Tuesday, November 17, 2009

I have a metal shed in my garden with a concrete screed floor. The floor is not drying out. Why? What to do?

is it concrete or screed ? screed is a sub flooring and should really only be used indoors. the temperature will also affect it. try sprinkling a little more powder over the surface to help dry it out. you can always rub down any rough surface bits when its dry.

I have a metal shed in my garden with a concrete screed floor. The floor is not drying out. Why? What to do?
Do you mean that you have a concrete slab with a screed topping? And do you mean that the floor is not curing rather than not drying out? I assume that the floor has cured, because otherwise you would have had a few problems fixing the shed to it!

If the floor is cured (fully set) but wet, then the chances are that not enough hard-core was put down before the slab. You might also have needed a physical damp proof membrane between hard-core and ground.

How long has the floor been down and how thick is it? Concrete can take months to dry!

Try either leaving the shed well ventilated as much as you can and possibly use a de-humidifier - easily hired from most hire shops.

And - if you really have a screed floor, then you probably really have a problem.

There are too many possibilities here with too little information.

Whatever you do, don't try to put a floor covering on it just yet - it will just bubble up!
Reply:If the floor is recently laid it will only dry if moisture is allowed to evaporate/escape so the doors/windows of shed need to be kept open as much as possible until the floor has dried. If no moisture/damp proof membrane was incorporated under the floor then it will never dry out completely, always be damp to some extent, drawing moisture from the ground underneath it. Is the shed smaller than the base it stands on? or the same size? will make things different for fixing the problem if there's no membrane.
Reply:There are several reasons for the concrete not to dry out.

1) lack of air flow in the shed. If moisture cannot escape it won't evaporate

2) The humidity of where you are is very high. This will slow the evaportation down and prevent it from happening all together in some cases.

3) There is moisture wicking up through the concrete

4) There could be new moisture introduced from somewhere else. A sprinkler or hose?

If there is adequate airflow in the shed, then locate the moisture source.

If there isn't adequate ventilation, then cut a few holes in the opposite ends of the shed and put screen or louvered vents over the holes. This will increase air flow in in turn increase evaporation.
Reply:You have damp rising through the concrete, which was installed without a damp prood layer. You could remove the shed, and put a building polythene damp proof course, with a 2" screed on top, then replace the shed.
Reply:What base is your shed on? You could try using a greenhouse heater for a few days. Also, you could try lining the floor with a dampcoat paint. Firstly though, I would check underneath and make sure there is an airway for the base to dry out.
Reply:My thoughts are that the floor may not be drying because of the shed being metal. It's like mini-ecosystems we built in science class as kids. The moisture on the floor isn't able to evaporate and escape the shed because the metal is non-porous.

I would open the door to the shed and place a fan in the doorway facing out. It might seem like you're trying to cool the outside, but it'll pull the evaporated moisture from the shed and push it outside while pulling fresh air into the shed. However, make sure that you do this on a day when it's not rained for a few days and isn't going to rain that day. If the air outside is drier than the air inside then you should be good to go. May take a few days of doing this, and make sure you close up the shed at night before the dew falls so you don't get extra moisture in there.
Reply:does the concrete floor have a membrane under it ?

If not, the floor will always "draw" moisture from the ground below.
Reply:Dear Welshy,

pamela is the only one on the right lines. It sounds to me that two things are wrong here, one easier to repair than the other.

The easiest is ventilation. I'm guessing that there is no ventilation in the base of your shed or at the bottom of your shed perimeter. At least 1 or possibly two airbricks or air vents need to be inserted in the walls of the shed at the bottom.

Better still, cut a channel in the base of your concrete insert a small piece of rainwater drainpipe and 90 degree bend in it so that one end (the 90 degree end) comes up level with your shed floor and put a grate or mesh cover over it. The other end, to exceed the perimeter of your base and put something like a pelican beak at that end to stop the water getting in and allow free air flow.

It sounds to me as though your shed base may have been built slightly or even a lot below the level of the rest of the ground its standing in. Even if not, you should ideally have put a non permeable membrane in the shed base before pouring the cement.

Did you slump test the cement if you laid a sharp sand screed?

e-mail me at statusquo44@hotmail.com and I'll go through it with you. Above is really guess work as I know nothing about the location and size and what cement mix you used?

Regards, Peter
Reply:from your name i m assuming you live in Wales, if so your weather is the problem, high rainfall damp conditions will not make the concrete easy to dry, but the mix of concrete may be a factor and being a metal shed will not help allow a throughput of air on dry days, if it is a handmade mix then you got it wrong and you can rectify by adding /sprinkling the surface with cement, brush in with a yard brush ,this will mark the surface but you can use it to your advantage by making it a non slip floor, if it is a bought in mix the water content is always high and takes longer to go off, regards LF
Reply:put a heater in there for a day.

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