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External renders are often referred to as "stucco" when applied to historic buildings.
The finished stucco can be either self colouring or subject to a masonry coating. Stucco repairs are achieved using reinforced mesh cages overlaid with a specialist natural mortar. Skilled masons are required to line and finish profiles using a horse and template.
Stucco techniques were elaborated by the Italians during the Renaissance and spread through Europe. This marble dust compound could be molded into decorative shapes, polished to a sheen or painted.
The terms 'Render' and 'Stucco' are almost interchangeable; Render generally being UK terminology and Stucco, American. However there are differences, such as :
- Stucco tends to be thicker than render (20mm against 10-12mm).
- Stucco tends to be built up from 3 layers while render tends to be one or two.
- Render tends to be only applied to masonry while techniques allow Stucco to be used on other structures.
- Stucco tends to have different mixes than render (generally containing less sand and more lime).
This page is written around Stucco, however the general principles may be applied to rendering.
Traditionally, Stucco has been applied to both internal and external masonry or timber wall construction as it requires a solid backing to give it strength. This page is based around the external use of stucco. Modern techniques have enabled the use of Stucco on timber frame walls. Although Stucco is at first sight a very simple technique, its incorrect or inappropriate application can result in major problems causing the Stucco to bulge, separate, crack or allow the entry of water causing timber frames to rot out.
When applied to masonry, the Stucco can be applied directly onto the surface. As masonry walls tend to be fairly substantial and are not susceptible to significant movement, the applied Stucco is unlikely to cause problems.
On stout wooden structures, the surface needs to be covered in some form of damp proof membrane (dpm) to prevent moisture passing from the Stucco to the timber. Wooden laths are then fixed over the dpm to provide a key for the stucco. The wooden laths should be horizontally attached to vertical wooden strips so that there is a gap between the laths and the dpm. Providing that the wooden subsurface is stout and the laths are adequately secured, the Stucco should last a long time.
On wooden frame structures, a dpm is required
and the horizontal laths can then be fixed to the frame. An
alternative method is to use some form of metal mesh to provide
the bonding key instead of the wooden laths. A problem with
timber frame buildings is that the strength of the Stucco
is almost entirely dependant upon the Stucco itself, and its
application on these sorts of structures should be left to
specialist contractors.
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