Is there a reason i cant build this DPM detail

[Caversham Build]

Hi all, me again, 

 

Thanks for all your help so far.  I have another question 

 

I want to protect against ground water / rain water / holy water the lower 500mm of my outbuilding, as it is below GL in a quite wet area. 

 

My first detail was this :

 

But i have come up wit this detail instead which, to me, will do a better job and much cheaper:

 

 

 

pretty please please could you tell me if this can work / share your wisdom  apart from the 'change your design so its above ground level' wisdom

 

thank you

[tonyshouse]

I would not use rockwool below ground, not fail safe, suggest EPS instead 

 

drainage channel ideally below lowest point of the base dpm 

 

I would go floating floor

 

Probably you want a cavity tray above the external dpc 

 

 

[Rob99]

My initial thought is that your external DPM, sealed at the top edge with Bituthene, is a significant weak spot and any breach at ground level will allow moisture to track down and into the internals of the building.

 

Your first detail seems to be the right way to do it and doesn't look to me like it is much more expensive. For things that I never want to have to deal with again I tend to go the belt and braces route as it provides peace of mind.

 

I'm not sure what BCO's view would be of your second detail, have you spoken to them about it?

[tonyshouse]

Fo me first detail runs risk of sweating and picking up some condensation on the inside of the dpm at the coldest places. 
 

don’t like rockwool below g/l 

[Caversham Build]

19 hours ago, tonyshouse said:

I would not use rockwool below ground, not fail safe, suggest EPS instead 

 

drainage channel ideally below lowest point of the base dpm 

 

I would go floating floor

 

Probably you want a cavity tray above the external dpc 

 

 

@tonyshouse thanks for your response can i please ask:

 

Can i use EPS for below ground level and rockwool above/ - my bricklayer who i have been trying to keep sweet as he is very good and they are in short supply  told me to use Rockwool i think it makes his life easier.

 

Floating floor - as in lose the wooden joists bridging the slab?

[Caversham Build]

19 hours ago, Rob99 said:

My initial thought is that your external DPM, sealed at the top edge with Bituthene, is a significant weak spot and any breach at ground level will allow moisture to track down and into the internals of the building.

 

Your first detail seems to be the right way to do it and doesn't look to me like it is much more expensive. For things that I never want to have to deal with again I tend to go the belt and braces route as it provides peace of mind.

 

I'm not sure what BCO's view would be of your second detail, have you spoken to them about it?

@tonyshouse@Rob99 and anyone else - how about his detail below i've just drawn? Taking into account your feedback - my question is will this many bits of plastic (DPC/ DPM) in the outer leaf of bricks / blocks cause any structural issues?

 

thanks for your help

 

 

 

[tonyshouse]

I like a drainage membrane between the wall and the stones. 
 

I like fail safe design which might mean membrane up the inside of the inner skin to the first block then up the cavity. 
 

I was I’m Caversham today, recently involved in building a hydro there 

[saveasteading]

I would fear that the gravel against the wall could work in reverse to your intentions and bring in water. 

Standard building details work, and this seems over complex to me.

If water was by any means to get into thecavity then there is no means of escape other than into the floor slab.

[Caversham Build]

4 hours ago, tonyshouse said:

I like a drainage membrane between the wall and the stones. 
 

I like fail safe design which might mean membrane up the inside of the inner skin to the first block then up the cavity. 
 

I was I’m Caversham today, recently involved in building a hydro there 

 

Thanks @tonyshouse yes my house is about 500m from the hydro, great little scheme.

 

Self building is a steep and stressful learning curve.

[Caversham Build]

3 hours ago, saveasteading said:

I would fear that the gravel against the wall could work in reverse to your intentions and bring in water. 

Standard building details work, and this seems over complex to me.

If water was by any means to get into thecavity then there is no means of escape other than into the floor slab.

 

thanks @saveasteading do you mean the water will more easily travel out of the soil and into the gravel channel around the outside of the building?

[Caversham Build]

 

I think i am losing a sense of why i was trying to do this in the first place.🤯

 

I thought it was to try and stop water getting into the building in the first place in the event of very wet ground or flood - but that's tanking right?

 

So with a DPM detail thats more standard - i'm, in general, obstructing the transference of moisture from the outside to the inside. BUT if the building gets very wet or there is a flood a more standard detail will allow me to dry things fix things out more easily?

 

I'm so confused 

 

 

[Iceverge]

Just have it in such a way that the water can go down and out. Relying on DPC only for keeping water out of your floor is foolhardy in my book It will get punctured during the build. . A french drain lower than the bottom of the cavity with a drainage pipe would be best.  I would make your brickies life really easy and get rid of the rockwool entirely and use EPS blown beads. Above and below ground. Take the cavity as low down as your SE will allow.  It will help with thermal bridging and any water will really struggle to cross the EPS beads.  

 

Here's what I think we should have done.  To be honest I can't see what the overlapping yellow DPM is achieving in the real world if the french drain is operating correctly but people will get angsty if I don't draw it! 

 

 

 

 

EDIT the DPC in the outer leaf low down is a mistake! 

 

 

 

 

Edited March 3, 2022 by Iceverge

[saveasteading]

1 hour ago, Caversham Build said:

soil and into the gravel

Yes. Or along the pipe into the  gravel,  and certainly down the wall and into the gravel

[Caversham Build]

Hi all, 

 

okay i have gone back to the drawing board and simplified. 

 

how does this look? 

 

I cant make the drain lower than the DPM unless i put insulation under the DPM rather than over as i have already poured foundations and  hardcore

[saveasteading]

To me that should be designed as a basement, regardless of whether it is 300mm or 3m underground. Also  that lower length of wall is acting as a retaining wall.

So I suggest completely tanking the outside up to  ground level for the former, and making the outer skin of the wall heavier for the latter.

That keeps the water out and the wall stable.

Then deal separately with dampness from below, so dpm and dpc all linked in the normal  way.

 

PIR is twice as effective as EPS (at twice the cost) so perhaps better in the cavity at this point where I am suggesting a narrower cavity.

 

For interest, why correx under the slab?

[Iceverge]

The PIR encompasses the concrete slab. I don't understand why?
 

A 25 mm land drainage pipe would be very narrow, We used 110mm, The pipe was below the lowest level of the DPM. 

[CharlieKLP]

I don’t understand why you’re reinventing the wheel on these things. Just use a standard detail and buy a window. I don’t think you can do these things better or cheaper than professionals.

[Caversham Build]

 

@Iceverge @CharlieKLP

 

These comments are making me more stressed and less confident. So i'm going to bow out of this discussion and make the best decisions i can without any more assistance.

 

Thank you to everyone who offered me constructive feedback. 

 

 

 

[Iceverge]

Sorry for my abrupt post. Tone is unfortunatly often lost in narrow converstion channels like forums, it certainly wasn't my intention to dissuade you or knock your confidence. I hope you find a good solution.

 

Good luck with your build.

 

 

[CharlieKLP]

I just said to check out some standard details, but sure good luck.