How to Apply Roughcast?

Discussion in 'Builders' Talk' started by Self-build Pete, Jun 8, 2008.

  1. Self-build Pete

    Self-build Pete New Member

    I have to roughcast an extension built using block. I think I'm okay with applying a scratch-coat, but have heard that the roughcast topcoat itself can be applied either dry or wet. (It's going to be painted with white masonry paint so colour isn't an issue.)

    What's the easient way for a relative novice to get an accepatable roughcast finish? I have a few tonnes of crushed stone that I'm considering using as the aggregate as the larger particles match the size of existing roughcast on the older parts of the building.

    Would appreciate any advice.
  2. -AbFab-Vincent

    -AbFab-Vincent New Member

    I'd recommend actually applying white roughcast rather than painting it white, it will last for many, many years as opposed to painting which will need to be re-done every few years and painting roughcast is a pain.

    Whatever you decide application is the same. The method I use is as follows. Apply a thin coat of render (cement/sand/lime) to a few square meters at a time, as much you think you can apply the roughcast to without it going dry. Then apply the roughcast or splatterdash as it's sometimes known directly onto this base coat. The mix for the top coat (roughcast) is 4 chippings/stone/ 2 sand, 1 cement and half of lime. It's needs to be well mixed and relatively wet. Too wet and it will not hold its shape on the wall. It's absolutely crucial to get the mix just right. You'll need an open ended dashing shovel too.
    Best bit of advice I can give is to make sure you do no apply the roughcast in exactly the same pattern, i.e. vary your 'throw' so that you don't get waves. Also, you'll know if you've put the roughcast on properly as it makes a nice 'smack' as it hits the wall, it needs to be applied quite firmly.
    Personally I would not consider it a job for a 'beginner' so make sure you start on a wall that isn't too obvious just in case it takes a little while for you to get the hang of it.
    Many 'so-called' professionals still can't do roughcast properly.

    Hope that's of some help and good luck.
  3. Self-build Pete

    Self-build Pete New Member

    Thanks for the advice. I'll practice where it won't show first and be prepared to scrape it off and start again if I make a total mess!

    I have heard that you can get devices for throwing tyrollean finish, pebble-dash etc. at walls by turning a handle. Would one of these work with roughcast made as you describe or are they only suitable for dry material?

  4. -AbFab-Vincent

    -AbFab-Vincent New Member

    I have only applied Tyrolean finish with one of those machines and I've never heard of any other machines for applying Roughcast. Of course that doesn't mean there isn't, I don't claim to be the most up to date builder. I would be amazed if a Tyrolean machine would work for roughcast, the mix is completely different, I'd guess all the pebbles in the roughcast would sink to the bottom and you'd have a right mess. (imho)

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