Suspended Timber Floors
When we refer to timber floors, we are primarily interested in the structural construction of the floor not the floor finish. The construction of the floor is an important design decision despite the fact that it is not something you can see or is rarely discussed during the design phase.
The floor construction is a fundamental decision as it is not easily changed and the options should be properly considered in terms of the immediate requirements but also its adaptability and the practicality of its construction. Concrete beam and panels are very heavy and require a crane. Restricted access through terraced houses also limits the length of the joists or beams which also informs the floor design.
The majority of houses in and around London are constructed during the Victorian period 1837-1901, the years of Queen Victoria’s reign which was a period of high national self-confidence and wealth and access to more trees than there are today. Today most, if not all timber is imported in the UK.
During the Victorian period timber joist floors were the only practical choice as concrete was not in widespread use due to its relative complexity and quality control issues. Despite this concrete floor slabs were still used in sculleries and hallways because there were more susceptible to flooding and water ingress from occupants.
Suspended timber joist ground floors are still in widespread use today due to the perception that it is more sensible to match the existing construction as far as possible. There are benefits of using this type of floor. It is easier to gain access below the floor for running pipework and electrics; timber joists a lighter so cranes are not required and fewer people are required to carry them.
The easiness of adding in pipes for central heating systems or underfloor heating in obviously beneficial for current and future requirements and as our homes become smarter our need to integrate modern technology will increase. Currently we need to accommodate additional electric cables for electric power sockets, USB points, TV and Ethernet sockets and an increasing plethora of Wifi gadgets.
As our homes become smarter the question regarding their environmental responsibility will become louder. Today Designers are preoccupied with complying with legislation related to environmental efficiency, in reducing heat loss through the external fabric of the building whilst paradoxically juggling with the issue of ventilation.
Whilst technology has attempted to address the environmental issue of increased consumption borne of the increasing population, it is often failing us. From Smart meters and Wifi smart phone apps which allow us to configure our heating and lighting to ever more compact toilets through to ‘electric consuming entertainment’ we are often oblivious to the environmental consequences of our way of life.
Durability of suspended timber floors
To prevent the decay of timber joists, the suspended timber floor should be constructed in such a way that:
- All joists and wall plates are above the DPC level.
- A minimum void of 150mm is provided betweenthe joists and oversite.
- Air bricks are provided to give adequate crossventilation to the floor void.
- Joists have adequate bearings and do not protrude into the cavity.
The oversite should be either:
- 100mm thick concrete oversite (GEN 3) onwell-compacted hard core, or
- 50mm thick concrete oversite on a 1200g DPMlaid on 25mm sand blinding and well-compacted hard core
For sites that are susceptible to gas migrations, the oversite should incorporate gas protection measures designed by a suitable specialist.
This gas is Radon gas and is a radioactive gas which can cause lung cancer. An informal search can be done to determine if you are in an affected area at https://www.ukradon.org/information/ukmaps but this is not conclusive. We recommend paying a small fee of around £50 for a Radon test kit from Public Health England. This is comprised of a sensors which you position around your house for 2-3 month and then send it back to get the results.
The structure shall, unless specifically agreed otherwise with the Warranty provider, have a life of not less than 60 years. Individual components and assemblies, not integral to the structure, may have a lesser durability, but not in any circumstances less than 15 years.
Whilst many builders will prefer to use suspended timber floor construction due to its easiness of installation it may not always be the right choice. If a stone or ceramic tiles floor finish is to be installed it could be argued that a more rigid beam and block floor construction would be preferable as movement is reduced and any cracking of the grout and eventual movement of the tiles can be avoided.. However, timber floor can be reinforced to reduce movement and together with flexible grouts and adhesives problems can be mitigated. Suspended Timber floors are more suited to carpet and timber floor finishes where movement is less problematic.
Clearly, most extensions are built onto existing houses which have suspended timber floors and it would seem acceptable to match the existing floor and not end up with a floor less effective than the existing floor. At the same time it would appear more sensible to match the existing construction so that the build- up detail for floor finished and underfloor heating is the same throughout. This makes installation consistent and simpler.
It can be seen from this article that such seemingly simple and inconsequential decisions are not as straightforward as one might think. The usual approach is to let the designer or the builder make this decision which is not always the best approach because designer need more information to make this decision and builder more often than not like to ‘value engineer’ in their own interest.
Our approach at G. A. Architects is to explain the choices and their consequences and let our clients decide.