Beam and Block Floors
Suspended concrete floor construction is increasingly the most common flooring system specified in residential construction.
When selecting a floor construction there are a number of functional requirements that need to be considered. These include:
Durability
Strength and stability
Resistance to ground moisture
Fire safety
Resistance to passage of heat
Resistance to passage of sound
In the UK the Building Regulations Approved Documents provide guidance on some of the requirements of ground floors.
There are two types of concrete floor system commonly used in residential construction; ground bearing and suspended. In this article we will be focusing on the suspended concrete floor, particularly beam and block floors.
The suspended floor does not rely on support from the ground below but instead is supported by the external walls, and therefore the foundations below.
Ground bearing floor
Suspended floor
When is a suspended floor used?
Suspended concrete floors have become popular in UK house construction. They are typically used in situations such as sloping sites, areas where the ground has poor bearing capacity, or there is a likelihood of ground volume change. If the ground water table is high, or there are aggressive chemicals present in the soil, a suspended floor is often specified.
Different types of suspended concrete floor
Suspended concrete floors can be constructed using different methods. They can be cast in situ reinforced concrete slabs, precast concrete planks or slabs, or the focus of this article, beam and block floors.
Beam and block floors systems
beam and block layout
The beam and block floor system is a popular form of domestic floor construction. It is an economical option due to off site manufacture of the beams and blocks, fast assembly and little requirement for specialist labour or equipment. The installation of a beam and block floor is not usually weather dependent and therefore specifying this form of construction can reduce site delays that other methods might encounter. Beam and block flooring demonstrates good noise reduction and fire resisting properties, and is also suitable to host underfloor heating systems.
beam profiles
The precast concrete inverted T-beams are supported by the perimeter walls and internal load bearing walls. The beams vary in depth, and can be 130mm to 250mm made up to lengths of around 6m, they are usually designed and manufactured to span specific distances. Beam depth and profiles are selected according to span and loading requirements, along with the type of block specified.
Infill blocks are then positioned between the T-beams, these blocks are usually lightweight concrete. A sand cement grout is often brushed over the surface, filling any gaps providing a good load distribution and avoiding movement in the blocks. The grout also keeps insects and vermin out and assists with air tightness.
Insulation is usually positioned on top of the beam and block floor, a concrete topping or screed is then laid onto the insulation. An upstand perimeter insulation is installed around the edge of the concrete topping or screed in order to prevent thermal bridging. Some insulation manufacturers recommend a separation/vapour control layer between the insulation board and the screed to prevent interstitial condensation and the seeping of screed into insulation board cracks.
Timber deck finish can also be used – if this is specified a vapour control layer must be included in the design under the timber deck.
A void is left under the floor of at least 150mm between the soil and underside of the beam. If heavy clay soil is present it is good practice to increase the void depth to as much as 225mm or possibly more. All organic material must be removed from the soil and the surface should be treated with weedkiller. This void must be ventilated with air bricks or ventilation sleeves positioned in the external walls in order to avoid the potential build up of gas. Ground levels under floor should be relatively level, otherwise external walls become retaining walls and would need to be designed accordingly.
A DPM or concrete oversite in not usually required, unless needed for gas or vapour protection as per the Building Regulations. Some manufacturers do recommend a DPM over the beam and block floor before insulation – be sure to check manufacturers instructions.
Expanded polystyrene (EPS) blocks are also used as infill between the beams. Not only do these blocks provide excellent thermal performance, they increase construction times as they are lightweight and easy to work with. The blocks are either designed to lap under the concrete T-beam in order to prevent cold bridging, or an additional insulation layer is positioned over the EPS infill blocks. More manufacturers are developing EPS block systems which have varying profiles and methods of construction. They are fast, efficient and would be likely to over take the concrete blocks in popularity over the years due to their enhanced benefits.
If we now revisit the functional requirements of a floor construction and apply the characteristics of the beam and block floor, we can establish whether the beam and block is a suitable option for our flooring system.
Strength and Stability
The beam and block system provides suitable strength and stability with specifically designed precast reinforced T-beams to suit the required spans. The structural concrete topping can also be designed with a reinforcing mesh if required.
The beams are suitably supported by the external walls and any load bearing internal walls or beams.
Resistance to ground moisture
The suspended system with suitable void, provides a natural resistance to ground moisture. If required a damp proof membrane can be positioned over the beam and block floor. A damp proof course installed on the perimeter walls will ensure ground moisture does not penetrate the structure.
Durability
Concrete in its nature is a very durable material, providing a sound floor construction system.
Fire Safety
The beam and block floor fire resistance can vary according to the beam and its applied finishes. An individual beam should provide up to one hour resistance depending on its size.
Resistance to passage of heat
The beam and block floor offers good flexibility with thermal performance. High levels of rigid insulation can be installed over the beam and block floor. However, if EPS blocks are used, the floor system can achieve excellent thermal performance.
Resistance to passage of sound
The inherent mass and damping qualities of concrete give beam and block flooring construction a good noise reduction. Additional sound insulation can also improve the performance of the flooring system if required.
There are many different configurations for beam and block flooring systems, with specifications varying according to different manufacturers. The images here offer just a few examples of the many solutions available.
Conclusion
Emma has written four popular books on construction detailing. Understanding Architectural Details is a series based on UK construction and standards and looks at both residential and commercial construction. Residential Construction Details is based on US construction methods and codes. All books are full of both 2d and 3d images, and it is possible to download all of the 2d cad files and 3d SketchUp files with the books.
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Amazing book!!! I will be buying.
I’m studying a MSc in Construction but I’m also a visual person, this book really helps.
Beams are not reinforced with “steel bars” as stated. Reinforcement is provided by high-tensile, pre-tensioned strands producing a prestressed concrete unit. The pre-tensioning exerts a contracting force in the bottom of the beam which in turn creates and upward curvature, or camber, in its length. This camber is typically 1/300th of the span.
This is important as a rigid timber floating floor can be unsatisfactory as the finished surface will displat the camber whereas an insitu screed can be laid to a flat surface.
Thank you John, appreciate your comments.
Is block and beam suitable for constructing a factory floor that must carry heavy machines and a forklift truck with a lifting capacity of 2 tonnes, therefore gross weight of 7 tonnes rolling across it.
If I want to span 4.8m by 3.6m, should my T beams be 4.8m length ways of 3.6m widthways, given a 5% slope lengthways.
Whether I use 4.8 lenghthways of 3.6 widthways, how many of each beam would I need. For each options how many 440mm block would I need.
am being a based of a Summer residentual Log Cabin on clay soil near trees on a slight slode. The log cabin foot print is 4m x 3m
hi we have a floor 4.4x26meter
Do you have a specification you could provide me for my scope of works to give to the contractor.
I’m after 1-2 paragraphs outlining sizes, types, and BS standards and codes.
Many thanks
How can an access hatch be formed in beam and block floor construction?
Hi Chris, drop me and email and I can help you with anything block and beam floor related!
Hi Luke. Is it possible to pass 2 x 68mm pipes through an existing block and beam floor? I’m interested in connecting to the indoor unit of an Air Source Heat Pump.
I’ve seen the recommended minimum void for this type of beam and block floor, but what is the maximum void recommended? Francis
I have a block and beam floor with a two storey passage floor space void underneath. The blocks have cracked in two areas leading to concerns the could eventually fall. How is this best remedied.