External finishes Flashcards
Render introduction
Render is a relatively thin layer of material normally applied to the outside of the building to enhance its weatherproofing or appearance. Render can be applied over various-based materials
Functional requirements of render.
Types of render:
Ordinary Portland cement (OPC)
Cement mixed with sharp sand, sometimes lime or other appropriate additives. Normally applied in a to Coates system a minimum of 20 to 25 mm thick
Types of render:
Resin based systems – acrylic or silicone render
Types of render:
Lime renders
Made by mixing lime putty or bags hydraulic line sand and aggregate. Applied in a two coat system with a total thickness of 20 to 25 mm.
Relatively slow hardening process 2 to 7 days, unpopular for general construction
Binders - The part of the render that holds everything together
Normally cement or lime.
Cement goes through a chemical reaction when mixed with water and results in the relatively rapid setting/hardening.
Line goes through much slower chemical process involving urbanisation and can take a long time to fully strengthen. Both differ in the long term flexibility and ability to allow moisture vapour to pass through
Binders – aggregate.
Graded sand mixed with the binder; grains of Sand & Gravel help prevent the render from changing size cracking due to changing moisture content
The effects of particle size in aggregates in render diagram
Binders – additives
Used to change the characteristics of the render to improve the application or how it performs in use
Vapour permeability of render
The render coat should be highly resistant to water penetration but the vapour permeable allowing moisture to be evaporated out. This allows the structure behind to dry out
Cracking of render.
Cement base renders are prone to cracking and do not have sufficient vapour permeability to allow penetrating moisture to dry out readily
Movement. Expansion/contraction due to thermal or moisture changes.
The render needs to be able to accommodate this movement otherwise it will crack. Movement joints should be included in the structures
Corrosion of inbuilt metalwork
Bond between render and substrate failing
The bond between render coats failing
The bond between the coats within the render system may also fail, this is almost inevitably, due to poor application techniques
Professionals involved in assessing the condition of buildings need to build to identify:
I need a clear understanding of materials including:
Problems that may arise can be a result of:
Environmental factors to consider may include:
Substrate supporting the render finish. Once render is cracked windblown rain can easily enter and cool significant problems. Issues to be aware of include:
Making good of defects to render.
Depending on the scope of the report, we may need to highlight:
Works or investigation required fall into a few categories including:
BRE produce a range of useful documents:
