Traditional ground floors have consisted of little more than a few flagstones or bricks placed directly over the soil. The Victorian era saw the widespread introduction in mass housing with suspended timber floors, alongside rudimentary solid floors in hard-wearing areas such as kitchens and hallways. This combination persisted well into the 1930s, with solid concrete predominating throughout the second half of the 20th century, until the advent of the modern ‘beam and block’ suspended concrete floor in the late 1990s.
The suspended timber ground floor has, over time, given rise to a number of potential problems. This has been largely due to the change in our life styles. We have tended to seal up our homes trapping moisture in and reducing natural ventilation. We have blocked up our chimneys. We have painted our walls (inside and out) with impervious paints. We have re-plastered our houses with dense sand and cement renders. We have put down patios and concrete footpaths which raise up the ground levels and trap moisture beneath. We have filled our cavity walls with retro-fill insulation. We have allowed our air bricks to block up. We have covered the timber floors with impervious flooring.
In light of the above, it’s worth taking a moment to reflect on how these floors were built. A typical example might comprise joists supported or resting on timber wall plates over brick ‘sleeper’ walls to support the sleeper plates and joists. To reduce the risk of damp and timber decay, a good flow of air under the ground floor is important, so short brick sleeper walls were built in with a ‘honeycomb’ pattern; the gaps allow air to circulate. The air enters via small vents or airbricks sited in the lower walls. Suspended timber floors to kitchens and bathrooms can be particularly at risk from hidden plumbing leaks and condensation behind fitted units, too. Other sources of damp include defective water supply pipes run in from the street under the house.
If the oversite appears to be wet or if the flooring timbers are in the ‘at risk’ range of moisture content, the walls are more likely to be significantly damp in places. This is because moisture-laden air in the floor void may rise and condensate under the floorboards or behind skirting’s, giving a false impression of a rising damp problem. This phenomenon is more accurately termed ‘rising damp in the air’.
There are choices when it comes to timber floor repairs. You can replace like for like and this has several advantages: It is much easier to repair or adapt a timber floor and a timber floor will make the routing of service pipes and cables much more safe, economical, efficient and straightforward to install. One additional benefit is that it is much easier to supervise a suspended timber floor installation as its various components are more exposed and visible for inspection than a concrete floor would be.
Rotten wood should be cut out and remaining timbers treated. Once dried out, new treated timbers can be fitted. Joist ends in walls should be protected with a DPC (damp-proof course) or new joists hung from steel hangers. Clear any blocked airbricks, replace damaged vents, or fit additional terracotta or plastic airbricks. Where an extension has blocked off old airbricks, it may be possible to improve airflow by fitting ‘periscope’ vents channelled to the exterior.
If insulating the floor then use breathable insulation (as this allows the whole of the floor to breathe) and also a breather membrane (this allows you to create an airtight seal to reduce draughts) and also for any moisture to pass through and dissipate naturally. Care needs to be taken when fitting the membrane as it needs to be taped together to form a consistent barrier and also to be attached to the underfloor walls using a long lasting sealant. Look for products like Orcon F rather than relying on silicon sealers.
Concrete replacement floor
The trend now is to replace these traditional timber floors with concrete floors with a damp-proof membrane (DPM). The membrane is supposed to link to a damp-proof course in the walls which in practice consists of just turning the DPM up the face of the wall and not tucking it into the DPC course in the wall as is the practice with a new build. This could create a moisture pathway between the wall and the concrete floor slab.
In any case, the DPM will stop moisture evaporation through the floor in an historic building, forcing any moisture to travel under the non- porous floor until it reaches the walls. If the moisture cannot escape through the walls because they have a cement render or a waterproof coating on them, it will accumulate in the wall, causing deterioration. External concrete paving will only compound the problem. Water will always find a way to escape and if the only option is up an internal wall, so be it.
The installation of a solid floor can also block airflow to other rooms. It is common for one room to have its floor replaced. This then can cut off the underfloor ventilation to other rooms. This is turn can cause more problems for the other rooms. At the risk of repeating myself, installing a concrete floor inside a building to replace an older breathable floor means ground moisture is forced out to the walls, increasing the damp within the walls. After all, a large previously ventilated void has been filled by a solid, dense and obstructive material.
Limecrete replacement Floors
If you really have to remove your timber floor and replace it with a solid ground floor then one option is to use a slab based on lime (‘limecrete’) which is breathable, rather than Ordinary Portland Cement, which is virtually impermeable. A limecrete floor can be designed to meet modern insulation requirements and can incorporate under-floor heating (UFH). It may be possible in some cases to re-lay the original surface on top of the new slab if desired, although this can be difficult to achieve successfully and requires a methodical approach if the character of the floor is not to be altered. The greatest danger associated with limecrete floors is the potential need to excavate to a greater depth than the foundations. This can destabilise the building, so great care should be taken to establish how deep the foundations are before any new floor is considered. Two or three test pits by the walls could be dug to allow a plan to be formulated.
Having decided on the Limecrete floor, preventing ground water penetration is a major consideration and requires some form of capillary break. This can be achieved using recycled foam glass or expanded clay aggregate insulation as loose-lay insulation. Being made from re-cycled glass or clay, these are more durable and widely perceived to be more environmentally friendly than an insulation based on petro-chemicals. Furthermore, due mainly to the open pore structure of these aggregates, they have low capillary attraction to moisture.
The installation is as follows: the ground is first excavated and levelled – the greater the depth available, the better the insulation. A breathable geotextile membrane is laid and the loose-lay insulation is added followed by another layer of geotextile membrane (a slightly more expensive option than using expanded clay aggregate as loose-lay is foamed glass which is around 20 per cent more thermally efficient and more structurally stable as it requires compaction). The limecrete slab is then cast using a mix containing expanded clay aggregate or sand, and natural hydraulic lime. This is usually left for around three weeks before any Underfloor Heating Pipes (UFH) pipes are fitted and covered with a lime mortar screed. Having created a breathable insulated floor, it’s important to avoid an impermeable covering if possible e.g. glazed ceramic tiles, cementitious adhesives and grouting, rubber carpet underlay.
For more information visit: www.limecrete.net/
- Energy Efficiency and Historic Buildings: Insulating Solid Ground Floors, English Heritage, London, 2012 (www.english-heritage.org.uk/publications/eehb-insulatingsolid-ground-floors/eehb-insulating-solid-groundfloors.pdf)
- Fabric Improvements for Energy Efficiency in Traditional Buildings, Historic Scotland, Edinburgh, 2012 (www.historic-scotland.gov.uk/fabric_improvements.pdf)
- Limecrete: The Vapour Permeable Solid Floor (includes a method statement for laying a limecrete floor), Mike Wye & Associates Ltd (www.limecrete.net/limecrete-solid-floor/)
- A Wright, Care and Repair of Old Floors, SPAB Technical Pamphlet 15, London, 1999