The Screed Scientist® would like to offer some yuletide greetings and would wish that 2013 can become a prosperous year for our readers.
We asked the Screed Doctor for his top tips to avoid winter floor screeding problems.
Screed Scientist (SS):
What is your top tip for the winter?
Screed Doctor (Dr):
Simple. Do not let your screed freeze!
There is some confusion over the temperatures that we should be respecting as screeding professionals. Broadly there are two:

• Air temperature should be a minimum of 3°C and rising
• Floor temperature should be 5°C and rising

SS: Just for the day of installation?
Dr: No, you need to maintain these temperatures for at least 5 days in order for the screed to gain strength so that it can resist temperatures unaided and for minimum impact.
SS: Any other considerations
Dr: Oh yes. Where have you left the sand? Don’t let it freeze, same for the cement. The cement needs to be acclimatized so that the chemical reactions that take place during mixing are not impacted. You also need to be careful of the fact that screed temperatures will rise during mixing due to the friction forces.
SS: Warmer screed can’t be a problem, can it?
Dr: Don’t forget if the ambient temperatures are very low and if you add warmer screed on top of floor insulation then you will find that the insulation will keep some of the heat. What we are trying to do here is to avoid the extremes on a job where one part of a job that might be frozen and another might be at a higher temperature which could cause problems where they meet.
SS: Can frozen screed be that much of a big deal?
Dr: I am not the Scientist here and I am surprised that you asked me the question! As you know water expands by 9% during the freezing process. Imagine how the screed will behave! The volume will increase in all the dimensions and who knows how the chemistry will be impacted.
SS: Yes, I get the picture, I was simply asking the question for clarity.
Dr: The technical term is friable, the screed will break up and become dusty.
SS: So in summary?
Dr: Well, these are the issues

• The screed will take longer to set
• In colder temperatures the cement will not set properly because the chemical processes can be adversely impacted
• You will get the end client calling you back to deal with a whole heap of problems.

SS: What are the solutions to a freeze impacted screed project?
Dr: Yes, that is the question most people want to know. It will cost you some mince pies and a sherry.
SS: Come on Screed Doctor, where is your seasonal spirit? Health and safety means no alcohol on the job.
Dr: Just kidding, usually the most appropriate remediation is to use a penetrative hardening system.

• A special resin is poured into the screed
• This floods into the pores of the screed and becomes harder than the screed itself.
• Quite expensive, and could cost more than the original screed, however what is the cost of ripping out the screed and replacing it?

SS: Anything else to add?
Dr: Sure, some other ideas that come to mind are:

• Keep the building envelope water and wind tight
• Use materials such as hessian cloths to overlay the screeds to act as a winter blanket. (To be used as soon as possible after the screed installation.)
• Don’t take risks or assume the screed will be fine. Test with a BRE drop hammer test to check for screed soundness before applying expensive finishes.
• Always test for moisture content before applying final floor finish
• Specifically for UnderFloor Heating applications, you must pressure test before and after the screeding works. You must screed with the UFH pipes under pressure. Don’t use air in the pipes, use water that contains anti-freeze. If you use air for pressurising you may not be aware of the location of any compromised piping. (If you don’t use antifreeze in the pipes – well, let us say that you could end up with puddles and wet screed – don’t risk it.

SS: Wow, the real world is really full of potential snagging issues
Dr: Yes it is, but paying a little bit of attention to all the details can go a long way!
SS: Thank you Screed Doctor – Merry Christmas and a Happy New Year.
Dr: Thank you. Be prepared and protect your screed.