Render mix & thicknesses

[woodburyboy]

I'm constructing a workshop from dense block and now I have the timber roof on, I want to render the exterior blockwork. Can anyone please advise me of the correct mix ratio's for the 50/50 sand, cement and lime? Also, If I apply a scratch coat and final coat what are their approximate thicknesses? Finally as a novice at rendering I was anticipating using batens and a straight edge in bays to gain a flat final surface surface. Do you just guess the thickness of the scratch coat and use the batens on the final coat?

 

[oilman]

Cement gives a lot of long term problems. Look at a sand/lime mix of around 2:1 to 3:1. Do some research as you'll need to add some fibre to the mix to support the base coat. These ratios are for volumes, and though it sounds extravagant, the cost will probably be a bit less than using cement. Lime can flex and so is less prone to cracking than cement renders, and if you use limewash for the final covering, the maintenance will be very cheap and quick. On no account use masonry paints. There's no need to cover the blocks with PVA .

 

[jeds]

I would agree with a lime based mix for any areas where there is no pedestrian traffic - i.e. the rear and any sides that are not against a path.

Where people might brush against, this will be vulnerable to damage for about 30 days or so. (because it takes a long time to set) In this case you should consider equal quantities of cement to lime in a 1:3 mix.

For example: one cement - one lime - six sand = 2:6 (or 1:3)

 

[woodburyboy]

Thanks for the advice guys.

I've founf a mix reference that I've gone ahead and used. The scratch coat went on yesterday.

1st coat = cement ~ 50/50 ~ lime in the ratio's of 2:7:1

2nd coat = 2:9:2

I'm just about to apply the 2nd coat 15hrs on and have just sprayed the scratch coat with a mist of water, leaving it to soak in so that it doesn't suck the final coat dry. Also I'm using a waterproofer/placticiser to give a bit of protection to the overall coat. As this is the first time I've rendered I'm using 12mm timber batens at 1m centres. I pluged and screwed these to the wall for easy removal (this is my 1st time !!)

I've bought one of those yellow rigid polurethene foam darby's (1200 long) to level off the final coat (£13.56 inc from Tool Station !!)

 

[TexMex]

woodburyboy
Could you clarify which order you are quoting the ratios.
Also, double check the instructions on the Waterproofer/Plasticiser to ensure that it is suitable for use with Lime.

 

[woodburyboy]

Sure I'm using the mix at the bottom of the table on the following link:

http://www.diydoctor.org.uk/projects/mortarmixes.htm

 

[TexMex]

Right o,
Do double check that admix though. Normal plasticiser should not be necessary with lime and interferes with the setting process. Also lime render should be allowed to breath so a waterproof additive doesn't seem qutie appropriate. Check on the container for any "Limitations"

 

[woodburyboy]

No limitations on the label. It's the B&Q product in the red 1 gal container. I'm off for a final coat on the east wall !!

 

[oilman]

......It's the B&Q........

Oh sh1t! Seems like what you've done is 2 cement, 7 sand, 1 lime.

Pity you hadn't done some research first. This is nothing more than a 3.5:1 mix with some lime as a plasticiser. It'll be nice and hard so the slightest anything will crack it. Oh well

If you think I'm being a bit hard, there's a wall just up the road from me done by an expensive builder for himself, about 8 years ago, and the amount of repairs needed is going to cost the present owner a lot.

 

[johnegrey]

Hi woodburyboy,

I've just pulled this from the archive as I'm about to emark on a similar job (also for the 1st time) I wondered how it went with tha battens and foam derby etc. Any tips for a fellow novice renderer? Cheers, j