Mortar Mix for Below Damp Course?

paperclap

Hi all,

I've got some pointing to do below our damp course.

Is there a sand to cement ratio to adhere to... or is a typical 4:1 mix the right thing to do?

Thanks!

stuart45

The NHBC recommend a mix of 3/1 below DPC. 
Or 3.5/1 with plasticizer.
Or 4.5/0.5/1 sand/lime/cement 
However that is for new builds, and for an older house you could have softer bricks below DPC.
Rule of thumb is that the mortar is weaker than the bricks.
Re-pointing the mix slightly weaker than existing mortar. This allows for movement in the wall not damaging th bricks.

paperclap

stuart45 said:

The NHBC recommend a mix of 3/1 below DPC. 
Or 3.5/1 with plasticizer.
Or 4.5/0.5/1 sand/lime/cement 
However that is for new builds, and for an older house you could have softer bricks below DPC.
Rule of thumb is that the mortar is weaker than the bricks.
Re-pointing the mix slightly weaker than existing mortar. This allows for movement in the wall not damaging th bricks.

So therefore a 4:1 mix might be more suitable?

Don't know how to tell if our bricks are "strong" or not. But, it is a 1955 bungalow.

FreeBear

laurencewhymark said:

stuart45 said:

The NHBC recommend a mix of 3/1 below DPC. 
Or 3.5/1 with plasticizer.
Or 4.5/0.5/1 sand/lime/cement 
However that is for new builds, and for an older house you could have softer bricks below DPC.
Rule of thumb is that the mortar is weaker than the bricks.
Re-pointing the mix slightly weaker than existing mortar. This allows for movement in the wall not damaging th bricks.

So therefore a 4:1 mix might be more suitable?

Don't know how to tell if our bricks are "strong" or not. But, it is a 1955 bungalow.

You can often tell (with a little experience) just by looking at them. Or try scratching one in an obscured spot with a screwdriver. If it is a hard brick, the screwdriver will barely mark it.

And yes, a 4:1 mix will be suitable below the DPC (in my opinion).

stuart45

Make sure you rake out the joints properly, and damp down before you repoint.

paperclap

About that, what is the best way to rake out the joints without a disc cutter? And to what extent should the joints be raked out?

stuart45

Plugging chisel and lump hammer for the harder bits. If the mortar is soft enough a mortar rake will do it. A pointed bit of metal will work. 
A depth of about 20mm is usually fine. Rule of thumb is double the joint size.