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Tiling over retrofit underfloor heating systems

Alan Collins and Martin Pouncey, technical training managers at Instarmac, address some of the commonly asked questions around fixing tiles to XPS cement faced or plain and aluminium faced polystyrene boards.

OK, so the retrofit system has been installed and you are ready to get tiling, So what’s first?
Step One: Ensure the boards are fully bonded to the substrate. Make the time to go round and check there isn’t any deflection on any of the boards. Although you might not have installed the retrofit boards, it’s important that you check they are fitted correctly before tiling.

A test we often do to check how well retrofit boards have been installed, is to put a bit of duct tape on the corner and pull it. If the board doesn’t move, you are good to go. If it does move – run and don’t tile or level!

Step Two: It’s time to prime! Prime using a primer that’s suitable for non-porous surfaces – Prime IT Multi-surface primer is perfect. Priming will ensure that any cement-based products, whether it be a leveller or adhesive, will bond securely to the surface of the boards and the pipes.

Failing to prime will result in debonding, cracking, and tears at a huge repair bill.

When priming, it’s important you don’t over prime the area or let the primer puddle in any of the joints, unused channels or surface of the board. Puddling causes a film that will inhibit the primer from drying underneath, resulting in a primer blister.

Step Three: Once the primer has fully cured, it’s time to choose your adhesive or leveller. As most people reading this will be tilers, let’s leave levellers to one side and focus on selecting the ideal adhesive. That being said, if you are levelling to fit LVT, or another floor covering, please feel free to give us a ring and we will guide you through choosing the right leveller for your installation.

So, adhesives. Our recommendation is to use an S2, highly deformable, adhesive, as per the EN 12002 classification. This reason for this is high thermal change the deflective movement within the boards. It is essential your adhesive can cope with these challenges.

We would always advise that a combed coat of adhesive is applied to the board and that the tiles, regardless of their size, are back buttered to ensure 100% coverage with zero voids, as per BS5385 part 3.

Let’s talk about grout baby…
Your grout must be flexible and suitable for joint widths a minimum of 3mm. Trust us when we say that using a non-flexible grout will only cause you headaches later down the line, as it will crack and come out of the joints.

When grouting, get the grout all the way to the bottom of the joint. Your joints should be completely full and not just topped off.

Allow your adhesive and grout to cure for a minimum of 7 days before commencing the commissioning cycle. To heat commission effectively you need to slowly increase the water temperature by 5°C per day, up to the system’s maximum temperature. When you have reached this temperature, slowly decrease the system by 5°C per day until it reaches equilibrium. This needs to be carried out on the manifold and not the room or boiler thermostat.

After that, you are good to go!
Underfloor heating can be a bit of a minefield, so why not join us on 13 April for our Underfloor Heating Academy Day. Our Academy Days are a great way to build your confidence and knowledge, expand your network by meeting other tilers and get hands on, practical advice – and lunch is on us!

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