At Bostik we have an enthusiastic, young(ish) flooring sales team that is always asking me technical questions. From my point of view this is great, as I believe no question is too small or too insignificant to be asked. As the basis of this article I think it would be appropriate to share one of the more typical questions I’ve recently come across.
So today’s question is: What should I advise for the contractor when there is a joint across the floor?
The answer, as is often the case, is that more information is required. This is because joints come in various guises and perform different functions.
There are three main joints found in subfloors (NB by subfloors I am referring to solid substrates such as concrete, screeds, terrazzo etc. Wooden substrates, for example, will have totally different requirements for movement).
These three joints can be summarised as:
In a similar vein there are, in effect, joints around walls, columns and parapets, which can also be included in the discussion.
Movement joints must not be covered over by smoothing compounds, DPMs, floorcoverings etc., but should be covered over with an Expansion Joint Cover (EJC), enabling the flooring systems either side of the joint to move independently. EJCs come in a wide range of designs and can often compliment the flooring being used so as not to be too apparent. However, architects and designers occasionally get this wrong and have expansion joints at wholly inappropriate places. I have seen one approximately 100mm from a partition wall so leaving a thin strip of vinyl with an EJC running right along the room (if only they designed it so the partition sat over the expansion joint!!). Don’t be tempted to try and overcome the aesthetic problems by covering over the expansion joints unless by written instruction from the main contractor and with no warranty offered. You will also see that when ceramic or porcelain tiles are installed they ensure there are wide joints introduced within the design to do a similar role. In my opinion these can look awful.
Day joints can also be referred to as bay joints in some circumstances. A bay is simply an area designated as the appropriate size to lay a screed or concrete. The bay size can vary depending on materials being applied and often, with the likes of pumpable screeds, particularly calcium sulphates can be almost non-existent by design. A day joint can be a couple of mm to up to 10mm wide and is basically the point where the screeds have shrunk back on curing.
The treatment of day joints is less clear. Will the joints stay in the cured position or are they likely to move further? If it’s a fully dry floor of 75%RH or less and without any Underfloor Heating (UFH) then they can generally be considered stable and can be infilled with a suitable repair mortar, or even a fluid smoothing compound of suitable strength. It’s important to allow the infill to dry fully before continuing.
If the subfloor is still above 75%RH the moisture gradient in the subfloor when covered over can equalise causing the joint to close at the surface. This will be a problem as it compresses the floor covering and smoothing compounds causing the all too familiar “ridge” in the flooring.
Fully securing the joint could require a full resin stitching approach involving cutting and bonding metal pins at right angles using an epoxy resin system. Success may also be achieved by infilling the joint with a strong moisture tolerant product such as a bulked out epoxy resin compatible with the chosen surface DPM.
If UFH is present then this can create potential issues, particularly if it is switched off and then brought back on line. Always advise the client of this possibility. If necessary, get the screed installers and UFH suppliers to offer their advice too.
The situation with regards to what is required here is as per day joints. If the subfloor is still drying out then joints may close up. We have found from experience that the use of epoxy resin bulked out is sufficiently strong to maintain the integrity of the substrate but again ensure the client and installers are aware that closure of the joints can cause materials above to crack and “ridge up”.
I also mentioned joints around parapets and walls. These are to ensure screeds and concrete do not exert undue pressure onto the fabric of the building. They should not be covered over and are usually hidden by skirting and similar materials. They are particularly important in screeds incorporating UFH as there will be some thermal movement under the temperature changes exhibited.
Of course sites vary and each project should be evaluated on its own merit, so this should only be seen as a general overview of some of the issues that could be faced when it comes to joints. If you see random, nonlinear joints on the floor then these are ‘cracks’ and should not be there. Involve the main contractors, screed installer and technical support if required whenever this arises… the answers may not be so straightforward.