Gypsum Plaster Floors

Why do Gypsum plaster floors fail?

Last week I carried out a pre-purchase survey on a 250 year old cottage in the East Midlands and one of the most significant defects noted was significant deflection or dishing in the Gypsum plaster floors at first floor level. This is an issue I’ve encountered on many occasions, since these floor systems are prevalent in the East Midlands, or a number of other areas that historically operated Gypsum quarries.

What is a Gypsum Plaster Floor?

In new construction, Gypsum, Anhydrite or Calcium Sulphate pumped screeds are now commonly installed at ground floor level; the three terms are interchangeable but refer to the same product. However, Gypsum floors were commonly installed in Derbyshire between the 17th and 19th centuries and were sometimes referred to as, plaster concrete, lime plaster or lime ash floors, but Gypsum plaster is the correct terminology and these floors were installed predominantly at upper floor level, though they can be found at ground floor level, installed over a solid substrate, as opposed to a timber base.

Gypsum Plaster Floors
Figure 1: Image Credit: SPAB Regional Technical Advice. Gypsum Plaster Floors

Gypsum floors are formed from a calcium sulphate binder mixed with aggregate, which is wet trowelled in a single layer (35mm to 100mm thick) over riven or sawn timber laths, which are nailed to the underlying floor joists. We often see three variations on this theme:

  1. The timber laths are laid closely together with a circa 5mm gap between them and the Gypsum is wet trowelled directly onto the timber laths. (I’ve chiselled up Gypsum floors to expose this configuration in the underlying substrate)
  2. An additional bed of water reed, hay or straw is laid across the floor joists and secured with timber laths above each floor joists. It is important to note that the bedding material adds little strength to the finished floor and acts in a similar manner to concrete shuttering; it is there purely to support the Gypsum throughout the drying process.
  3. Where joists are exposed to ground floor rooms, lime plaster is often applied directly to the underside of the reed or straw bedding material, as opposed to having plaster and lath ceilings applied to the underside of the joists.
Failed lime plaster applied to underside of reed bedding
Failed lime plaster applied to underside of reed bedding

How do Gypsum Plaster Floors Fail?

Commonly we see significant deflection in these Gypsum floors, with floors being significantly out of level, often to such a degree that serviceability of the floor is severely affected. This deflection is almost always mirrored in the underside of the floor joists or ceilings below.

Interestingly, it is said that the relative lightness of Gypsum floors allowed for slimmer floor joists to be used, and yes, they are relatively light when compared to concrete floor slabs, approximately 40% lighter, but they are still significantly heavier than floorboards and it it my opinion that floor joists were sometimes undersized and incapable of supporting the combination of live and dead loads they would encounter over their lifetime. As a result we see often see excessive stretching and deflection in the supporting joists which can push these floor slabs beyond acceptable limits of serviceability.

We often discuss floors in terms of serviceability, and in simple terms we ask, are the floors still largely functional? Do door bases bind on the floors, are trip hazards created, can we place furniture without noticeable problems or does the floor feel sound and level underfoot? In old or historic buildings, we can accept some degree of deflection as a quirk or character of the building; part of the charm in owning a period property. However, we must also recognise when serviceability has been affected to an unacceptable degree. There comes a point where floors are no longer functional and even the structural integrity of the floor is severely compromised.

Floor sloping to a degree that affects furniture placement in the room
Floor sloping to a degree that severely affects furniture placement in the room

Material Incompatibility?

Whilst Gypsum is relatively flexible, when compared to a concrete floor slab, its flexibility could never match that of the underlying floor joists, so when the floor joists start to stretch or deflect under load, then the likely outcome is that you will see severe cracking or crazing to the Gypsum slab; again, this is something we commonly see.

Severely cracked Gypsum Floor
Severely cracked Gypsum Floors Encountered on a Previous Pre-purchase Survey

In the figure above, the floor was so badly bowed and structurally unsound that we could see no available option for cost effective localised repairs and in period properties of this type, conservation and repair would always be the preferred option. As you can see from the image above, the Gypsum floor slab is incapable of flexing to the same degree as the underlying floor joists.

Gypsum Floor Repair Options

A thorough visual inspection of the floor would be needed, and this will usually entail clearing the room and taking up any finished flooring, such as carpets. Where deflection is seen to the underside of the floor, this visual inspection to the topside will usually reveal cracking in the floor, which can range between hairline cracks to severe cracking through the full depth of the slab. Gypsum floors are susceptible to moisture and can break down in its presence. Impermeable floor coverings are often in place, such as foam backed carpets, which drive up moisture content in the slab and you may want to consider having the floor slab moisture content tested to assess whether drying is required. I would generally look to see moisture content in these slabs of 0.5% w/w or less, as would be expected for a modern pumped Gypsum floor screed.

Where severe deflection is seen in the floor, then attempts to level the floor using additional cementitious screeds would be misguided, as these can add significant additional loads for which the underlying floor joists were not designed, or it is likely that de-bonding or delamination would be seen at the interface between the original Gypsum floor and the repair screed.

You should always look to conserve historic building fabric, and carry out repairs using compatible materials wherever possible, and repairs can be completed using traditional methods. Where severe deflection is seen, it is sensible to employ a qualified structural engineer to assess the structural integrity of the existing timbers, because any defects in the underlying supporting timber structure, must be addressed prior to repairing the Gypsum floor. You should note that floor timbers can provide lateral restraint to external perimeter walls, which is another reason to ensure that repair protocols have input from a structural design engineer.

Minor deflection in the timber joists can be addressed through the use of new reed bedding which can be feathered to account for these minor deflections, before the new Gypsum plaster slab is wet trowelled in place. The new organic bedding material is often first dressed with lime putty.

When taking up the existing Gypsum, it is sensible to have a sample analysed for composition in the labs, so as to inform the choice for a compatible repair mix. It will be virtually impossible to replicate the original Gypsum composition due to the large variance in original manufacturing processes, but two types of Gypsum are commonly used for repair:

  1. A high strength hemihydrate plaster
  2. A high-impact finishing gypsum plaster, which gives a longer setting time.

Where possible, the existing Gypsum should be retained and crushed for use as aggregate in the replacement Gypsum slab.

It goes without saying that repairs are of a specialist nature and can prove to be very expensive but as always, when considering the purchase of these period properties, it is about understanding their historic nature and significance and making informed decisions as to whether you are prepared to invest in maintaining a period property, in the appropriate manner.

2 responses to “Gypsum Plaster Floors”

  1. Lee avatar

    “Do you have anything else that I could read? I really admire your writing style, and I’d love to read more.

    This is what’s stressing my husband, luckily I showed this to him and this article helped him a lot! Do you have other blogs about this?

    What is the best compliment you’ve received for your writing?”

    1. Joe Malone avatar
      Joe Malone

      Thanks very much Lee. I appreciate the kind words. I actually do get told often that people like my style of writing and I guess I just try to make sure it flows in a coherent manner and try to make it accessible to the layperson.



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