Rendering problem

[jonnybubu]

Hi, i bought a very old farmhouse(120 year old) 3 years ago, and re-rendered around the foundations etc. The brickwork behind the old render was in a poor state and needed alot of work before i could re-render.
I dug down so rendering as far down as the brickwork went.

2 years on, (after a bad, bad winter) a couple of cracks have appeared and i fear there could be more.

Can anyone tell me what i did wrong. I used a mix of cement to sand with a waterproofing. I also used pva to bond, and finished with a few coats of a bitumous paint.
The work involved, first repairing the brickwork, followed by a scratch coat and then top coat.

Untitled

• jonnybubu
• 9 Mar 2010
• 1

Untitled

• jonnybubu
• 9 Mar 2010
• 1

Any advice would be appreciated.

 

[roughcaster]

I think the weather's caused the damage to the render,,, moisture getting in behind it, freezing up,,,, snow building up against it, freezing etc etc. I wouldn't have used PVA outside to bond it either, but i wouldn't see that as being the cause of the cracking. Another thing that "might" be relevant is the bitumin sealer. Although it stops moisture/damp going into the render, it might also stop any moisture that has got in behind it, getting out/drying out,, especially if rain runs down the wall, and gets in behind the render at the junction where the brickwork meets the render,,, that's just a thought, but either way, i'd blame the weather.

 

[joe-90]

It looks too pale to me. if you want waterproof render you need more cement.

 

[roughcaster]

It looks too pale to me. if you want waterproof render you need more cement.

Yep,, Joe has a point there, or did you use white cement for the render mix?

 

[jonnybubu]

I used normal cement. OK, so when i repair it all, ill try more cement. Thanks.

Would adding lime help at all?

 

[mnb]

There's an enormous amount of info. about plinths, renders and DPC's if you search here or on the screwfix site. however:
1. If you use cement in render then regular cement is fine. Never go heavy with cement in masonry or render.
2. you have created a fillet and plinth that is a water trap and a bridge over your DPC (if one exist's). Hence the reason render is bell-stopped 150mm above ground level.
3. you would have done better replacing the damaged/spalled brickwork with engineering brick course(s).But if it's a solid wall then the new brick course must span the wall section.
4. Roughcaster is correct, the weather has caused expansion and will contiue to do so, hard cement mix will blow but plastic lime mix will give a little.
5. What to do now? Come back if you want to know.

 

[jonnybubu]

5. What to do now? Come back if you want to know.

 

[Micilin]

5. What to do now? Come back if you want to know.

In addtion to above -

If you are using this kind of plinth, the top fillet should cut into the brick joint at the dpc level so that the water coming down the wall is thrown off and the dpc is not bridged.

I suspect that the pva bond has not helped and may be where the water gets in behind the plinth, it's not meant to be used outside.

You can have a breathable render, with lime, or use a waterproofer. Don't mix your waterproofer with lime (read the container or spec sheet).

A lime render without waterproofer is good enough to throw water off, and teh lime gives it a bit of flex.

However, if you want a waterproof, sealed job use SBR. This is water proof, flexible and sticks like the proverbial to the blanket.

Slurry coat of 1:1:1 SBRpc:sand brushed on covering areas for seal and key, scratch coat of 4:1 sand cement with SBR or waterproofer as per spec sheet,
Finish coat 5:1 with water proofer or SBR, or a 5:1:1 with lime being the extra one

Either way, make sure it is well prepped and clean.

When you hack off, also look for cracks in brickwork, in case

 

[mnb]

mic. has given you some good suggestions, but they really apply to flat rendering not to an external plinth. No matter what remedial measures you try they wont take because you have a failed background.
For starters, if plinths were a blind bit of use they would have been incorporated into all masonry structures, they are not, and the major fault is, when the plinth is in ground contact, cappilary action will pull moisture up the wall to bridge any existing DPC. They also, of course, catch any moisture washing down the wall surface.
Even if the DPC is not bridged the retained moisture will - esp. in winter - cause spalling and degradation of the masonry face, and eventually cause blowing and cracking as in your case. The "remedy" if you want to bite the bullet is to remove all your work and begin to insert 2no course's of engineering brick in 5no brick-length staggered sections - in other words, inserting a two course brick DPC. This is major work.
Or leave alone and wait and see, but it will not "get better".

 

[Micilin]

As far as I know, historically this kind of plinth was used where were no engineering bricks ie the wall above and below dpc (usually slate)was the same or (similar less expensive stocks etc) It gave extra protection to the base of the wall

It was also easier to paint regularly as the bottom of the wall gets grubby from rain splashing up dirt.

Modern plinths are, as you say, just engineering bricks, or are rendered engineering bricks under a bellcast for purely aesthetic reasons - practically engineering bricks are the ideal. However, in this case we don't have engineering brick so IMHO this is the next best thing.

I personally would not go to the extreme of replacing everything below damp level in this case

A waterproof plinth, sealed up with an sbr slurry will 'tank 'the wall (where it contacts, without the 'laps' that are needed), you can fill in any cracks or damaged bricks and at least stabilize your damaged bricks. Done properly it will be an integral part of the wall, no weakness behind it ie fully bonded (which seems to have caused the problem here)The main water on plinths comes from rain coming down the wall, splashing up in a shower, or the rising damp as mentioned above.

Capillary action may work on the surface of this plinth, but not within it, - the same as the capillary action of water in the mortar of brickwork - except with brickwork the mortar is not usually waterproofed*, and can in theory pull in water through the joint, but this is obviously negligible. Will the water creep up the outside of the plinth any more than it would bricks? I doubt it would make much difference.

The fillet cut in under the next course of bricks will help throw the water off the wall - similar to a lead flashing.

In other words, you are making the outside of the plinth as good as having engineering bricks, water and frost will not damage this surface - but behind it you will still have the old bricks that will pull the water up to dpc level, but no further.

* as an aside, I often wondered why the mortar in bricks below dpc level , or maybe the first two course above it doesn't have waterproofer added -the cost is negligible, much teh same as plas - comments ???

 

[Micilin]

Also in days gone by, the plinth bricks were set back from the finished courses above to allow the render to cut in under them and make a more efficient seal with the chamfer/fillet on top.