Fixing damp proof course which is leaking over the TOP

[Fromey]

My work shed was build by an unknown numpty lost in the mists of time but unfortunately I'm stuck with it. #-o It's a concrete slab with two courses of red brick, a damp proof membrane and then concrete blocks. It leaks water around the DPM in a couple of places when there is heavy rain. I've discovered one particular area inside which is a constant drip of water coming in over the TOP of the DPM (so much so that I have to put a drip tray there). This happens even in mild rain.

The culprit appears to be where the rain hits the external wall (the one that usually faces the prevailing weather), runs down the wall and then encounters the DPM where it pools and wicks into the wall, eventually reappearing inside.

I have a couple of questions of this esteemed forum (where I always get sterling advice).

i) Should the DPM be cut right back very close to the external wall or is it suppose to be about 5 millimeters proud?
ii) Would you suggest layering in some cement above the DPM to stop it leaking or,
iii) Squirting in a bead of external silicon sealant or,
iv) Fixing some sort of drip guard bead along the lower level of the wall (not very practical as it's the neighbour's yard).

My optimum solution would be to tear the whole abortion of a shed down and rebuild in timber, but I'm afraid that's not really an option.

Thanks in advance.

 

[Harbo]

I would try some sealant and paint with bitumen paint to seal the blockwork/brickwork?

Rod

 

[Graham Orm]

Sounds to me like your wall is single skin and porous. This means that it's nothing to do with the DPC, but is just soaking through. In fact it proves that the DPC is doing it's job as the water is stopping when it gets there and coming out of the wall.
You need to seal the outside either with a purpose made sealer (requires re-doing every 2 years), or render, then weatherproof paint, or you could just try weather proof paint.

I've used this sucessfully before. Read the reviews here http://www.screwfix.com/p/thompsons-wat ... Au0w8P8HAQ

 

[Phil Pascoe]

Is the blockwork skimmed or bare? Sounds like it could do with a bell cast and or some silicon water seal. How high is the damp inside? It might be coming in higher up than you think.

 

[CHJ]

i) Should the DPM be cut right back very close to the external wall or is it suppose to be about 5 millimeters proud?

If you are saying that currently the DPM protrudes as per this sketch:-

Then yes it needs cutting back level with or very slightly undercut of the upper layer of mortar to allow water running downward free passage without pooling.

The blocks as mentioned really need skimming or sealing, course aggregate cement blocks can readily allow driven rainwater passage through the blocks.

 

[Phil Pascoe]

My friend's garage actually floods through an unrendered single 6" block wall. :shock:

 

[Fromey]

Wow, testament to a wet and dreary Bank holiday Monday; I went to the shed to oil some shelves and returned to 5 responses! Thanks all.

Firstly, the concrete blocks are indeed unrendered and at least looked to be very porous (although they're super hard to drill through). Thus, I painted all of the walls with Sandtex about 6 years ago. I'm pretty sure I gave that wall about three coats as it faces the prevailing weather. The paint looks sound. There are just two hair line cracks in that wall that have subsequently opened up in the mortar joints. I certainly wouldn't expect this much water to come through those (the leak point drips at a rate of about 1 drop per second). So I'm pretty sure the walls should be well sealed. In addition, there are no damp patches on the walls anywhere in the shed other than a few entry points at the DPM level. Dripping happens only when it rains and stops pretty much when the rain stops.

Below are some pictures of the area directly opposite the leak point (granted, that doesn't mean it's the entry point for the water);

You'll see that the DPM does indeed extend out as Chas has drawn. I think I'll firstly try cutting the DPM back and when the weather gets dry I'll try oversealing with something like bitumen paint as Rod has suggested.

Thanks again.

 

[Graham Orm]

That DPC (Damp proof course not Damp proof Membrane DPM) does not need touching. If you paint over it you risk bridging it. As for the inside not showing damp, I would suggest that's because the wall is soaked. Try a damp meter on it, I think you'll find it goes off the scale. Paint the outside with Thompsons sealer. Job done.

 

[Hemsby]

That DPC (Damp proof course not Damp proof Membrane DPM) does not need touching. If you paint over it you risk bridging it. As for the inside not showing damp, I would suggest that's because the wall is soaked. Try a damp meter on it, I think you'll find it goes off the scale. Paint the outside with Thompsons sealer. Job done.

+1 For the Thompsons

 

[Fromey]

Hmmm, the plot thickens. I just so happened to have purchased an Axminster moisture meter only last week.

The walls on the lee side of the shed (have never had water problems) show readings between 8% and 12%. That's the same as my internal walls in my house.

The walls on the weather side show readings between 10% and 18%.

Where the water comes in is in fact the base of a pillar which has been built inside the wall of shed to hold a piece of railway line that acts as an internal beam (convenient as a storage rack). This chunk of metal sticks out of the shed by about two inches on both sides. I've long suspected it as a bad idea but had only sandtex painted into the cement around the metal. The moisture meter shows progressively increasing readings as I go up the wall and get to the brick that is immediately under the beam. At that point it reads 20%.

I have a damp wall in my house which is most likely rising damp, and that goes off the scale, so the shed wall is not as damp as that.

This suggests that water may well be getting in around the beam where it exits the shed wall and is channeling internally down to the DPC. However, if that's the case I would have expected a higher reading.

As to the Thompsons sealer. I have some as I painted the bricks below the DPC to help prevent moisture penetrating through the bricks into the shed. Were you suggesting painting around the DPC area in general, or the whole wall?

 

[Hemsby]

Do all the single skin areas, for the time & cost it has to be worth it. Stopping any water ingress at its source is always the best =D> .

A nice big single skin brick/rendered wall can be like a giant sponge .

Regards

 

[Phil Pascoe]

I would do the whole building. To my mind anything that is meant to be waterproof should shed water as quickly as possible. I must admit I cannot see that stone chip and pebble dash are good for that. A friend bought an old cottage some years ago, and had to have a chemical DPC put in to satisfy the lenders. Suddenly there was a water ingress problem where there hadn't been one for 150yrs - instead of the rain soaking into the gable end and going to ground, it started to run indoors above the DPC.

 

[Graham Orm]

Do all the single skin areas, for the time & cost it has to be worth it. Stopping any water ingress at its source is always the best =D> .

A nice big single skin brick/rendered wall can be like a giant sponge .

Regards

+1

 

[Fromey]

OK, but I'm wondering if Thompsons will work on top of a thick coat of sandtex, which the whole shed has. The literature on it implies that it's for bare brick or concrete before overpainting.

 

[Hemsby]

Don't think Thompsons will be any use on anything other than bare brick/concrete. If the walls have already been treated with external paint finish and that is in good condition it should stop the water penetration =D>

 

[Graham Orm]

I'd do it anyway

 

[Harbo]

Looking at some of your photos there's very little clearance between the soil/ path? and DPC?

Rod

 

[Graham Orm]

Looking at some of your photos there's very little clearance between the soil/ path? and DPC?

Rod

Agreed Rod. There should be minimum 2 bricks showing below the damp course.

 

[Phil Pascoe]

OK, but I'm wondering if Thompsons will work on top of a thick coat of sandtex, which the whole shed has. The literature on it implies that it's for bare brick or concrete before overpainting.

The last silicone I looked said the opposite, which makes more sense. There'd be little point in trying to use a water based paint over the silicone - it would throw it off. Actually, bite the bullet - buy oil based Weathershield (or another). It's way, way better than water based (and easier to use).

 

[Fromey]

Regrettably, yes there is little clearance. It's the neighbour's yard so I don't have too much control over it. In between owners, I slipped in and dug a type of French trench along the wall to help clear the water (you can just make out the gravel in the picture). I also think the brickwork in general throughout the shed has a lot to be desired.

My plan is to seal the couple of fine cracks in that wall, dig out a little bit around the protruding railway line, clean up the rusting metal and then paint it with rust reverter, then a spray of zinc paint and finally external sealant around the metal. If that doesn't work, I'll try to devise some sort of complete covering for it. I may also trim the DPC a bit and add a line of silicone sealant above it in case it's the culprit.

The builder was the same wally who cemented the ground at the side of the house and sloped the surface towards the house and with no drainage holes. Thus, the kitchen wall has been absorbing all of this run-off for the past 30 years or so (the building is c. 1890s and so only has a slate DPC). Jacking all of that up, disposing of it and laying brick paving is next year's project :roll: