Dovetail, tenon… do you know your joints?

Martin Cummins

Published by Martin Cummins - UK Technical Support Manager, Bostik

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential.

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:

  • Movement joints
  • Day joints
  • Stress inducing saw cut joints

In a similar vein there are, in effect, joints around walls, columns and parapets, which can also be included in the discussion.

  • Movement joints. These are critical from a design and structural point of view. They are created as part of the building fabric to enable the building envelope to move. It may be hard to appreciate but buildings do move and the stress induced by the movement needs to be taken up within the design. Expansion joints are also found in the walls and are often a continuation of those on the floor, so always look sideways as well as downwards when assessing a site. Expansion joints are often the widest joints you will see on a floor and can be anything from 8mm to 20mm typically. They are often filled with a compressible and recoverable material such as a polysulphide sealant or similar.

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. These are the junctions between different applications of screeds or concrete. They are called day joints due to the fact that it is usually a point at the end of the day where the screeders have finished and then started again the following day so creating a non-continuous screed. Historically there would have been requests to slurry bond the exposed face of yesterday’s screed so that today’s application bonds. I have never seen this as a successful way to create the continuous floor and cannot even be sure it is still done.

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.

  • Stress inducing saw cuts. These are cuts or joints introduced into the subfloor after the initial installation. Rather than do small controlled bays it is often better to install a full floor plan and then, after initial cure, cut through the subfloor (concrete or pumpable screed) to effectively introduce locations on the floor where curing tension can be released. The cuts are not generally all the way through the subfloor but seem to work very well in preventing random cracking and curling of screeds and concrete.

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.

2 Comments

  1. The information above is incorrect…i have been in touch with gradus many times any they only cover very small joints with a restricted movement which is hardly relevant.couple of mm i seem to remember.My last experience with this was at gsk and the joints were no where near 20mm….you suggest that a joint between 8 and 20mm can be covered with a ejc strip….id like to see you get gradus to back this up in a statement please..

    1. Hi Mike, long time no see. Hope all’s well with you. With regard to the comments below I am somewhat surprised at your response and wonder if there is a misreading of the article. The reference to Expansion Joints Covers has been shortened to EJC for ease of reference. There are indeed several Expansion Joint Covers in the Gradus range designed to be placed over expansion joints in subfloor. These are the JM881 range, and are generally aluminium products.

      The standard products I believe you are referring to are the plastic strips used over construction joints aimed at accommodating a few mm of movement. The fact that Gradus refer to these as EJCs is probably what is causing the confusion here.

      Quotes from Gradus website:
      “JM881-A01-025 expansion joint cover is designed to cover an expansion joint between two floors up to 25mm wide. At its maximum coverage it will provide 30% total movement and withstand loads of up to 250kg (calculated using a 25mm diameter contact surface)”
      “EJC 50 is suitable for use with 2mm resilient floorcoverings. EJC profiles should be installed using contact adhesive applied to the rigid outer sections only. Note: EJC profiles are not suitable for use over structural expansion joints”

      I trust this clarifies the article and removes the confusion. Kind Regards. Martin

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