Damp in garage workshop

[fingerless]

Hello all,

I'm after some tips on what to do with my garage to overcome a major damp problem. I had it extended in length last year, and a new roof put on. The roof is ply on joists, then covered with felt, then roofing felt. This seems to be watertight. The extension consisted of pouring a concrete base which butted up to the previous concrete base. The walls are brick. During construction, after the concrete was poured and semi-set, but before the roof was put on, it snowed. After construction the builder painted the concrete floor with garage paint. This has lifted over the new patch of concrete, but not on the old concrete. The major problem is that the underside of the ply panels are now covered in mould, and the damp is unlikely to be doing my tools any good either. I'm guessing the moisture is mainly coming from the concrete floor, but maybe also from the generally humid air due to the terrible weather this year. We did try to air it as much as possible through the summer. The windows and doors are double glazed UPVC.

So, what do you advise? I can try heating it (oil radiator or convection heater?) or I can try a dehumidifier, although they seem to be extortionately expensive. I'm open to any advice, as I don't want a new roof destroyed by damp...

Thanks for any input.

Colin

 

[DIY Stew]

I am not a builder but I believe you have to put down a damp proof membrane before you pour the concrete, did the builder do this?

Stew

 

[undergroundhunter]

I would 2nd the damp proof, if the paint is peeling on the new section and not the old it would suggest that there is no damp proofing under the concrete. Give the builder a ring and get him to come and have a look to see what he thinks.

Matt

 

[Harbo]

Lack of DPM I think - lots of buildings are constructed during bad weather and once weather tight dry out without any problems?

Go back to the builder.

Rod

 

[Pond]

It certainly sounds like you have a damp membrane problem; either you haven't got one, or it has failed under the floor somewhere.

If there is a DPM (Damp Proof Membrane) it is most likely there is a gap between the old and new.

You need to install a damp barrier somehow. The easiest way is to lay a dpm, either liquid or polythene sheet on top of the concrete floor, making sure it is cut into the lowest brickwork mortar course or the damp will creep up the walls. A floating overfloor, T&G is the easiest, can then be laid on top of the DPM. Floating is best as you cannot put fixings into the concrete or you will puncture the DPM and it won't work. The hardest part is damproofing where the walls meet the floor!

Heating will make the problem worse, as the moisture will evaporate and rise, condensing on the coldest and highest part, ie the roof! A dehum won't really help if you have constant moisture entering, as it will just be condensing a never ending amount of moisture.

Personally I would confirm the problem by having a report done by a surveyor or similar, then get the person who did the job round and ask him what HE proposes to do about it.

HTH

 

[Fromey]

I have a shed with a poured concrete floor. I didn't do it, the numpty who built the whole shed many years ago did. I suspect it doesn't have any damp proof membrane underneath it (I can't know for certain). When it pours I get water actually rising up through it and the lower bricks of the shed along two edges.

Similarly, I have a roof made of external grade plywood laid on joists with two layers of felt. It seems quite water tight. I should have put a breathable membrane up there when I did this, but at the time I didn't have sufficient knowledge and I was racing against time to get it done before it rained (plus I had a broken wrist, don't ask, and a friend was finishing the job for me).

Almost immediately I also got mould forming on the ceiling plywood. In addition, in the winter, in one corner (the last bit of the shed to be hit by the sun) I got so much condensation forming it actually ran down and dripped off significantly.

Gaaa! If I knew then what I know now, I would have demolished the shed when we bought the house and built a proper timber job. Anyway, we live and learn.

My first update, other than the roof, was to completely pain the exterior concrete walls with sandtex paint.

Next, I decided to insulate the roof with some foil backed bubble plastic they sell in B&Q and such places. It provided a bit of insulation but the glue I used wouldn't stick to the plywood so the insulation kept pealing off.

Eventually I got around to buying some fairly thick celotex rigid foam insulation (double foil backed), cut it to shape and rammed it up between the joists on top of the previous insulation. I then battened it in so it's a tight fit. Now that fixed it! Of course I don't know if there is insidious water collecting immediately under the plywood, but so far there's been no mildew or leakage anywhere.

I still have the rising water occasionally, but I don't think there's anything I can do about that other than laying down a water proof membrane and pouring another layer of concrete on top.

I have a humidity monitor in the shed at all times and habitually look at it. It never gets above 75% relative humidity and normally it's down in the low 60% range. I periodically take out our household dehumidifyier and give it a couple of days of treatment. That can lower the humidity down to 50% but it soon rises back up to 60%. I figure that's going to be the general background level simply coming in from the British air and rising from the floor.

From what I understand of rust on tools, the main culprit is letting the tools get too cold and then warming them up. I now have an oil heater in the shed on at all times to maintain a temperature of about 10 degC (stops the glue and paint going off as well) and it rises to 15 - 17 degC when I work in there. To date I've had no rust problems other than a bit in the early pre-insulation days.

 

[pip1954]

is the garage insulated? is it boarded out on ceiling? is it double or single skin brick work or block work? all these things can have an effect on the situation, if you say it is condensation you can use dehumidifier .
pip

 

[fingerless]

Thanks for all the replies. Just to confirm, this is a slab of concrete that forms the foundation for the walls and the floor of the garage. Are you saying there should be DPM under this concrete (it would probably then be below ground level outside)? There is DPM in the walls, but not under the concrete as far as I'm aware (although I wasn't there when the concrete was poured, so I don't know). The garage isn't insulated, is single brick walls, and the roof isn't boarded out.
The builder is coming around next week to "have a look".
Thanks,
Colin

 

[Lons]

Hi Colin

I'm a builder and I can't add much to the advice you've already been given.

A properly laid concrete floor with the correct thickness DPM does not allow moisture to ingress from below. (If a membrane has been installed, it is possible the sub base hasn't been consolodated and blinded sufficiently and has punctured the membrane.) the DPM must be fully lapped under the DPC of the walls to be effective.

Having said that, it is also possible that the floor paint has lifted because the builder didn't allow the concrete to cure fully before painting. To be on the safe side, 1 month is recommended.

As an aside, I always take WIP photos when doing groundworks and foundations so I can prove compliance. Have never needed to but better safe etc.

cheers

Bob

 

[Pond]

Yes there should be a damp barrier under the concrete. It doesn't matter what the floor level is outside. The idea is to prevent moisture rising through the concrete from the ground below it. This is then 'tied in' to a DPC in the brick mortar which should be approx 2 courses (6") above the highest ground level outside. The 6" rule of thumb is to stop large amounts of rain splashing above the dpc level on a regular basis, or vegetation growing above this height and bridging the dpc.

If installed correctly, you would have a 'tray' like shape, ie the DPM is laid under the concrete, then laps up the walls to be buried in the brickwork at DPC level, creating a single barrier.

sorry to repeat Lons but the posts overlapped (same as a DPM should )

 

[Lons]

Thanks for all the replies. Just to confirm, this is a slab of concrete that forms the foundation for the walls and the floor of the garage. Are you saying there should be DPM under this concrete (it would probably then be below ground level outside)? There is DPM in the walls, but not under the concrete as far as I'm aware (although I wasn't there when the concrete was poured, so I don't know). The garage isn't insulated, is single brick walls, and the roof isn't boarded out.
The builder is coming around next week to "have a look".
Thanks,
Colin

OK, it's in effect a "raft" foundation. It should actually be deepend all the way around to carry the weight of the walls and is usually reinforced as well to prevent possibility of ground shrinkage breaking its back.

The concrete should very definately have a DPM of something like 1200g and should be lapped and protected at the edges. Exposed DPM should be covered and protected from UV.
I assume you mean DPC in the walls, so where is that? Is it under the bottom brick course or 1 or 2 bricks further up. Wherever it is, if it isn't lapped into the DPM, you will get water through, if not the floor then under or through the bricks.
Looking outside, is top of concrete above, level or below ground level?

Single brick walls if not rendered or treated will always allow driving rain through btw. Can you be sure your moisture is not from the walls rather than the floor?

Bob

 

[kostello]

The peeling floor paint is due to the damp being trapped in the concrete.
Dtdt

The cheapest way to dry it out is to keep circulating fresh air..

Keep the doors or windows open and maybe a fan on..

A dehumidifier will only work if the ambient temperature is above a certain point as it works by cooling the air to dry it.

 

[graduate_owner]

I'm not a builder but I believe oil heaters (also gas and LPG) produce moisture as a by-product of combustion, so if you intend to heat your workshop then electricity may be the best option - although probably the most expensive to run.

As a matter of interest, I'm working on damp-proofing and insulating my workshop too. Polystyrene, glass fibre matting and urethane foam all being collected ready for the job.

K

 

[Dusty]

Evening Fingerless
One thing you have to remember is that when a garage is built , it was never really deemed to be important to have the same level of damp proofing as a house , hence why yours hasnt either . If it is a raft footing as was cast as one and normaly above ground level to prevent water running in ,and as Lons said there is still ways to prevent damp and protect the Membrane overlapping the base from damage and UV light , but they woudl never have bothered with a garage plus the footings / raft would of been higher than the ground .

I have had to deal with a few garage conversions over the years in the same situation as you have got , both masonary and pre-fab ie woolaway concrete . The only way I would say to be 100% if you have the head height is to lay a 1200 gauge membrane or use a liquid membrane like synthaproof or similar ( bitumen ) and then lay a 50mm screed over the whole floor , the membrane will need to continue up the wall and sealed up to the damp course or above .

If you lay a floating floor , the floor will either have to be very very level / flat or protected with Kingspan or similar to prevent the floor moving ( expansion ) and damaging the membrane or bitumen . you could lay a screed floor down to about 35 mm if you use a liquid poor screed but the cost of that system is big compared to traditional screed .

And as for the paint peeling on the floor , if you pour a 100mm concrete floor , once it is protected from the weather ie an extension , it will take in the region of 4 to 6 months to fully dry out , if your floor has no membrane then it will suffer from rising moisture and never fully dry , causing the floor to sweat and push the floor paint off . This can work but it is necessary to use good quality paint and to prime the floor first , not DIY products like wickes .

Hope this helps

Dusty

 

[fingerless]

Thanks again for the extensive responses, especially from the builders (are any of you in the essex area?)!
I've had a closer look and in the extended section there is DPC in the wall two bricks above ground level. In the old section, the DPC is at the same level as (or maybe just below) the concrete floor. There is no damp proof membrane from the floor joining to the DPC in the extended section. It looks like the builder painted black rubbery paint on the floor, followed by the garage floor paint. This was done within a week of the roof going on, so probably 1-2 weeks after the snow was gone. It sounds like this was far too soon.
I'm fairly sure the damp is coming in through the floor as if I leave something on the floor the underside gets wet.
I agree that garages may not be done to the same level of damp proofness as houses, but the builder did know this was going to be a workshop, so it is annoying. In addition, I don't see the roof lasting very long with the amount of mould that's growing on it. Given the drying times mentioned in your posts, and the fact that the floor was painted, I could believe that the concrete just hasn't dried out yet, but if there's no DPM then it sounds like there will always be a damp problem, unless I do something extra.

I do have the headroom for a screed, so that is definitely an option, although the doorways / thresholds may be a bit awkward. I'm guessing the peeling paint is not a good surface for applying any form of damp proofing paint, and it needs to take wear from foot traffic, wheelbarrows etc. so maybe screed is the only option. Roughly how much is a 50mm screed / m^2?

I will see what the builder says when he visits on Thursday, but I have a feeling a real solution is going to be expensive.

Cheers,
Fingerless

 

[Dibs-h]

By 50mm screed, I am assuming folk are talking about anhydrite screeds. These aren't cheap and must be professionally laid and are laid via pump. I entertained it when building my shed, but gave it a miss due to the additional cost of the pump, etc. (when I say - I mean a lorry pump).

I don't think a traditional sand\cement\concrete\etc screed at 50mm will last - but folks mileage may vary.

Regards

Dibs

 

[Peter Sefton]

I have pumped free flowing screeds in our barn conversion over underfloor heating - these screeds are excellent for that but I would say OTT for a workshop. Our furniture workshops have some old concrete floors which never had a DPM as they were agricultural in a former life. We do have a DPC 150mm above outside ground level and have layed a new DPM and wrapped it up the walls 300mm before laying a floating floor of Kingspan covered with 18mm chip board as a glued floating floor, which we painted with Koovar floor paint and have a very warm and dry workshop now. The walls and ceiling may take more work to sort out the problems of condensation and water ingress. If you are in the Worcester area come and see us and I can show you how we overcame similar issues.

Cheers Peter

 

[simocco]

Try this site for all things damp...

http://www.heritage-house.org/pages/dam ... -damp.html

 

[vinnie_chip]

If the builder missed out the DPM he should do whatever is necessary to fix it.. at his expense

 

[fingerless]

So, the builder came around and admitted there was a problem, saying the concrete and mortar had not had a chance to dry out properly.

He left a dehumidifier with me for 2 weeks, which I ran along with heating (he'd hired the dehumidifier at his own cost). He has also offered to lay a wooden floor over a damp proof membrane, where I pay for materials, and he covers the labour. His plan is to lay the DPM on the existing concrete floor, lay 2x2 joists on that, and then fix ply to those. I'm aware of the issue of breaching the DPM when fixing the joists to the floor from Dusty's response. Can the breaches be sealed, or should the joists not be fixed to the floor?
I was also wondering if it is worth putting insulation under the boards to warm the place up, and I'm planning on boarding up the ceiling as well, potentially with insulation between the rafters. Do you think this makes sense?

Overall I'm happy with the response from the builder, and hopefully the workshop will be a cosier more pleasant place to do stuff at the end of all of this.

Cheers,
Fingerless