What is a double tee slab?
A double tee slab is a precast concrete floor or roof unit with a cross-section shaped like two letter Ts side by side: a relatively thin top flange with two parallel stems (ribs) underneath. The units are cast in a factory, usually prestressed, and lifted by crane onto beams or walls on site. They are also called double-T units or TT slabs; in Dutch the floor is a TT-plaatvloer and in German a TT-Decke or π-slab, after the shape of the Greek letter pi.
Key features
- Long spans: according to manufacturers’ data, prestressed double tees span up to about 20 metres.
- Wide units: standard widths from manufacturers are around 2.4 to 3 metres, so a few units cover a large floor area.
- Heavy loads: the stems make the unit stiff, which suits heavy traffic and storage loads.
- No propping: units sit as simply supported members and carry load immediately, so temporary propping is usually not needed.
Uses
- Multi-storey car parks and parking decks.
- Warehouses, logistics centres and mezzanine floors.
- Roof units in industrial and commercial buildings.
Double tee or hollow-core slab?
A hollow-core slab has a flat soffit and voids running through it. A double tee has exposed stems underneath. That costs structural depth and gives a less tidy ceiling, but leaves room for services between the stems and copes better with very long spans and heavy loads.
| Soffit | Hollow-core: flat soffit, voids running through it. Double tee: exposed stems underneath. |
|---|---|
| Structural depth | Double tee: the exposed stems cost structural depth. |
| Ceiling finish | Double tee: gives a less tidy ceiling. |
| Services | Double tee: leaves room for services between the stems. |
| Very long spans and heavy loads | Double tee: copes better than a hollow-core slab. |
Practical tips
- Structural topping: a reinforced in-situ topping is often cast on top. It evens out tolerances and makes the floor act as one stiff diaphragm. Agree with the structural engineer whether it is needed.
- Bearings: units usually sit on corbels or beams with elastomeric bearing pads. Check bearing lengths and corbel dimensions before delivery.
- Openings: larger holes can only go through the flange between the stems. Never drill or cut the stems without approval from the manufacturer and engineer.
- Crane and logistics: units are long and heavy, so plan crane position and delivery route early.
Related terms
- Hollow-core slab floor
- Structural topping
- Precast concrete panel
- Span
- Reinforcement
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.