BS 1264 gives some very dangerous advice if you follow it to the letter as it misses out the gradual heating up process. If you follow this advice it is likely to thermallyshock the screed which will crack it as sure as eggs is eggs.
It ought to say put on at ambient for 3 days, increase by maximum 5 degrees per day up to maximum operating temperature and the reduce by 5 degrees per day to normal operating temperature. Run heating until screed is dry. With Anhydrite the process can start after 7 days. With sand cement it should not commence for 28 days (21 days following initial curing and all cement based screeds should be cured for 7 days following installation using a suitable curing membrane - usally a sheet of polythene -although this advice is hardly ever followed).
The CFA are currently lobbying for changes to BS8203 (resillient floor coverings) to be changed at the next revision to reflect the corrct method of comissioning. As other standards come up for review I guess they will be harmonised (at least that's the hope)
As the screed dries it shrinks. It also moves about in various directions as it shrinks depending on the bay sizes. The bay size should be geared to an aspect ratio of no more than 2:1 and if heated no more than 15m2 is the NHBC recomended maximum bay size - it is here where people often say to me that the screed is reinforced with chicken wire or some such - This is completely irrelevent.
If the screed is placed around a restraint point e.g. a door liner it needs it will be shrinking in opposing directions. The effect of this is to pull at approximately 45 gegrees to the point of restraint. This causes the classic Y shaped crack you have seen here. The best way to avoid cracking in this instance is to place a joint in the screed through the door way.
If no joint is present and the heating is then turned on the effect is to create alternate expansion and contraction. The expansion strip is there to allow the screed to do this effectively unrestrainedly. If it is not there restraint occurs at the door liner also causing the screed to crack in the classic Y shape. The best material for this is expanded polyethylene (etherfoam) as it is easy to install around door liners and other re entrant conrners without breaks. This needs to be of sufficient thickness as to allow for the maximum expansion of the screed. I can't remeber off the top of my head what this is for sand cement but for anhydrite it is 0.012% - a minimum 5mm is therefore suitable for most applications increased to 8mm or 10mm for larger bays. The expanded polystyrene often used in the "cheaper" underfloor heating systems are difficult to turn round corners. They are therefore often cut into peices with a break at the corners which reduces their effectiveness considerably.
Ditra is a great product and deffinitely has a place in the market particularly for fast tracking projects or for helping to stabilise unstable surfaces. If you were uplifting the tiles in this installation and completely re laying I would suggest it in this one. However if the rules are correctly followed it would not really be necessary.
You are right that the underfloor heating being commissioned prior to the floor coverings is absolutely essential and something I actvely encourage as part of my work.
It ought to say put on at ambient for 3 days, increase by maximum 5 degrees per day up to maximum operating temperature and the reduce by 5 degrees per day to normal operating temperature. Run heating until screed is dry. With Anhydrite the process can start after 7 days. With sand cement it should not commence for 28 days (21 days following initial curing and all cement based screeds should be cured for 7 days following installation using a suitable curing membrane - usally a sheet of polythene -although this advice is hardly ever followed).
The CFA are currently lobbying for changes to BS8203 (resillient floor coverings) to be changed at the next revision to reflect the corrct method of comissioning. As other standards come up for review I guess they will be harmonised (at least that's the hope)
As the screed dries it shrinks. It also moves about in various directions as it shrinks depending on the bay sizes. The bay size should be geared to an aspect ratio of no more than 2:1 and if heated no more than 15m2 is the NHBC recomended maximum bay size - it is here where people often say to me that the screed is reinforced with chicken wire or some such - This is completely irrelevent.
If the screed is placed around a restraint point e.g. a door liner it needs it will be shrinking in opposing directions. The effect of this is to pull at approximately 45 gegrees to the point of restraint. This causes the classic Y shaped crack you have seen here. The best way to avoid cracking in this instance is to place a joint in the screed through the door way.
If no joint is present and the heating is then turned on the effect is to create alternate expansion and contraction. The expansion strip is there to allow the screed to do this effectively unrestrainedly. If it is not there restraint occurs at the door liner also causing the screed to crack in the classic Y shape. The best material for this is expanded polyethylene (etherfoam) as it is easy to install around door liners and other re entrant conrners without breaks. This needs to be of sufficient thickness as to allow for the maximum expansion of the screed. I can't remeber off the top of my head what this is for sand cement but for anhydrite it is 0.012% - a minimum 5mm is therefore suitable for most applications increased to 8mm or 10mm for larger bays. The expanded polystyrene often used in the "cheaper" underfloor heating systems are difficult to turn round corners. They are therefore often cut into peices with a break at the corners which reduces their effectiveness considerably.
Ditra is a great product and deffinitely has a place in the market particularly for fast tracking projects or for helping to stabilise unstable surfaces. If you were uplifting the tiles in this installation and completely re laying I would suggest it in this one. However if the rules are correctly followed it would not really be necessary.
You are right that the underfloor heating being commissioned prior to the floor coverings is absolutely essential and something I actvely encourage as part of my work.
