I had my floor filled and the filler cracked - why?

I had my floor filled some time ago and all the filler has cracked/fallen out. Why?

Firstly the primary purpose for filling gaps in any floor is to allow the lacquers/oils/hardwax oils etc. to work properly without contamination/side bonding issues. Sometimes it is desired for the visual improvement it gives (and it can be a massive improvement) but if it is being done for this reason it is important that both the contractor AND the end user are aware of a few facts and responsibilities.

The main issues are with resin fillers that are mixed with fine wood dust and trowel filled across the entire floor and so this section is dedicated to those.

Firstly the contractor needs to make sure that the resin filler is not “too dry” when applying it, it is very tempting to mix the product this way so it dries quicker, but this is a false economy as a dry mix will not go far enough down into the gaps and will either come straight out during sanding or shortly after. The mix needs to be between single and double cream and needs to be trowelled on correctly to force it into the cracks or gaps. DO NOT try and fill in one go, even in fine gaps it does not generally work and any time saving will not be a real time saving.

Secondly, try and use water based resins, avoid pure solvent based products and alcohol if possible as they do not have the flexibility of water based products nor are they as good colour wise. Good resins such as Pallmann Pall X Filler contain fibres that help reinforce the filler for larger gaps.

Third, take wood moisture, air temperature and relative humidity readings before use. These will tell you how the filler will perform. Low wood moisture means the filler will dry quicker and make short term cracking more likely, high moisture will slow down drying. However, what is really important is the RH reading, this will give you a lot of info especially when used in conjunction with your other measurements.

In a recent scenario the filler was applied at a time of extreme high humidity during the summer. In theory the wood should be tight, but in this case there were still gaps that the contractor filled. This was fine until the winter came and the heating was turned on, all the moisture that had transferred from the air into the wood floor now evaporated over a couple of months and the wood shrunk meaning the gaps were wider and the filler fell out. It is important to understand that NO RESIN FILLER can cope with this situation and therefore it is better not to fill during times of extremely high RH. If you don’t take readings you will not be aware of the potential problems!

So at the beginning I said that both the contractor AND the end user have responsibilities, I explained the contractors (use good quality filler, mix correctly, take readings and act appropriately) but what are the end users responsibilities? If you have a wood floor you need to be aware of the relationship between the wood and the atmospheric conditions in your premises. Only you can control this, the contractor cannot so do not blame them if they have done the job correctly and the filler has still cracked, it means that at some point your property has exceeded the parameters of what the product can handle. Learn to recognise if the air is too dry or the opposite to wet and humid, look for damp problems, don’t blast your heating and by the same token do not leave the property unheated during winter time. If the humidity is high open the window and turn the heating up or use a de humidifier, if the humidity is low turn the heating down and allow moist air in through an open window or use a humidifier.