Plaster and render
How to replaster old, damp walls
Property Repair Systems
|Replaster old walls. Render mix for damp walls|
How to plaster old damp walls - Order and Free Technical Help - 01626 331351 - Kathy, David or Angela answers your call - discuss your building problem with a Qualified Dampness Expert (C.R.D.S.)
Quick DIY Master Class in Dampness and Condensation - become your own Expert in 1 hour! Buy our "Dampness in Buildings" reprinted book by the great expert Graham Coleman. A great read, short and to the point.
A 'damp meter' and surface thermometer (see www.dampness-info.co.uk/meters.htm)are useful tools for this diagnosis - if not available use the back of your hand to assess the relative dampness of one affected area to another unaffected area..
Indoor Walls - ground floor, a quick summary checklist:
Symptom - white, fluffy salts, plaster possibly 'blowing' off the wall - 'efflorescence' - you have a water source which needs to be found and eliminated. Use our Water Leak Checklist (click here).
Answer - Salt Neutraliser might work if you can find and stop the leak, or the plaster may be too damaged and need replacement to our Specification.
Symptom - damp patches (at low level,l right down to the skirting board) that come and go - you have 'hygroscopic' salts that are attracting moisture from the air.
Answer - you may have Rising Dampness, or your damp proof course may be 'bridged' by high outside ground, plaster on the inside or debris in the cavity (cavity walls only). Rectify any defects and if necessary inject Quick Cream at the correct level to form a new Damp Proof Course. The plaster will need replacement to our Specification.
Symptom - damp patches on or around chimney breasts - sometimes showing yellow or brown stains in wallpaper or paint - you have 'hygroscopic' chimney salts that are attracting moisture from the air, which may include tars and other chemicals from burning wood or coal.
Answer - these 'salts' are very difficult to hold back in the wall, even with the replastering to our Specification carried out to perfection. We recommend using Mesh Membrane - a type of dry lining - to allow you to replaster using normal, lightweight plaster, with no risk of further 'salting'.
Products mentioned above:
Salt Neutraliser - a clear liquid used to neutralise 'salts' in render, plaster or masonry, either to attempt to save otherwise sound plaster or as a preparation prior to re-plastering to our Specification. Apply two coats, wetting in between with fresh water.
Mesh Membrane - easy to install dimpled plastic 'air gap' dry lining membrane, supplied in rolls like wallpaper. Designed to cover damp or salty walls and provide a fresh surface for replastering with a modern lightweight plaster (usually 'bonding').
Quick Cream - new, easy to install, cream damp proofing product - no pump required, no mess, no smell - includes injection nozzle extension. Designed to gently diffuse into the wall to form a water repellent layer - a 'damp proof course' (DPC). It is approved by the British Board of Agrèment - BBA Test Certificate No: 02/3961 - as used by the professionals. The BBA is a Government Approved organisation which has been testing building materials for over 30 years. Note - not all damp proofing materials have passed these tests - beware of cheap imitations).
380cc tube with extension nozzle - makes a maximum of 3.5 metres/11 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Can be applied with a DIY, standard 380cc version mastic/sealant gun or a professional 380cc skeleton gun (see our Order Form)
1000cc tube with extension nozzle - makes a maximum of 9 metres/30 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Requires a professional 1000cc skeleton gun (see our Order Form)
3 litre bulk pack, for professionals - makes a maximum of 27 metres/90 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Requires a pump up dispenser.
* Most Frequently purchased Items:
- Replastering Additive (Renderproof) - part of the Replastering Specification
- 'Tanking' waterproofing liquid (BondAcryl) - for walls and joints below the DPC
- Salt Neutraliser - to kill 'efflorescing' (fluffy, white) salts
- Anti Mould paint - to prevent Mould, Guaranteed
- Certificate of Product Guarantee
Installing a Quick Cream DPC - the modern Professional method used throughout the UK.
As used by the Trade - Quick Cream is inserted into 12mm holes (380cc mastic tube type illustrated - uses standard skeleton gun)
1. Free injection nozzle extension with every Order
(charged as additional cost from other suppliers)
2. Comes in a standard 'mastic' tube, ready to use
(no need to transfer the cream to a pump)
3. No special tools required
(special pumps needed with other makes)
4. Quick to install - just drill & gently inject
(no pressure required)
5. No electric pumps to hire or messy, smelly fluids to spill
(does not flow, or spread, or smell)
6. Effective - fully BBA tested and approved
(your guarantee of a quality product)
Replastering old walls - indoors specification, the Professional standard in the UK.
1. Hack off all old plaster to at least 300mm clear of all signs of damp or salt and at least 1 metre above ground floor level - use a meter to test the walls and remove plaster at least 300mm above the highest level that readings can be obtained.
Reason - This is to ensure that the residual 'salts' do not climb above the new plaster in the future.
2. Brush the walls to remove all plaster residue, particularly around angle beads.
Reason - old plaster will have 'salt' in it and will cause damp spots to appear in time.
3. Repair any holes or poor joints with sand and cement (4:1) using washed, sharp (means slightly gritty) plastering sand (sometimes called screeding or rendering sand). Do not use fine, unwashed sands.
Reason - fine, unwashed sands often contain salt and the fine particles are too numerous to find enough cement particles to bond together tightly - these two problems make for a weak render, prone to the easy passage of 'salts'.
4. Special Note - walls made with non-porous stone or brick, like granite or blue brick, will require an adhesive to help stick the render coat to the wall - use BondAcryl concentrate)
Reason - normal renders and plasters stick to the wall by suction - the surfaces need to absorb some water. Without this suction the render or plaster will not grab the wall and may become loose and hollow as it dries.
5. Damp the walls lightly (to reduce 'suction', which can cause excessive drying and cracking) and apply a thin coat (maximum 1/2 inch, 12.5mm) of render consisting of 3 parts sand - dry, washed, sharp (means slightly gritty) plastering sand (sometimes called screeding or rendering sand) with 1 part of fresh (free flowing - no lumps) Portland Cement.
Reason - thick coats are more likely to slump down the wall during application and crack during drying out.
6. Scratch the render surface liberally all over with a nail board, trowel, metal float or similar object.
Reason - without these scratches the natural drying shrinkage will cause cracking, crazing and hollowness to develop - the next coat will probably pull the first coat off as it dries.
7. Use only Renderproof water proofer/plasticiser in the water that the render is mixed with, at the rate of 1 part Renderproof to 40 parts of water. Do not use fine, unwashed or wet sands.
Reasons - Renderproof binds the sand to the cement and prevents liquid water passing through. It also makes the mix stickier (plasticising) which help to hold the render together. Wet sand weighs more than dry, so it will make your mix weak. Fine sands produce a weak, powdery render, which will not resist 'salts'. Do not allow the plasterer to add plaster (usually 'browning') or washing up liquid to the mix.
8. When the render surface is firm enough (but not bone dry, or it will need re-wetting) apply a second coat to exactly the same specification - if further coats will be needed to reach the desired thickness don't forget to scratch liberally. In hot weather spray the render surface with water to slow the drying process.
Reasons - excessive drying out increases the suction and can prevent one coat sticking to another. Rapid drying always increases shrinkage, which gives rise to cracking of the render or finish plaster.
9. Whilst still damp (or re-wet again) apply a skim coat of Universal or Board Finish. Do not polish or add water.
Reason - this will produce a shiny, glazed finish which looks good, but is prone to condensation, black mould growth and poor drying.
10.. Delay any decoration for at least one month and then only apply a thin coat of breathable emulsion paint (not a heavy vinyl). Do not repaint or wallpaper for at least three months.
Reason - the paint or paper will fall off the wall due to the water vapour that will be trapped underneath in the new render.
Blown plaster due to Salts - copyright TNO
Salt Neutraliser in 5 litres and 1 litre
Additive - waterproofer, salt inhibitor, plasticiser 5 litres & 2.5 litres
|How to install a damp proof course|
First, check for high ground levels, leaking gutters and downpipes, water leaks.
A Damp Proof Course must be at least 6 inches (150mm) above abutting ground level
(whether internal or external), to avoid splash up or 'bridging'. |
1. Drill 12mm diameter holes at 120mm intervals in the mortar course (or via the brickwork, angled down to meet the mortar course) selected to be at least 150mm above outside or abutting ground level
2. Fit the extension nozzle to the cartridge tube and load into the skeleton gun
3. Inject cream from the bottom of the hole outwards until the hole is full.
4. Replaster to our specification.
Treat any skirting boards or joinery with Boron Ultra Gel.
Holes can be capped with mortar or fitted with a plastic plug
Replastering to a minimum of 1.2 metres internally, to our specification, using sand and cement and Renderproof integral waterproofer, is required to finish the job and to prevent 'salts' damage to plaster finishes and decorations.
Our Products required: Quick Cream DPC, Skeleton Gun - either a 380cc or professional 1000cc
Optional Products: Renderproof Replastering Additive (5 litre), Wall Plugs, Boron Ultra Gel (2.5 litre), UltraProof exterior wall treatment (5 litre or 25 litre), Anti-Mould Paint (2.5 litre), Professional Skeleton Guns (380cc or 1000cc), Professional Extension Nozzle (for repeated use with 1000cc cartridges)
Tools Required: 12mm masonry bit, electric hammer drill, eye protection
Typical usage rates:
4.5 inch thick (115mm) single leaf wall - 1000cc (1 litre) per 9 metres, inject from either side
9 inch thick wall (230mm) double leaf solid or cavity - 1000cc(1 litre) per 4.5 metres, inject from one side or from both sides.
18 inch thick wall (460mm) solid or random fill - 1000cc (1 litre) per 2 metres, inject from both sides
Replastering Notes: don't forget that replastering is essential if dampness has affected your plaster because of the 'salts' that come with the water: you have two options - either replaster to our specification using sand and cement, with Renderpruf salt inhibitor/waterproofer in the water or look at the Wall Membrane option. Wall Membranes offer a considerable improvement over sand and cement - they produce a warmer surface, can be insulated easily and can be directly plastered with lightweight, modern plasters, like Carlite Bonding.
For easy fix Membrane information go to: www.drywallandfloor.co.uk
How Rising Dampness is Blocked
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