feedback welcome...
NT
=Render or not= A good first question is do you really need to render? Render has downsides, so if its not needed its as well to leave it off.
Good reasons to render:
- the masonry is an ugly mess
- extensive masonry damage
- construction type requires render, eg earth wall
- to [[Insulation|insulate]] a 4" wall
- lightweight blocks used for exterior wall
Downsides of render include:
- need to renew it all occasionally
- sometimes increases risk of damp problems
- frequently doesn't look as good as unrendered walls
- sometimes can damage the masonry underneath when it comes off later
- cost
=The Law= Rendering can be illegal.
- When rerendering more than 25% of a wall, its now required to bring wall [[insulation]] up to a certain level. This often requires fitting insulation.
- Rendering an unrendered wall in a conservation area, or rerendering a [[Listed Building Basics|listed building]] with a different material mixture can both be illegal.
=Mixes= Cement render is often 6:1 with plasticiser. [[Mortar Mixes|Other mixes]] also get used.
Lime render is usually 3:1, though mixes are sometimes made a bit richer, anything up to 2.5:1. Sometimes people include horsehair in the mix.
The [[sand]] used in either case can be builder's sand, sharp sand or a mixture. A mix seems to be a popular choice.
=Formal and informal= Rendering can be done in a formal or informal style.
Formal uses accurate straight lines, beaded edges, and ends up dead flat & level. Generally this suits modern buildings and formally styled old houses. For this style, # beads are fixed # level strips of wood are laid down, adjusting them to get them all level. 3 feet spacing is good # The strips between these are filled in, using a timber stick or feather edge to get them dead flat # remove timber and fill
For a more informal result, no such procedure to get everything precisely aligned is used, and the result is that lines and surfaces aren't entirely level and flat. Rather than setting up accurately level strips, one simply begins wherever and progresses over the wall at will, not worrying about precision. This style suits less formal old properties much better than the more precise approach.
Each method looks out of place on some buldings.
=Lime render= Lime render suits some types of old building, both stylistically and functionally.
Earth walls and timber frame should use lime, not cement. Lime is more permeable, more tolerant of movement and softer, all of which are important on such walls.
Lime is also often recommended on old soft brick walls, though discussing the use of lime versus cement on such walls often results in disagreement. Spab on this.
Lime is a softer material than cement, and lime render should use round corners to avoid unsightly damage.
=Lath= Render sticks well to some substrates, to some it doesn't. Some substrates give good firm support to the render, some don't. Where stick or support are insufficient, lath is used. Lath provides a material that render can adhere to, and it reinforces the render.
Expanded metal lath (EML) is by far the most common lath in use today. Some historic buildings use nailed wooden laths, often oak, or woven sticks.
=Strength= If house movement occurs, which is no rarity on old buildings, the best outcome is for the render to break. It may be tempting to make the render stronger so it won't, but...
- this doesn't work, a layer of render can't support 100 tons of masonry
- the weakest element of the wall breaks, ie the bricks, causing greater damage than cracked render
So in short the render should be weaker than the wall its applied to.
=Wall preparation= Again opinions differ... there are a few ways, all of which work in practice.
There are 2 issues that prompt preparation: suction and a friable or broken surface. A porous wall can suck a fair bit of [[water]] out of applied render, causing very weak render that fails quickly. A friable surface can result in poor adhesion, as can a broken surface.
==PVA/EVA== A coat of dilute PVA blocks suction and glues the surface together. [[Adhesive|PVA]] is not water resistant, [[Adhesive|EVA]] is.
==Water== Wetting the wall before rendering solves the suction problem. [[Water]] should be applied more than once, a lot gets sucked up. This reservoir of water in the wall also keeps the new render damp for longer, helping ensure it cures properly.
==Lime== A watery solution of lime is used where lime render will be used, and the surface is in poor condition. It soaks in well and works as a slow acting [[glue]], and is more compatible with some old building types than [[Adhesive|PVA]]. Several coats can be needed.
=Shaping= Flares: Where water will drip off where render stops, such as above a window, the render is usally flared out to move the drips further out, reducing wetting of masonry directly below. Metal bead is used to help form the shape (plastic is also available)
Top edge: Where there is bare wall above rendered, the top edge of the render should be sloped to shed [[water]]. Failing to do this can lead to freeze/thaw cycle damage to the render or the brickwork, increased risk of damp and earlier render failure.
Bare bottom: its generally advised to leave the bottom foot or so of walls unrendered. This acts as an evporation zone, reducing risk of damp problems.
=Texture= ==Smooth== A smooth finish stays cleanest longest, but has no visual detail. Its achieved with a float or a wetted paintbrush. The surface is floated to remove roughness and more or less even it out, without being fussy. Then a wet paintbrush is gently run downward to get rid of the float edge ridges, and smooth any slight indentations. This is all done before the render sets, so is inevitably done in patches (except by superman).
==Dry brushed== Tamp with a dry brush once the render has dried to give a mild texture. Using a dry sponge in a circular motion is another option.
==Patterned== Patterns may be made with trowel & comb. Typically the wall is combed, and the edges floated flat. This is another finish losing popularity. Less often a simple diamond shaped scoring of the render is seen.
==Pargetting== Sometimes 3d artwork is incorporated into the plaster. Small plastering tools may be used. Obviously this takes a fair amount of time and skill to do well.
==Hurled== Render can be applied by throwing it on, and leaving it untrowelled. This leaves a very rough craggy surface. Such a surface does attract dirt, and takes more work to clean or paint.
==Pebbledash== Pebbledash is a pebbled mortar finish. Its popularity has declined.
There are 2 ways to apply pebbledash: # apply mortar, trowel it more or less flat, throw pebbles on # mix pebbles in with the mortar, throw the mix on, and gently water the surface when it has set to remove the thin film of cement from the face of the pebbles.
==Imitation stone== In some areas its popular to sculpt render to look like ashlar blocks. Grooves are made in the render to imitate mortar joints.
==Scratchcoat== Any non-final coat should be scratched after application to enable a good key to the next coat. Scratching tools include a scoring comb, a block of wood with some [[nail]]s, or for small areas even a stick will do.
=Colour= [[Paint]]ed render may look nicer, but it needs repainting at times.
Lime render is near white, or light pink if made with red [[sand]]. Lime also sheds slightly (microscopically) over the years, making it to a very limited extent self cleaning.
Incoporating just a small amount of subsoil into lime render removes some of its brightness, giving a broken white colour. This isn't often done.
For render that won't be painted, coloured aggregate is occasionally added for decorative effect. Gently brushing the render the next day exposes more of this. Running [[water]] gently down the surface once its set exposes rather more.
=Quantity= An inch (2.5cm) of render uses about 50kg [[sand]] and 7.5kg cement per square metre.
=See Also=
- [[Special:Allpages|Wiki Contents]]
- [[Special:Categories|Wiki Subject Categories]]
[[Category:mortar]] [[Category:Building]] [[Category:Construction]]