Repairing cement stucco render

[michael]

I live in an early 19th century Baptist chapel (1817) which has a late 19th century gable end frontage/ extension (approx 1880). The frontage has a classical appearance of imitation block work, a pediment, arched windows etc formed from a very hard cement render. I know that the render is cement based as I sent a small piece off for analysis by ROJ. This stucco render wraps around the sides of the building by about 6ft before changing back to plain brick elevations i.e. the extended stucco frontage is about 6ft deep forming an entrance lobby with balcony over. Part of the cement render half way up the building along one side had to be cut away to rebuild an adjacent lead roof valley i.e. the lead upstands had to be chased into the brick work wall which meant cutting away a section of the render along one side. With the lead now installed there remains a channel in the render which is about 6ft long by 2 inches high by 11/2 inches deep which needs to be back filled. My question is should I back fill this channel with a sand cement mix to match the existing or should I use a lime mix?

 

[Raylea]

Hi Michael,

My understanding that original stucco would of been lime, sand and water, therefore if yours is cement based maybe not original. Many would say repair with Lime and I'm on that understanding too, however you may get cracks around the repair because of two different materials, thus letting in water. So you will have to keep on top of repairs regularly.

Hope this helps, others my have other opinions.

Raylea

 

[andrew gayton]

that advice is not quite right. a lot of 18th and 19th c stucco (lined out or rusticated render to simulate stonework) was in cement. many types - keenes,parian, roman etc etc - were used before the use of o.p.c. i have only seen lime 'stucco' over a much earlier t/f building.................... go with the results of the analysis for a repair.

 

[BMcD]

Unless you know the sand used and the cement and in which quantities then it is virtually impossible to get an exact match but with abit of experimenting with various mixes and perhaps some toners then a good match can be achieved.

For a natural cement try Vicat quick setting

http://www.vicat.com/

http://www.virginialimeworks.com/limesandmortars/nc.htm

Sold by Wormleys as well as a retarder

http://www.womersleys.co.uk/acatalog/Building_Products_1_1.html

I dare say other outlets and cements are available

 

[BMcD]

Unless you know the sand used and the cement and in which quantities then it is virtually impossible to get an exact match but with abit of experimenting with various mixes and perhaps some toners then a good match can be achieved. For the area you need to do I wouldn't worry overly about using cement but if you want to use natural cement you could try Vicat quick setting

http://www.vicat.com/

http://www.virginialimeworks.com/limesandmortars/nc.htm

Sold by Wormleys as well as a retarder

http://www.womersleys.co.uk/acatalog/Building_Products_1_1.html

I dare say other outlets and cements are available

 

[chuckey]

I found matching the colour is cheap and easy, just a lot of mixing various amounts of dye and sand/cement - just be careful to label what you are doing so you where to go next. The difficult thing is to match the texture/finish. As your render is old I guess the finish will be sandy. Don't ask I don't have a clue how to replicate that (acid etch?). In a large area of render your repair I fear will could look like a scar, but hopefully you can get it down to more like a shadow.
frank

 

[Gervase]

I've not used it myself, but I know that Rose of Jericho does a matching service on Parker-style Roman cement, which could be just the ticket. Details here.

 

[BMcD]

Chuckey wrote:

As your render is old I guess the finish will be sandy. Don't ask I don't have a clue how to replicate that (acid etch?).

Once floated but before the render has dried if you gently rub a slightly damp sponge over the surface you will achieve a uniform sandy texture

 

[michael]

The entire stucco frontage of the chapel is painted off white (a modern water based weather shield paint I’m afraid). This was done long before I came to the property. The repair therefore will have to be painted over to match so I’m not too worried about the colour of the render. What is important to me however is the durability and the avoidance of blowing and cracking.

There is an adjacent building to the chapel, an old Sunday school, (hence the lead roof valley separating the two) which is also entirely covered in an original cement render (again painted off white). This building is about 1880 and I suspect that the fancy stucco frontage on the 1817 chapel was added at about the same time (when the parishioners had a bit more money).

From what I can see the stucco on the Sunday school has been built up with three coats; a render coat , a floating coat and a finish coat. I have had to do some repairs to the finish coat where a few small areas had fine cracks and a hollow sound. The repair entailed breaking away the blown finish coat (about 2 square ft) to reveal a very sound floating coat and refinishing with a 3 to1 mix of sharp fine grain Westerham sand and NHL 2 hydraulic lime and then sponging over to reveal the grain. This repair was done 2 years ago, it matches in very well and there has been no further cracking or blowing.

Unfortunately, I did not pay ROJ the £180 + for the full mortar analysis but they were kind enough to advise me just by looking at the sample that it was a plain and simple sand cement render mix. I am trying to keep things simple and inexpensive and so I am proposing going down one of two paths:
(a) Fill up the hole with a first coat of 3 to 1 sharp concreting sand and NHL2 hydraulic lime followed by a finish coat of 3 to 1 sharp fine grain Westerham sand and NHL 2 hydraulic lime
(b) fill up the hole with a 5.1.1 mix of sand, OPC and hydrated lime.

Does anybody agree or disagree with either of these solutions?

 

[Gervase]

I would go with the two-coat hydraulic lime approach, but probably use at least a 3.5 to approximate the hardness of the original render. Two coats - with time for the first to cure - lessens the risk of shrinkage and subsequent cracking.
When patching, undercut the old stucco if you can to provide a better key for the new, and use a scratch-comb to get a good key for the second coat.