Screed Expansion Joints Underfloor Heating?

I’m Andy Parkin, Managing Director of the Multi-Award Winning Speed Screed. I’m here today to talk about screed expansion joints underfloor heating.

Screed Expansion Joints Underfloor Heating

There are two considerations that you’ve got to think about with underfloor heating and in screed.

Screed Shrinkage

Controlling shrinkage through expansion joints.  If you are looking to minimise the effects of shrinkage cracking, adding screed expansion joints minimises the chance of getting shrinkage cracking in your screed.

Thermal Movement

The other consideration is thermal movement, the screed heating and cooling of the screed causes thermal movement.  Adding thermal expansion joints in the screed will help minimise the risk of thermal cracking.

The screed will only move once by itself, as the screed dries it shrinks.  The screed is then inert, and will not move by itself.

The only other time the screed moves once inert is through outside influences, such as the structure of the building moving, any construction joints in the substrate should be mirrored in the screed so that movement is matched and managed.  Finally the screed may move through thermal movement, with the heating and cooling of the screed.

Sand and Cement Bay Size Guide

The bay size guidelines for underfloor heating are:

  • Maximum of 40 m2
  • Based on an aspect ratio of 2:1, if the ratio is greater the m2 is reduced
  • Reduce bay size if there are restraints, such as service pipes, columns, centre islands, and re-entrant corners

40m2 is the perfect bay size, always bear in mind this will need to be reduced to compensate for any of the other factors increasing stress on the screed as it shrinks and moves.

Flowing Screed Bay Size Guide

The bay size guidelines for underfloor heating are:

  • Maximum of 300 m2
  • Based on an aspect ratio of 2:1, if the ratio is greater the m2 is reduced
  • Reduce bay size if there are restraints, such as service pipes, columns, centre islands, and re-entrant corners

With flowing screed (calcium sulphate), the bay size is larger than with sand and cement screed. There is less natural shrinkage in the product and you can actually go up to an impressive 300 m2.

Be it thermal movement, be it shrinkage, any movement in the screed builds up stress and then it looks for the weakest point for it to release that stress and tension.  If you have expansion joints in the screed the stress is reduced, if there are no joints then the screed may form it’s own joints by cracking the screed.

Zonal Temperature Control

One other reason you may wish to include screed expansion joints underfloor heating is to literally separate zones. If you are heating different zones at different temperatures, you could have a situation where you have areas 2,3,4,5 degrees different.  For example with student accomodation, where the rooms are 2, 3, 4 degrees warmer than the corridors.  This situation causes differential thermal movement, one zone expands more than another, which may lead to cracking due to the stresses placed on the screed.  Placing a screed expansion joint minimises this possibility.

Adding a thermal joint also minimises the migration of heat from the warmer zone to the cooler one, giving you more control of your heating zones.

Vertical Control Joints

In flowing screed expansion joints underfloor heating, we would recommend a vertical control joint. The joint is a 10mm rigid upstand, a T shaped product that sticks to the membrane and then is rigid enough so flowing screed can flow up alongside to it and but wouldn’t push it over. It is made with ether foam, and has a rigid plastic upstand either side of the foam to give it enough strength to withstand the force of the screed. When the screeds finally dries, you can then just trim to the level of the floor screed.

When laying sand and cement screed, there isn’t the issue of it being a fluid product. Therefore you can just use a loose ether foam, at 5 or 10mm. You can just effectively sandwich it in between the two screed bays and this will then minimise the heat transference between those two slabs.

It is always worth seeking advice on individual projects. Quite often the design engineer will get involved and will give advice as to where screed expansion joints underfloor heating should be.

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