A Nehobo floor is a Dutch system floor built from hollow fired-clay blocks, with reinforcing bars laid in the mortar joints between the blocks and usually a concrete structural topping over the whole. The name comes from the manufacturer, Nederlandse Holle Bouwelementen. The floor type was produced from 1956 to 1984 and is therefore found in a very large number of post-war Dutch houses — worth knowing about for anyone buying or renovating property there.
Construction
The hollow blocks were laid up like little walls to form floor strips, with reinforcing bars set into the joints between the blocks. That reinforcement carries the tensile forces. The strips were delivered as precast units or assembled on site, after which a concrete topping tied the whole into a composite floor. The result is light and quick to build, which is what mattered in the reconstruction years.
The well-known problem
To speed up hardening, chloride compounds were added to the mortar at the time. Combined with moisture and carbonation — which makes the cement paste less alkaline and more porous — the reinforcement loses its protective environment and starts to rust. Rusting steel expands and bursts the mortar and blocks apart. Damage starts on the underside, in the crawl space, and stays invisible from inside for a long time.
Damp, poorly ventilated crawl spaces accelerate the process considerably. That makes the Nehobo floor, like the later Kwaaitaal and Manta floors, a survey point when buying a house from that period.
What to do about it
- Have it inspected. A surveyor or structural engineer assesses cracking, rust staining and spalled mortar from within the crawl space
- Light damage: monitor, and above all get the crawl space drier and better ventilated — clear the air bricks, fix leaks, and consider a ground membrane
- Moderate damage: support with steel or timber beams on pad foundations. Cheaper, but the floor stays weak
- Severe damage: replace with a renovation floor or a beam-and-block floor. More expensive but final, and the opportunity to insulate and add underfloor heating
Points to watch
- Do not drill or cut openings without an engineer; the reinforcement sits in the joints and is decisive
- When buying a property built 1956–1984, ask specifically for a crawl space inspection. Standard surveys do not always look closely enough
- Costs. Replacing a ground floor in a terraced house typically runs into the tens of thousands of euros; propping costs a fraction of that. Always deal with the cause as well as the symptom
Related terms
- Hollow block floor
- Hollow brick
- Structural topping
- Reinforcement
- Carbonation
- Crawl space
- Cement
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.