Gary Chapman, Technical Advisor at Flowcrete UK explains the importance of a joint in screed and resin floor finishes, especially its ability to prevent cracks in the floor.

An essential element of most types of screed and resin finishes, joints can prevent erratic and extensive cracking in the floor.

Why is a joint required?

Installing a joint helps to protect the floor from cracks; in the event too much strain is placed on the floor, the cracking will be located along the line of the joint in a controlled manner. In the absence of a joint, the cracking could be spread across a wide area. 

Joints are also an important flooring consideration in areas with certain service conditions or where architectural factors have a role to play. For example, some parts of food production plants may be prone to thermal cycling or thermal shock, such as in front of hot ovens where the floor will be frequently exposed to intense blasts of hot air. Steel tiles or dairy grids are typically installed in these areas to withstand the harsh heat levels. A joint is required around the edge of the tiles or the dairy grid to isolate them from the rest of the floor and to ensure that the thermal impact doesn’t spread.

Joints are also required across door thresholds as a separation for the different materials that will probably be used on either side. Within thin rooms such as a corridor, a day joint should be installed every five linear metres in order to accommodate the movement and shrinkage stresses experienced by the floor. At corners and right angles of corridors a joint is required diagonally across the floor.

If the joints in the substrate are subject to movement, then this must be reflected in the floor finish and therefore a joint should be formed in both the screed and the finish. Similarly, joints should be considered along the lines of rigid support to any precast plank constructions.

How is a joint installed?

There are two different types of joints for screeds and resins, and construction and day joints; the type of joint will be based on whether the material in question is a bonded, unbonded, floating screed or resin system.

Day joints are typically used for cementitious screeds (such as Isocrete K-Screed) and are made at the end of a bay by creating a straight edge, priming the edge (using an agent such as Polymer 70/ Isocrete Primer) and then butting the next section of screed against it. Day joints can also be used for resin screeds such as Flowfresh HF.

If the screed is unbonded or floating then a D49 steel mesh at ¼” thickness should be applied along the day joint at the mid-height of the screed before applying the next bay. This mesh acts as a tie between screed sections. Similarly, for resin screeds a steel strip can be placed up to the joint; when this is pulled away the screed stiffens and the day joint is formed. The applicator should saw-cut the joint the next day to a straight edge with a diamond blade before continuing with the floor. 

Construction joints utilise mastic sealant to form a connection between flooring materials that moves effectively with the building. To create this, the joint is saw-cut out and a foam rod placed into the gap. The foam rod controls the thickness of the jointing mastic while also ensuring that the face of each screed/floor section is fused to the one opposite.