example details of Type A construction

Insulated concrete basement detail type A
Insulated concrete basement detail type A

Found this interesting? You can get the full set of information for Type A, B and C basement construction, along with all the dwg details and the sketchup files!

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