why has my render cracked?

Discussion in 'Builders' Talk' started by Purple Steve, Aug 30, 2006.

  1. Purple Steve

    Purple Steve New Member

    Hello,

    I rendered a wall at the back of my house, after reading about rendering on this forum, and the render has cracked. I have plastered a few rooms and ceilings, and rendered about 5 other walls in the garden using the same technique.

    Here is the wall prior to rendering:

    http://i2.photobucket.com/albums/y21/pimpdriver/IMG2861.jpg

    I hacked off as much as I could, but the remaining render was really well stuck, and as the brick wall is single skin, and I think the wall would have come down if I attacked it any more.

    Onto the painted render I painted a mix of SBR, holm (washed sea sand - what people use in Bristol) sand and cement, and let it dry. I then did the scratch coat at 3 holm sand to 1 cement and added "rendamix" at the correct proportion, after giving the wall a very very very good soaking with the hosepipe. The weather wasn't too hot either, it was a cool evening and the wall wasn't hot.

    The scratch coat is well bonded to the wall.

    The "floating coat" I did 4:1 sand/cement with rendamix in the correct proportions, again after giving the wall a very good soaking. The weather wasn't hot, it was the evening again, and the wall was cool.

    The floating coat has cracked, and "blown" off the wall, see photos.
    http://i2.photobucket.com/albums/y21/pimpdriver/IMG3008.jpg
    http://i2.photobucket.com/albums/y21/pimpdriver/IMG3006.jpg
    http://i2.photobucket.com/albums/y21/pimpdriver/IMG3007.jpg

    Is it because the floating coat is too thick, at about 10mm?

    What should I do now, hack off the blown render, and fill in the holes? Use lime in the mix??? Hack off the whole of the floating coat and start again?

    HELP!

    Thanks,

    Steve
     
  2. Astramax

    Astramax Super Member

    Looks amazingly thick, maybe too thick. Could be that your problem is trapped moisture. Only a guess though!
     
  3. Purple Steve

    Purple Steve New Member

    the photo is deceiving, the top coat is about 10mm thick max.
    Steve
     
  4. -chippy_john

    -chippy_john New Member

    I'm no expert with wet stuff but 3:1 sounds far too strong, 6:1:1, sand/cement/lime would be more normal for rendering.
     
  5. sinewave

    sinewave Screwfix Select

    Them lectrix in the first picky look a bit dodgy! :O


    I hope you've notified Building Control! ;)
     
  6. gardm1nt

    gardm1nt New Member

    My thoughts,

    The mix is too strong, The coat probably a bit thick and possibly you didn't damp down sufficiently before starting
     
  7. Purple Steve

    Purple Steve New Member

    what's the electrics got to do with my rendering?

    That cable has been there for about 3 years and is in "old" colour wiring so is Part P exempt. And its not connected to anything, its a decorative feature.

    That's what I like about the Screwfix forums, the helpful attitude of people. Ask one question, and get either:
    a. the p*ss taken for something else
    b. threads hijacked by "regulars" (i.e. 5000 posts, what else do you do with your life???) with no other lives having a go at each other (which I'm sure this thread will become once the "Part P boys" come along).

    The wall was absolutely sodden when I rendered it, I soaked it for 10-20 minutes.

    Will probably hack off the top coat and re render it in 2 coats, with lime added.

    The guy at college who taught me to render said 4:1 then 5:1 with a shovel of lime, but I did the scratch coat at 3:1 because I wanted it to "take" to the previously painted/tyroleaned wall.

    Where's "Super Robbo" to the rescue when you need him?

    Thanks for the (sensible) replies chaps,

    Steve
     
  8. mudhut

    mudhut New Member

    robbo is the man for sure, but, in his absence here i go.

    many posters here feel that using a waterproofer (ie rendamix) in the first coat is a no no as it drastically reduces base coat suction. if your problem is the top coat key, i suspect this is your error.

    give it a year, hack away at dodgy areas (will probably be well stuck mostly) and patch up carefully with cement scud key, but thats just my amateur opinion.

    lovely wiring btw.
     
  9. nearnwales

    nearnwales Member

    How much feb did you put in because it looks very spongy, 3-1 is to strong, 5-1 with waterproofer first coat and 6-1-1 the second coat with lime and waterproofer
     
  10. Just back from my Jollies, so been busy getting nagged, seperating warring kids, and spending a small fortune on tack, over priced eateries and long stay car parks et al !!!!! cant wait till next year.
    Hi Steve, from what I see in your photos I think perhaps the problem could be the lack of key you put into the scratch coat. looking at picture 3 of 4 the scratch is almost non existant in the patch that has blown and in 4 the scratch to the right is a little light and hit and miss. The scratch needs to be deep and uniformed. You could have caused the situation whereby the floating coat will not adhere to the backing coat as it may be too wet, with little or no key....did it slide at any time during application (patches of mortar slide away from the wall and need reapplying) This would be certain if the renermix you used was a waterproofer (scratch) too and you proceeded to saturate it prior to floating. The mix was strong but not the main reason for failure, though it will not have helped. The problems seem to be with the finish coat only and it seems down to poor preperation from what I can see, poor scratch? over soaked walls? waterproofer in the scratch?.
    Lime would help alot in a floating coat but not necessary for a scratch. When you use the differing plastercisers I find by using recommended measures it can make a mix far too soapy and tacky (hard to spread and float) paricularly if the gauge is left to run and overmix in the drum. Cut back on the admix a little, mix it up stiffish then allow the drum turn for five minutes and let the admix do its stuff, the mix will reach a nice wet consistancy after a few minutes.....giving a lighter easier to work gauge of mortar.
     
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Not that I'm an expert, but I agree with most of that.

    And the thing to remember, your new scratch coat will soak up a lot more water than your old brickwork, so a dampening down may be all that's required, not a very good soaking.

    I think that was your main problem. Your top coat was drying top first, and much quicker than underneath.


    Mr. HandyAndy - really
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice