Damp proof rendering

[AndyDavis]

I want to take off all the pebble dashing and render with damp proofing cement, and then paint over. Can anyone tell me what is the best damp proofing additive on the market. Saw a liquid additive in Wickes but not sure how good it is. Heard of some thing call Damp Guard but not sure what it is or where to get some.

Ideally, how thick should the render be so that it lasts and doesn't need looking at regularly.


Andy

 

[masona]

Why not remove all the loose stone with a shovel , seal the wall with Unibond, and rendering on top of that ?

 

[AndyDavis]

Do you mean UniBond Super All Purpose Silicone Sealant, or another UniBond product?

 

[jeds]

Andy, you shouldn't waterproof external render. I promise you it is the worse thing you can do and will just cause problems.

If you waterproof render you will not keep water out - it is impossible to keep water out - whatever you do, the water will get in. So what you will be doing is trapping water in - not keeping it out.

In addition, you will prevent vapour from inside the building escaping. The vapour will condense and build up under the surface causing spalling of the render and sub-masonry.

It depends on the quality and properties of the surface to be rendered but a good average mix would be 1:2:9 cement:lime:sand

If the ground level is a high traffic area you could substitute half lime for half cement for a slightly stronger mix.

 

[AndyDavis]

Yikes, now I really am confused! I was told that putting in some sort of additive in the cement mixture was essential to get a good damp proof surface. Is that really true, or am I just being led up the garden path?

 

[jeds]

It's a commonly held misconception Andy. Buildings must be able to breath and waterproofing only causes problems. This link is the best I can find at the moment. Scroll down to C.13

http://www.odpm.gov.uk/stellent/groups/odpm_planning/documents/page/odpm_plan_606900-12.hcsp

 

[tim00]

never render over a DPC it will bridge it and cause rising damp. it will also invalidate any DPC guarantee. any future mortgage will require a retention for remedial work. it's best to set a bell cast approx two inches above the DPC and render down over it. plastic bell cast is sufficient.
as regards your render mix, use lime and go easy on the cement in the second coat. if you are Tyroleaning come back for further suggestions.

 

[tim00]

just noticed your further Q's. each render coat should be approx. 10mm thick. Never use liquid/chemical damp "solutions" they create further problems. after removing pebble-dash you will have to render to create a respectable surface, if you are going above ground floor height then its no longer a job for an diy'er.