Wooden shed condensation

12 Dec 2012
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United Kingdom
Hi all
Recently has landscape gardeners in to overhaul the garden. A new concrete shed base was laid and the existing shed repositioned.

Since its been moved I'm having real issues with condensation. Inside roof is very damp, all bottles of liquids on the shelves are covered in water therefore damp staining wooden shelves.

This shed is not air tight!! I can't believe I have an issue with inadequate ventilation. The shed is placed straight on the concrete base with no DPC. Is this my problem? A temporary issue as the concrete dries out or one I will suffer until I put a DPC between the base and the shed joists?

I find the need for a DPC odd as haven't had one on any of my other sheds and never suffered this problem.

If a DPC is the solution, anything else to do as I cant move the shed to install one! I see no use in using a dehumidifier given levels of ventilation I have to the outside world... How about an extractor fan of some kind?

Thanks for advice.
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no a DPC is completely useless.
is there a lot of water on the shed base?

other than that I would think theres a serious leak in the roof. or the shed has become damaged when you moved it.

did you dismantel it when you moved it?
if not then you may hav some cracks in the roof that you haven't noticed.

other than that it's difficult to say without seeing it myself.
Unless the concrete base incorporates a DPM it will never be dry and will constantly absorb water from the ground and it will evaporate into your shed.

If the shed is small enough to lift onto bearers and fit a floor with a ventilated airspace underneath you can prevent that.

Current cold nights will mean that the inside of your shed is colder than the damp slab so condensation is inevitable. It will stop when the weather warms up.
Thanks both...

Shed was moved as built by 6 guys, straps running underneath the shed at the front, middle and back, held at each end... Since its move I've had the old felt off, inspected the roof (no damage) and refelted with heavy duty felt.

On to the DPC, I'm surprised landscape gardeners don't know how to lay a concrete base if a DPC is a requirement but I can pick his up with them. Clearly if this will happen every winter I'll need to do something about it as have expensive bikes / lawnmowers / tools stored in there.

When you say "ventilated airspace" what do you mean? The shed floor is already on rafters to provide some airflow as you'd expect of a shed. It's on rafters probably an inch high. Are you suggesting an entire frame needs to be built for it to to sit on? If so, what height / design is recommended?

I'm assuming the DPC should be placed underneath the concrete, on top of any hardcore? Clearly that can't be fitted now it all layed. Could I just lay one on top of the slab and place the shed on top of that?

Thanks for advice.
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if you already have a wooden floor on joists (I had the idea the shed just had the concrete as floor) then ventilate that airspace well, and prevent rain or pests getting underneath. Expanded metal may suit. Water run off from the building must not be allowed to drip or stream onto the concrete.

Metal tools and machinery will get dew forming on them, mostly in mornings, as they will get cold overnight.

If the shed is insulated, you can maintain the temperature a little using an oil-filled radiator. It only needs to be warmer than the outside. Don't use any flame heater; apart from fire risk, they generate water vapour.
So to be clear, no need for a DPC if using a shed with a wooden floor on joists? I'm trying to determine if I need to get the trade back....

Shed has guttering around it, and is clear of debris as only moved 3 months ago...

I'm at a loss as to where the moisture is coming from then? I'm sure it's condensation as all roof boards feel damp. It's not an isolated issue as you would expect if the felt had failed. And as stated earlier, bottles of e.g. Liquid weed killer are always dripping wet in the mornings and staining mdf shelving.

I'm also nervous of the moisture as have electrics in there for garden lighting (wise box) so don't want water causing issues.

Maybe this problem was always evident but I'm only aware of it now.... Who knows.

No insulation so heating would be a waste.
if you have a ventilated subfloor space, a dpm in the slab is not necessary, because the breeze will whisk away most of the water vapour that comes up through the concrete. I originally thought you had a damp concrete floor.

If your shed floor is ply, without big gaps, not much will be coming up through it.

IMO you are suffering at the moment from dew as we are going (down here anyway) though a spell of warm wet, and dry cold periods. The new concrete will change from dark grey to light grey as it dries out. This may take months in winter.

But keep looking for any signs of rainwater penetration. Do you have good gutters, and a downpipe taking the water away from the slab?
Still suffering with this annoying problem. Entire roof of the shed is damp, particularly around any knots in the wood so am convinced this is dew or condensation and not a leak.

Is there any sense in putting a simple air vent at the top of the apex of the roof to try and let the warm air out and reduce the temperature difference?

Must be something I can do as things are getting damp...
i'd also go with more damp air is rising through your floor from the slab than the ventilation can cope with. the under floor air gap/ventilation is needed so the damp air is blown away instead of entering...

wrong time of the year for it, but i'd suggest....

empty the shed. get a crowbar/gorilla bar, a small bit (eg 6-9 inches long, 1 inch thick) of scrap wood and a large bit (2x4 offcut) of scrap wood. use the small bit up under the base of the wall/frame near a corner to spread the load. make sure you're lifting under the frame else you'll probably break the floor or cladding! crow bar over the big bit of scrap as a fulcrum with the tip under the smaller bit of wood. leaver a corner of the shed up. slide something under the joists to elevate. keep going around until the shed is lifted. you need to decide what you're going to use to keep it elevated. eg blocks under the existing joists or further treated timbers at 90 degrees to the original. getting blocks under the middle can be fun depending on size of shed! extra bit of temporary lift for installing, torch and a long bit of wood useful. depending on block spacing/joist size you might get a bit of bounce in the floor. newer prefab sheds tend to have lots of smaller joists and thinner floorboards, so doing it with block work becomes a pain. older sheds have thicker floorboards and joists, so block work is a bit easier.

my old small (8x10) shed has 2x2ish runners/joists, which sit on about 25 engineering bricks. gives a good void underneath for airflow. replaced the previous 1x1 floor runners a few years ago when the ends rotted. put bigger runners in and elevated it a bit better. lifted it twice using the method above. first time for the new runners, second time lifted it onto some long 2x4s then put rollers under that and moved it to another location in the garden! now on a new slab, no dpc in it, no condensation problems (yet).

i don't like the idea of wood directly on the slab. always likely to get some amount of standing water there so would rather it was a brick standing in it than timber, treated or not!
Is the floor damp at all? (stef99 asked this in the first reply to your original post but I can't see a reply to that.) I would have thought it would be if the damp was coming up from below the shed, rather than being generated inside it somehow.

Have you changed what you are storing or doing in it since it moved? Has it moved from in the shade to in the sun (or vice versa)? Not that I know why these things would matter, but trying to help you identify what might have changed.

If it's secure enough I'd be tempted to leave the door open a little bit to see if that helps. If so then I would think about increasing ventilation by drilling a few holes under the eaves.
Floor doesn't look visibly damp but I had plastic bags of compost and coal directly on the floor and on lifting them it was drenched underneath / woodlice crawling around... So they are now raised on a pallet in the shed to keep ventilation there...
Could the bags of compost be the problem? I mean, moisture coming from there? It has to go somewhere as they dry out and decompose further.

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