My concrete shed (workshop) base floods

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diyfiesta

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Hi All,

I've finally got to building my dream workshop, a 6m x 4m log cabin (Dunster House). No more dragging my tools out of my tiny shed before I can even start a job. I had an over sized concrete based laid, so 1m to the side, another 1m at the rear (giving access to the surrounding fence) and a 2m front "porch". The conrete is about 100m thick with rebar and is pretty much flat.

The trouble is its probably too flat.

I built the log cabin and it rests of 20mm x 20mm bearers but whenever it rains, the water just sits on the flat concrete and seeps under the shed and basically floods the underside. Although the bearers are guaranteed for 10 years, I don't think they'll last 10 mins if they're pretty much constantly sitting in standing water.

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I'm hoping for some advice about how to avoid the flooding and fix the problem. I've put guttering round which I'd hoped but it doesn't really help. I'm thinking about adding draining channels but don't relish chopping into the concrete or trying to find a contractor to help - I suspect they're as deep as the base!. I'm basically stuck!

Thanks in advance
 

Jameshow

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Why not put another timber tangential to the others sealing off the base, glue it down with a ebt or 3M 5200 type adhesive which will seal any gaps. Also paint with bitumen paint for belt and braces approach???

Or build it bigger to cover the whole concrete!!🤣🤣🤣

Cheers James
 

hairy

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So the slab is catching rain and feeding it under?
Is it possible to jack it up and put cut up paving slabs under the bearers?
Or hire a disc cutter, buy a diamond blade and cut round the edge of your structure. Either detach and break up the excess slab or cut a channel in it? That should be easy to cut, but use water to damp the dust down.
 

Fergie 307

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I think putting anything permeable under it is a waste of time. Either lift it and insert resin deck boards under it, or as hairy says cut it away.
 

okeydokey

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I like the resin decking boards idea I can use that on a small shed my son is building.
For this problem as suggested above hire a disc grinder and cut a slot and install drainage channel - all the way to the far edge of the 1 metre section at the top of the drawing with a slight fall that way and water should run off.
 

Craig22

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My shop is in the garage. But the guys who built the house around 30 years ago laid the concrete floor sloping slightly downhill towards the garage door, the aim being to prevent water ingress when it rains. And that works 100%. Leaves blow in in the Autumn, and I have the occasional battle with spiders, but no water comes in.

That is probably not what you want to hear. I don't think there is an easy solution, but I'm not a builder. Why not get the guys in who laid the concrete raft, explain the problem and see what they suggest?
 

focusonwood

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I've recently put up a log cabin, I built the base 100mm larger all round. I thought I was following the vendors guidance, although they're a little vague when it comes to the base - after all they're only providing the log cabin.

Water was getting underneath, a lot of water. As you said, I don't think the treated bearers would last if they're constantly sat in water, plus moisture would transfer from the bearers to the non-treated wood soon enough.

I have attached a DPM 'skirt' all around the base and added sealant where it's stapled to the cabin - the skirt drapes over the edge of the concrete base. This seems to have sorted it. I hope so anyway as I've just put the floor boards down. I also screwed the floorboards down with decking screws rather than use the supplied nails, I figured that if I have a damp issue it'll be easier to take them up and fix the problem in the future.

Given the distance your base extends beyond the cabin, this approach probably won't work.

I considered jacking the cabin up a little and sliding DPM under the bearers - I meant to do this anyway but didn't so was kicking myself when I discovered water was getting underneath. It should be easy enough to do for the bearers that aren't supporting anything other than the floor boards without jacking it up.

If you've already put the floorboards down, a drainage channel around the cabin as suggested above might be a good option, although once that fills up with water presumably the over spill would just make it's way under the cabin.

What about a little brick wall right in front of the cabin, one or two bricks high, and then as with my solution, a DPM skirt attached to the cabin and draped over the little brick wall?

As you can tell, I'm obviously not a builder so you should probably get advice from one rather than follow any solution I suggest!

In my opinion log cabin providers should be clear that a concrete base must be exactly the size of the cabin to avoid water ingress.
 

HappyHacker

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Hire a concrete scrabbler to remove concrete and create a slope away from the workshop. The hire shop should be able to give you advice on the best tool and the necessary heads for concrete removal. This would cure to ponding but not stop any windblown water which would require one of the other options proposed.
 

baldkev

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Or..... place a bit of 4x1 along the edge of the cabin where the water comes from and put in a sand/ cement screed 'ramp' going upwards towards the cabin to stop the water? It won't be perfect, but it'll be quick and easy......
 

Fergie 307

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Ideally your base should have been laid for the building only, and the apron added afterwards. Preferably leaving the base standing proud, and with the apron falling slightly away from it. Now I think you only have the option of raising the building on some impermeable support, or cutting out a section and putting in a drainage channel. Only problem with the second is that you will probably expose the rebar in the cut you make, very difficult to prevent this from then rusting and causing further problems in the future. If the contractors knew the plan then they really ought to have foreseen this problem. Might be worth having a chat with them. You might be able to cut a shallow trough in the surface to act as a drain, but would be difficult to do and would need to be shallow to avoid exposing the rebar, do you know exactly how deep in the slab the reinforcement is?
 

diyfiesta

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Thanks all for the ideas.

the base is about 100mm and the rebar presumably half way down, maybe a little lower.

the contractor wasn’t very interested, I don’t think I’ll be able to convince them to come back. They did suggest removing some of the shuttering but I dont see how I’d be able to do that.

Just gutted I didn’t forsee the problem!
 

Jameshow

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Can you not lift it and put it on engineering bricks?

Use a scaffold board under one end and lever with a brick under it. Get someone to sit there hold it down/shed up until you have done one side and repeat for other side's.

Cheers James
 
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RobinBHM

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you could screw down some aluminium angle all around, or some timber batten -with some sealant underneath to create a water seal (you would have to thoroughly dry and clean the concrete first and brush / vacuum any loose stuff. Any you would need to research suitable sealant.

possibly it might work by using 2 strips of butyl tape -or compriband tape -with a gap between. Then put a large bead of sixaflex sealant in between -then screw down



and also use a grinder to cut a few channels across the concrete from the angle out to the edge -so water runs away and doesnt build up.

obviously you know now a base should be the size of the framework and the cladding should overhang -any path should be lower and separate.
 

hunter27

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I have fitted wood sheds on thin flags that are just sat on the concrete base, just to lift the timber above the water, leave gaps between the flags for the water to pass through, and fit a bit of DPM between the shed and the flag. If the water gets deeper than the flag thickness you need to get your builder back.
 

hairy

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Needle scabblers or ones with solid heads are often used to remove the surface layer of concrete ideally the day after it's poured to produce a good joint with the next pour or maybe with other stuff that needs bonding. The laitance left on the top after pouring can give you a permeable layer that lets water through a join if not removed. Doing that even a few days later is not a nice job.
Scabblers with solid heads give you a lot of vibration, possibly too much to be healthy even after only five minutes. Needle scabblers don't do a lot anyway. A pole scabbler doesn't need to be held too tightly and is powerful with a big compressor but even then even only a few days later is not nice. To create a fall away from the shed with one would be very hard, leave a rough surface and is someting you want to get someone you don't like to do. Ideally avoided altogether, and a retarder sprayed on immediately after pouring followed by jet washing the next morning does the same job (making a good construction joint) much easier, horizontal or vertical.

I don't think leaving tiny rebar exposed would do much long term, minor cracking? You could dab the ends with hammerite? Sika do a paint over product they say gives the equivilent of 50mm cover IIRC. A thin slab with a small mesh kind of held up, hopefully not trodden on during the pour, maybe chucked in after pouring, gives you definitely how much cover off the ground anyway?
 

Terry - Somerset

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Again a non-professional thought but what I would consider in the circumstances.

The drainage slope recommended for patios or flat roofs is approx 1:80. So for the widest flat area (2M) you need a slope of about 30mm from the edge of the workshop to the edge of the slab.

Around the edge of the workshop install some 30mm thick slabs cut down to approx 50mm wide. This would butt up against the workshop - the workshop/slab joint could be sealed using mastic (may not be necessary). Alternatively use damp proof membrane?

Then lay cement from the edge of the slab to the 50mm slabs surrounding the base of the workshop. Any water falling should simply run off to the edge of the slab.
 

Steve_Scott

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If you want any longevity from the build I’d strongly suggest you alter the base to prevent water getting under the cabin. Either cut away the excess entirely or cut out slots and fix gully drains at the perimeter.

Any effort to jack it up might save the immediate problem but you’ll still have stagnant standing swamp water under the floor (assuming you are putting in a timber floor?) which isn’t going to do you any favours.

As an aside, if you haven’t already painted it, I highly recommend Sadolin Superdec… mine is coming on 7 years old and hasn’t given me any problems yet although its just about due a refresh.
 
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