misleading bolx. If you want info on gypsum screeds why ask a company that has absolutely no involvement in them. Ask people who make them...
Turning on Underfloor After Tiling
- Thread starter kiteboy
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You are quite correct. 7 days is the norm.
My initial post was clearly misleading, I have found the extract from BS 8204, see below. The point I was trying to make is that forced drying using a high heat can damage a gypsum based screed. Turning the heat up gradually after a week is fine, but do not ramp it up quickly or raise it to a high heat. It can also be very deceptive as to how dry a Gypsum screed is. The carbide moisture test is a very good way of establishing if the screed has reached the required moisture content.
See the last sentence of the BS extract.
Extract taken from BS 8204-7:2003 Screeds, bases and in situ floorings — Part 7: Pumpable self-smoothing screeds — Code of practice, regarding heated screeds including calcium sulphate based screeds;
“6.8 Heated screeds
Heated screeds are generally laid as floating screeds in conjunction with proprietary underfloor heating systems with the heating elements at the base of the screed. The heating pipes or cables should be fully secured to the surface of the insulation or to the base to prevent flotation during screeding. The manufacturer of the heating system should provide their installation details, but it is essential to ensure that the heating elements cannot float in the wet screed and that the wet screed cannot get between or under any insulation boards. The tying of heating pipes to a steel fabric will not provide sufficient resistance to flotation unless additional fixing is provided. The thickness of the screed should be as detailed in section 6.4 but also ensuring a minimum cover over the heating pipes or cables of 25 mm for a calcium sulfate based screed or 15 mm for a cement based screed. Cement based screeds should not be heated until they are cured and dried because of the increased risk of cracking. The manufacturer’s advice should be sought for suitable time-scales. Calcium sulfate based screeds may be heated once they are at least 7 days old, which can accelerate the drying of the screed.
In all cases, the screeds should be heated very slowly to their operating temperature and maintained at that temperature for several days before cooling down to room temperature, but not below 15 °C, before installing any flooring. The usual operating surface temperature of a heated screed is about 27 °C; however, some locations operate at higher temperatures, e.g. 35 °C. Higher temperatures than this can adversely
affect the floor covering and the stability of the calcium sulfate screeds.”
See the last sentence of the BS extract.
Extract taken from BS 8204-7:2003 Screeds, bases and in situ floorings — Part 7: Pumpable self-smoothing screeds — Code of practice, regarding heated screeds including calcium sulphate based screeds;
“6.8 Heated screeds
Heated screeds are generally laid as floating screeds in conjunction with proprietary underfloor heating systems with the heating elements at the base of the screed. The heating pipes or cables should be fully secured to the surface of the insulation or to the base to prevent flotation during screeding. The manufacturer of the heating system should provide their installation details, but it is essential to ensure that the heating elements cannot float in the wet screed and that the wet screed cannot get between or under any insulation boards. The tying of heating pipes to a steel fabric will not provide sufficient resistance to flotation unless additional fixing is provided. The thickness of the screed should be as detailed in section 6.4 but also ensuring a minimum cover over the heating pipes or cables of 25 mm for a calcium sulfate based screed or 15 mm for a cement based screed. Cement based screeds should not be heated until they are cured and dried because of the increased risk of cracking. The manufacturer’s advice should be sought for suitable time-scales. Calcium sulfate based screeds may be heated once they are at least 7 days old, which can accelerate the drying of the screed.
In all cases, the screeds should be heated very slowly to their operating temperature and maintained at that temperature for several days before cooling down to room temperature, but not below 15 °C, before installing any flooring. The usual operating surface temperature of a heated screed is about 27 °C; however, some locations operate at higher temperatures, e.g. 35 °C. Higher temperatures than this can adversely
affect the floor covering and the stability of the calcium sulfate screeds.”
W
WetSaw
I'm getting comfy..
The section you refer to is talking about the operating temperature for the screed once covered. I can see how this might be confusing so i will bring it to the attention of the standards committee to see if it can be clarified in the next publication.
The maximum temperature for force drying is 55 degrees C as this is the highest flow temperature acceptable for these screeds. Temperatures above 65degrees can cause partial desiccation of the calcium sulphate molecule causing shrinkage but this will only happen if applied for a prolonged period of several months. To force dry calcium sulphate screed you should switch on underfloor heating flow at 25 degrees or the minimum allowed by the system e.g 35 kn this instance. Leave atvthag temperature for 3 days then increase by 5 degrees per day up to a maximum of 55. Leave it there for minimum 3 days before reversing the process. Once back down to minimum leave for 3 days before wpswitching off and testing for moisture. A 50mm screed treated this way in good drying conditions will be dry by the end of this process.
If you switch on the heating too hot too soon it will indeed crack the screed just as it would any screed due to the effects of rapid thermal expansion or thermal shock as it is known.
The maximum temperature for force drying is 55 degrees C as this is the highest flow temperature acceptable for these screeds. Temperatures above 65degrees can cause partial desiccation of the calcium sulphate molecule causing shrinkage but this will only happen if applied for a prolonged period of several months. To force dry calcium sulphate screed you should switch on underfloor heating flow at 25 degrees or the minimum allowed by the system e.g 35 kn this instance. Leave atvthag temperature for 3 days then increase by 5 degrees per day up to a maximum of 55. Leave it there for minimum 3 days before reversing the process. Once back down to minimum leave for 3 days before wpswitching off and testing for moisture. A 50mm screed treated this way in good drying conditions will be dry by the end of this process.
If you switch on the heating too hot too soon it will indeed crack the screed just as it would any screed due to the effects of rapid thermal expansion or thermal shock as it is known.
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