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Why choose Rackhams



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Why choose Rackhams
Why choose 175mm over a 150mm beam

1. Manufacture

Rackham 175mm Prestressed floor beams are manufactured in steel moulds giving dimensional tolerances of incomparable accuracy which are not affected by variances in the production process. This ensures that concrete infill blocks, which are manufactured in a similar way, stand uniformly proud of the top of the floor beams, providing maximum support for board type insulation – an essential requirement for traditional ‘beam and block’ systems.

Conversely the Rackham Poly-Plus panels sit slightly below the level of the beams ensuring that there is maximum connection between the 75mm structural concrete finish and the floor beams – even where methane and radon barriers are used.

Some 150mm floor beams are manufactured by mechanical extrusion methods onto flat beds. Control over cross-sectional size tolerance is extremely difficult which can give rise to significant dimension irregularities. Any aberration in the concrete mix can result in deformation of the finished product meaning that concrete infill blocks (beam and block) can sit below the beams thereby not giving board type insulation the required support.

2. Camber and Deflection

The ‘stiffness’ of the floor beams is largely dependent on its span to depth ratio, in other words, the smaller the beam depth or the larger the span, the more deflection and movement there will be. In addition the strength of a beam is dependent on the number, size and position of the embedded wires.

In an extruded product it is virtually impossible to ensure accurate positioning of the wires and this results in large fluctuations with regards to the initial camber and once loaded, the final deflection.

The increased depth of 175mm standard beam allows it to combat the above mentioned problems. The wires can also be positioned accurately within the section of the moulded 175mm beam by virtue of the method of manufacture. This production process assists in minimising any potential camber which is beneficial where board insulation is used and particularly when using the Poly-Plus system ensuring a uniformity of finished depth.

3. Coursing


At a glance a 150mm beam courses perfectly. However, it must be noted that all housefloor systems use a 100mm infill block and where the 150mm is used a 50mm ‘make-up’ is therefore required. This is difficult to overcome except by using a purpose made 50mm Slip Brick or block.


The depth of the 175mm beam has been carefully calculated in order that a 100mm infill block together with a 75mm brick course overcomes all the aforementioned coursing problems. The only issued lies in levelling the top of the external and internal skins of the cavity wall. This is simply achieved by adjusting the coursing using one of the Rackham standard details which most builders find is of no consequence.

4. Ancillary Blocks – Items required

Whilst a variety of similar ancillary blocks, including Closure Blocks are available for both depths of floor beams (150 & 175), the aim of which is to make installation easier it is only Slip Bricks that are really essential for use with the 150mm beam because of the make up required.

The 175mm beam can easily be installed using standard masonry units available on all sites.

It must be borne in mind that the originator of the housefloor, Charles Rackham, designed the 150mm extruded beam as his first concept. The 175mm moulded product was a later developments only conceived after considerable practical experience.

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