Internal Plastering

Attached Piers

Detached Piers

If there are detached piers or square columns in a room, the following method can be used. First of all, you will need to decide the thickness of the floating coat on a column at the end. You then need to mark the floor and make a note of the width in between the marks. The floor marks need to made again on the other end column. Make sure that the width is the same and then use a chalk line to mark parallel lines. On each side of each column, draw lines at right angles. Make sure you check margin widths and floating thickness when you mark these lines.

Timber rules should be fixed to the column or pier at the two external angles with nails when making hard angles. Make sure that it is straight with the parallel lines. You can then float in between the two rules. The rule should then be removed and fixed on to the other side of the pier/column and then float in between again.

The rules for the sides can be made by using timber again and nailing them to straight with the right angle lines. Always make sure you check the margins and make sure that they are fixed square.

External Angles

Square arris, pencil round, 15 mm round, bull nose and splay are all external angles that can be formed. Fixed angle beads are made of metal with expanded metal lath wings. Timber angle beads are not really used much any more.

Metal angle beads and trims

Metal Angles beads are used to make straight lines at joints that are not affected by the usual damage. They come in all different lengths between 2.4 metres and 3 metres. If you need a longer bead, you can join two together by placing some galvanised wire in to the holes at the end of the beads that you want to join together. Doing this keeps the beads straight and in line with each other.

There are 2 ways to fix the beads. For the first method, plaster dabs are placed on either side of the external angle. The metal beads have wings on them that are pushed into the plaster dab. You will then need to straighten the bead using a straight edge while the dabs are soft.

The second method is done by applying a thin coat of plaster mix along the length of the wings of the beads. While the plaster is still soft, you need to straighten the beads using a straight edge. To make it firmer, you might need to apply another coat of plaster on top the wings. Galvanised nails need to be used on the wings both sides of the angle. To make sure the angle is strong, plaster dabs are pushed through the mesh in the wings onto the background.

A saw is used to cut the beads to the length you need. You will need a hacksaw to cut through the nose of the bead and the wings can be cut with tin snips.

The nose of the bead should not get any plaster on it, so when you apply the floating coat, it should be below the nose of the bead. If any of the floating coat does go past this, it should be wiped clean. When you apply the skimming coat, it should go slightly over the nose of the angle bead so that it does not get damaged when you use a trowel. You do not want to take off any of the zinc coating on the beads.

Perforated thin cot beads are made for work where the thickness is between 3 mm and 6 mm. They are used for jobs that only need a thin coat applied on to plasterboard and other external angles.

Plaster stop beads come in 4 sizes and are used on openings to make a strong and tidy finish. They are fixed using galvanised nails that go through the holes in the beads into the background. If you are fixing to metal lathing, galvanised wire can be used.

Thin coat stop beads are used if the thickness is 3 mm and 6 mm. They are used for expansion joints because they can be covered by two stop beads. A steel plate is placed under the beads to cover the joint. It should only be fixed to one side of the joint.

Movement beads are made for use on expansion joints but only allows for movement of 10 mm. Movement beads are fixed using nails.

Plasterboard edging beads protects and makes fixings for the edges of the plasterboard. Nails are used to fix the bead in place. They can be used with plasterboard that is 10 mm and 13 mm thick.

External render stop bead, sometimes called bell cast stop beads, are used for openings for windows and doors. External render stops are also used make firm, straight projections at the bottom end of external rendering. Masonry nails are placed through the mesh of the beads into the background to fix it in place.

Screed beads can be used to separate two different types of plastering or as a skirting bead. It is fixed using masonry nails.

Internal Angles

Internal angles can be coved for decoration but they are normally finished square. Using a trowel, square internal angles are made in the skimming coat. A flat brush should be used to get out any skimming mix that isn't perfect. Leaving one side of the angle to set first will make it less likely for you to damage the work. You might need to finish both sides of the angle at the same time. In this case, an internal angle trowel or a twitcher is needed to trowel both sides of the angle together.

A coved trowel can be used to make small coved internal angles.

For big coved internal angles are made by cutting a template to the exact size that you need and wiping out the floated angle with it. A trowel should be used to skim angles like this.

Finishing Coats

Finishing coats are always different because the coat will depend on the surface and the strength needed on the finished surface. The floating coat will normally be stronger than the finishing coat. If not, the skimming coat could start to fall off. Most skimming mixes are retarded hemi-hydrate gypsum plaster. The mix can have perlite or vermiculite added to it. A laying trowel needs to be used to apply the mix evenly onto the surface starting from the left side. Once this has been applied to the entire surface, a weaker mix should be applied. Once this coat begins to set, you need to trowel it up. You do this by going over the wall with a clean trowel and a little bit of water at least twice. It needs to be done until you get a kind of polished surface.

If the floating coat is left to dry, it can start peeling, blistering or become harder to work with.

Keene's Cement

Keene's cement is a type of plaster that is called cement because this type of plaster is very hard and it can be brought to a near perfect surface. This plaster can be used for external angles or parts of walls that have been damaged.

Patching Walls

Patching walls can be done in a few different ways but it is always a good idea to make the area around the patch is okay, the piecing should be scraped clean. Also, the background needs to have a key or have good adhesion and be dust free. To make sure there is not too much suction, a damp cloth can be used on the area.

Holes that are less than 75 mm can be treated by clearing any wallpaper covering the piecing. The patches background can be made damp and then fill the patch with a mixture of lime putty and plaster. As it starts to set, a tightening coat can be applied. Once this has set, a clean trowel and a flat brush that is wet can be used to trowel off. You need to make sure that you enough pressure to get the patch to be level with the rest of the plastered wall. To make sure that no dirt marks show on the surface when it dries, the patch should be washed with clean water.

If you need to patch a large area on a wall, you will need to test how sound the area around is. You can do this by tapping around it with a lath hammer. Loose plaster needs to be removed and this is done with a bolster chisel or lath hammer. If the background does not have a good key then you will need to create one either by applying a bonding agent or raking out mortar joints. A stiff broom should then be used to make sure there is no dust on the wall. To make sure there is not too much suction, the surface is made damp.

If you are using gypsum plaster or a lightweight aggregate mix then one floating coat needs to be applied to the background. It is then straightened by ruling off the plastered edge of the plaster that was already there. The floated part of the needs to be rubbed down and washed so that it is clean. Once this coat has set, you need to apply the skimming coat and make it level with the plastered edge. Over the patch surface, you then need to apply a laying down coat. The next step is trowelling off, you need half of the trowel on the old plaster and half on the new to make it completely level. The final step is to wipe the outside edge of the piecing clean.

Curved Walls

With curved walls, you need to find the centre by splitting any two chords of the curve into two. The line where they meet is the centre. You can do all of this by placing a straight edge that is parallel against two points of the curve. A scaffold square should then be placed in the middle of the straight edge. The edge of the square that is a right angle splits the chord in two and if the line is made longer, it will go through the middle of the curve. If the floor is wooden, you can drill a nail at the centre point. Dots can then be put Dots can then be bedded on the curved wall at the bottom. They should be spaced about 1.8 m. Dots are placed in line with the dots you have already placed at the bottom. Once this is done, dots are then placed in the middle using the top and bottom to make sure they are level.

A 2 metre long template is then cut exactly the same as the curve. This is marked onto timber using a radius rod and then cut. This template can be used to rule in horizontal screeds for each of the dots. You will need to let the screeds harden. Once this has happened, you can float the wall and rule in. You will need to use a floating rule to do the ruling in making sure the rule is horizontally so that you do not cause any damage.

Convex Curved Walls

If you are working on a convex curved wall, you will need to work out the radius of the curve. If there are drawings for the site then this should be on there. If there are not any drawings, you can find the radius by putting the middle of straight edge rule on the wall and measuring the same distance both ends of the wall, at right angles. The length of the rule is how wide the curve is. The measurement from the end of the rule to the wall is the rise of the arc.

The curve can be drawn accurately, using the radius measurements, onto the the floor. This can be done with a radius rod. You need to draw a line from the middle to the middle of the arc. The middle of the straight edge is then used to make a straight line that touches the curve by putting it at right angles on the radius you have drawn. Nails are used to fix rods to the rule at right angles. The end of the rods must be touching the curve.

Floating Ceilings

When floating ceilings, you can use the method below as long the walls have been floated level and the ceiling has already been rendered.

About 225 mm away from the ceiling, you should use a datum line the whole way around the room. If there are beams in the room then it is okay to make the distance between the line and ceiling more. You should find a point on the ceiling that you think is the lowest point, it has to be 150 mm away from the wall and use a piece of lath at this point as a dot. Tap the lath so that it is fixed in place using a scaffold square. Once the dot is fixed in place, you mark the where the datum line is on the square and you can bed more dots on the ceiling making sure they are level with the line you marked on the square. Doing it this way makes sure that the dots are level.

Screeds can be made in between the dots and the ceiling can be floated and ruled in to make it level.

If there are beams on the ceiling, there is another way to float it level, make it square and linable.
A chalk line can be used on one of side of the beam on the floated ceiling to make a line that is 225 mm away from the beam but parallel. You then need to measure the depth of the beam and cut lath 9mm longer than the measurement you have just taken from the beam.

You should start at one of the beam, bedding the lath tightly into the ceiling angle. The level ceiling needs to be squared off. This is done using a scaffold square. When one part of the square touches the ceiling, the other part of the square must be touching the lath. If this is the case, then you can mark where the datum line crosses on the square.

Patching Ceilings

There are a few different ways to patch wood lath ceilings with plaster. You can cut a piece of plasterboard the same size and shape as the patch and fix it to the ceiling joists using nails. To make sure the plasterboard is level with the original ceiling, you will need to skim the plasterboard surface. You can scrim the ends of the board if it is thick enough to reduce the chance of the joints cracking.

If you can not nail plasterboard to the wood laths, then you can get rid of them on the middle line of the outside joists that you can see in the patch. Dong it this way means that you will have to make good to make the patch the same thickness as the original ceiling before you can skim it.

Sometimes you can not use plasterboard at all. If this is the case, you can patch the ceiling by testing the area around the patch to make sure it is soundness.Once you have done this, you should make sure that there is no loose plaster or plaster that has no key. If there is then remove it by hand. If there are any broken laths or any that missing then you should replace them before moving on to the next step. A broom needs t be used to get rid of any dust on and around the area and the piecing should be made damp. For the plastering, lime/sand/gypsum plaster or vermiculite/gypsum plaster mixes are suitable to use. When you are rendering, you should start by applying it around the edges first. Then you should apply across the laths diagonally. You must make sure that enough material goes through the lath joints because this will make a key above the laths. The area then needs to be floated, making sure that you float it the thickness needed. This can be done with the same material that you used for the rendering coat. You might be able to do this with one coat, but you may need than creating a key on the first and letting it set before applying another coat. The skimming coat should be a lime putty and plaster mix if it is being applied to lime/sand/gypsum plaster undercoats. If the undercoat is vermiculite/gypsum plaster, the skimming coat should be a lightweight aggregate/gypsum finish plaster.

You can patch a plasterboard ceiling by taken away the broken/damaged board to the middle line of the outside joists that you can see in the patch. You can then cut new plasterboard that fit in the patch and fix it with nails. Board finish plaster is then used to skim the new board and it should then be made good.

Another way of patching plasterboard ceilings is to cut a piece of plasterboard that you are using to replace the old one about 100 mm wide and longer than the patch you are repairing making sure that it is narrow enough to fit in the patch. You will then need to clean above the patch to get rid of any dust etc. You then need to drill two holes into the new plasterboard and poke a piece of string through them. A plaster mix should then be applied around the edge of the patch. The new plasterboard will then go through the gap and be pulled into position using the string, making sure the board is pulled tight into the coat of plaster mix. Once this coat has set, you can cut another piece of plasterboard to fill in the gaps. This should be pushed tightly against the new plasterboard. Just make sure that you leave the normal skimming thickness to make good.

Barrel Ceilings

Barrel ceilings that have a small span and rise will need screeds to be made. This can be done using a gig stick with a square end, turning from a pivot point. For the pivot point, a nail that has no head put into a timber plug on the side walls half way. Screeds made of timber can be turned on a timber stretcher that is squeezed in between the walls. This is done at straight edge length along the barrel. You can float the bays by filling in. This can only be done once the screeds have set. Then the you need to use a floating rule to rule off the curved screeds. Skimming is done at right angles to the side walls.

Dots are needed for bigger barrel ceilings. The dots are placed in position for the screeds and ruling in from the dots is done using a template that is curved.There is another method that can used and it is called the pressed screed method.

For the pressed screed method, some of the floating material is applied in between the dots. The curved template you have made will be pushed to the two dots to get the curve that you need and then the extra material at the sides can be taken off.

Once the screeds on the ceiling have set completely, the bays should be filled in and ruled off.

If you can find the centre of both the end walls, using a line or steel tape the dots can be fixed in the right place.

If the centre point can not be found, a drawing to a specific scale of the barrel ceiling can be made. It will then be divided up into sections on the drawing. The measurements you get from the drawing can then be worked out to the full size, measuring the height of each section from a springing line on the end walls to find the position for the dots.

Lunettes in Situ

Openings in domed/barrel ceilings are called lunettes. They are usually window head openings that are curved to let light in the room.

Circular Domes in Situ