damp course question

[noseall]

you can treat the problem as if it were a roof abutment, as the scenario is exactly the same. install a lead flashing to the wall 150mm up from the floor making sure the flashing is secured against the wall well below ground level.

looks kinda' ugly though.

 

[Softus]

If you really think about it, it's not that important.

I agree.

The reason given for the 150mm distance is to prevent moisture bouncing up and over the dpc in heavy rain.
But it's going to get wet by the falling rain anyway.

But less wet. In fact, probably less than half as wet.

Come to think of it why install a dpc in a outside wall anyway which will be subject to the driving rain, hail, snow, etc.

For those who believe in rising damp, it's clearly very important, otherwise the water will rise up the wall and emerge like a fountain from the very top. For those who don't believe in RD, it isn't necessary. However, AFAIK it remains a requirement imposed by the BRs.

The important thing is to ensure that if its a cavity wall the cavities are clear.

I utterly concur.

If its a solid wall then just make sure that the external ground level does not exceed the internal floor level.

And again.

 

[anobium]

Softus said

For those who believe in rising damp, it's clearly very important, otherwise the water will rise up the wall and emerge like a fountain from the very top. For those who don't believe in RD, it isn't necessary. However, AFAIK it remains a requirement imposed by the BRs.

Without wishing to be pedantic, I am as you well know Softus, one of those who believe in rising damp.
However in answer my own question, the only practical benefit that I can see for a dpc in an external wall is to prevent the salts being drawn up with the rising damp, and which when they crystalise can result in severe spalling of the bricks.

 

[Softus]

A naive question then anobium: what are those salts and where do they originate?

 

[JohnD]

I'm guessing calcium carbonate or something like that

Groundwater plus limestone.

It's all round my house below the DPC.

by chance I have this pic of a wall where the DPC is 150mm above ground. You can see where the dissolved minerals (this is chalky ground) show in the bricks below the DPC, but above the bricks are clean and dry.

 

[Softus]

In that picture there are many bricks in the course immediately below the DPC that appear not to be showing any signs of "The Salts". How certain are you that the ones that are affected are those that get wet from rainwater bouncing off the concrete blockwork drive/path?

 

[JohnD]

I don't know that it is exclusively caused by rainwater bouncing up, but I have observed the lower parts of the walls to be wet after rain. I'm also prepared to believe that some of it is groundwater being absorbed by the bricks. I know that some people do not believe in this. I'd expect rainwater to be free of dissolved chalk.

Although there are some bricks below the dpc that have little crust on them, I notice:

there are bricks above the DPC and bricks below.

Of the bricks above the DPC, none show the mineral crust.
All the bricks showing crust are below the DPC.
Most of the bricks below the DPC show crust in greater or smaller amounts.
Bricks which are 10m above the DPC and are drenched with rainwater in storm conditions have no crust.
Some bricks which are below the DPC and are inside the garage where they are protected from bouncing rain have some crust.
I have not observed water gushing from the top of the wall or the point where the DPC interrupts its movement.


This leads me to suppose that the crust is derived from groundwater and not from bouncing rain.

However I have also observed wet walls as a result of heavy rain.

the bricks above the DPC being generally drier have more capacity to absorb temprary wetness from rain without becoming saturated.

 

[Softus]

Some bricks which are below the DPC and are inside the garage where they are protected from bouncing rain have some crust.

This is really interesting! How thick is the garage wall?

 

[JohnD]

13" cavity. the illustrated wall with the meter cabinet is the outside wall of the garage.

 

[Softus]

You could have a bridged cavity then?

 

[JohnD]

below the dpc, the cavity might be filled with concrete. Or it might not. It is certainly in contact with the ground.

 

[Softus]

below the dpc, the cavity might be filled with concrete.

That would be a poor method of construction, and isn't the type of bridging I had in mind.

 

[JohnD]

it used to be considered the right thing to do, I believe to guard against crushing of the wall by underground soil pressure or backfilling.

There must be an older builder here who will know.

 

[Softus]

We must have very differently principled LABCs, because more than 20 years ago it was already bad practice in the region where I was building at the time.

 

[anobium]

Softus said

A naive question then anobium: what are those salts and where do they originate?

Chlorides, sulphates, and nitrates, and they come up from the ground with the moisture, ie rising damp.
The levels of the various salts can easily be checked by a lab analysis.
There are varying degrees of each of these in different parts of the country.
Why? I would refer you to a geologist for the answer to that question or maybe somebody on this forum has the answer