New shed going mouldy

[greyam100]

Hi folks

We had a company build us a new garden shed February 2015, but we have a fair bit of mould appearing inside. Photos are here //www.diynot.com/diy/media/albums/shed.25655/

The shed sits on a raised timber deck foundation (see photo Shed0) due to the sloping garden. The top surface of the deck (not shown in the photo) is boarded out with 6"x3/4" planks as used for the new fence (see new fence in photos) and this effectively formed the floor of the shed. However, to seal the floor off from spiders etc getting in through the small gaps (2mm) between planks, I then overlayed the planks with 5mm thick plywood closely fitted. Mould/rot has been forming for a while now, mostly concentrated along the floor at the window side but also spreading along front and back sides of the floor.

Grateful if anyone can offer any advice as to how to remedy it.

Is it lack of ventilation to the underside of the floor (due to the external side panelling used to close off the deck)???

or is it rising damp, rising up the side panelling from the ground/grass?

You will see that it is stone chips (as opposed to grass) at the opposite side of the shed where the mould is negligible.

Thanks in advance

 

[greyam100]

another point of note........

there is often condensation (beads of water) on the inside of the window, but i dont think its enough to be running down the wall onto the floor

is the condensation a sign that some ventilation in the walls/roof of the shed is required??

thanks

 

[ntb]

It looks to me like the eaves are too short and water is running off the roof and down the walls, thus causing the damp issue. In which case, it needs modifying to give a greater overhang. Guttering may be another option to solve it.

 

[ellal]

Added to the eaves problem, the shiplap extending into the ground doesn't help.

No capping on the corners can cause further ingress.

I'd remove the shiplap up to just below the top of the deck.

And going by the first picture, having the deck buried directly into the ground isn't going to help in the long run either.

 

[r896neo]

Whilst it looks neatly constructed a number of small issues are combining to exacerbate each other i would say.

1. The semi buried sub-frame as mentioned above will not last long and will be sucking up moisture very quickly and the whole floor of the shed is probably very damp under your ply. The whole underneath needs better ventilated and needs free air movement. This semi buried part needs to have clear space under all timbers of at least a couple of inches. What are the joists at the front sitting on? concrete or just earth.

I would strip the fence boards out of there (assuming the walls are not built on top of them) and put a sheet of 18mm ply directly over your joists.

2. The roof overhang needs to be long enough to provide a nice drip edge well clear of the walls.

3. The ship lap on the lawn side needs to at least have a large vent in it. I assume the back is open? If not another large vent in the fence side to provide some cross flow for the air

 

[greyam100]

Thanks for the feedback folks

The main joist at the front is spanning between the corner posts but it may also be touching the backfilled earth.

The walls sit on top of the fence boards and the back is shiplapped down to ground level.

I guess i'll need to lift the ply, cut some of the fence boards out to get access to the offending earth and clear it out best i can. Then get some airflow through the underdeck area. Will then monitor whether gutters required.

Another thing I have noticed is that i have a plastic floor standing shelving unit in shed, on which sit various items. I've noticed small pools of water at the bottom of the item, obviously a related condensation issue.

Thanls

 

[JohnD]

Will then monitor whether gutters required.

Yes, they are.

Squirt your hose on the roof and see if the walls stay dry.

 

[greyam100]

Ok thanks. I take your point.

Do you think gutters or extending the roof overhangs should be the priority i.e is that the main issue?

The thing is, surely i would have similar mould issue at both sides, which is not the case.

Also, often the rain is more horizontal than vertical.......

 

[JohnD]

IMO gutters and a downpipe to take the water away from your footings will be a reasonably quick job, I don't know if it will be easier for you than opening up the base. Do the quickest one first. Wet footings will exacerbate the damp base. You could use fine mesh to keep rats out.

Are you sure rain isn't getting in round the window frame? I think I can see a wet mark on the inside, beside that batten.

 

[big-all]

very very bad design
overhang off about 4"/100mm give far more protection
all timber must be clear off the ground with at least a 25mm gap to stop water transference from the side or ground

 

[ellal]

I was thinking the same about the design big-all

The OP mentioned having a company build it, so I'd be inclined to call them back.

 

[bernardgreen]

All the above and

water running down the slope has no where to go when it meets the shed.

the hedge will keep the back wall damp, The leaves will deflect rain drops onto the wall and then prevent the sun / wind from drying that wall.

Possible solution. move the shed a foot up the slope and away from the hedge. Support the floor on 2 by 2 beams resting on the red path at one end and on an improved frame under the back of the shed. Rain from the slope will then run under the shed beatween the beams.