Tests at Monash University by plasterboard manufacturers and HIA confirm peaking occurs for a variety of reasons, some of which are beyond the control of the builder or subcontractor. It generally occurs when there are increases in temperature and changes in relative humidity, which cause dimensional changes in materials.
Where timber frame construction is involved, shrinkage occurs as the timber dries. It is possible for "kiln dried" timber to shrink, bow and twist causing the plasterboard in a partition or ceiling to be put under stress, either in compression or in tension, depending on the temperature or humidity conditions. These stresses are relieved when the plasterboard bends away from the joint. Once this bending takes place, the system takes a set and very rarely returns to normal. It can become progressively worse with each change of temperature or humidity. Thus deformation appears as a continuous ridge along the length of the joint, with a uniform fine, peak like shape at the centre.
Peaking is a universal problem, which can only be limited by ensuring correct moisture content of substrates, good ventilation and strict accordance with manufacturer's fixing instructions.
Peaking may only become apparent after the house has been painted, particularly with gloss or semigloss paints.
For best practice backblocking of joints is recommended to reduce the likelihood of peaking.
Potential Causes of Peaking
Peaking may occur some time after construction is completed. Peaking presents itself as a quite pronounced sharp V, either V or V and should not be confused with visible joints caused by glancing light.
The critical period for compression type peaking is during the spring months and especially when there has been an extremely wet period followed by a sudden rise in temperature. This may cause timber to contract resulting in the plasterboard moving in a downward direction forming a linear V shape.
The critical period for inverted peaking is in the winter months, as low temperatures combined with damp humid conditions may cause timber to expand. The consequence of this is the plasterboard will invert or move in an upward direction forming an inverted V shape. Under extreme conditions cracking may occur.
A 10mm space is essential to allow for settling or shrinkage of the wall plates and studs. This prevents pressure being put on the base of the sheets, which in turn transfers tension to the joints.
Expansion or Control Joints It is essential that adequate expansion or con-trol joints are provided where required.
Adequate ventilation to areas with concrete floor slabs is required to prevent released water va-pour from diffusing to roof structures and caus-ing moisture movement in timber. For example problems may occur if buildings are left closed and unoccupied without ventilation for prolonged periods immediately after completion.
In all cases of peaking, it is essential that the area be left as long as possible, so that it reaches its equilibrium condition, before undertaking remedial work.
- Smooth peak down to reinforcing tape without cutting through the tape. Fill areas either side of the peak with a light coat of compound. Leave to dry, then trowel another thin film of compound over the entire area.
- Examine the area. Additional coating of compound may be required. Peaking may recur, but is usually less severe.
- Gloss or semi-gloss paint will highlight surface irregularities, particularly under the influence of glancing light.
The following steps summarise how to reduce the incidence of peaking.
- Straighten any bowed or twisted framing timber. Pack, plane or replace as required.
- Plasterboard manufacturers recommend the use of height adjustable metal furring systems on roof trusses.
- Allow adequate time between layers of Jointing Compound. These can be either an air drying or setting type material specifically formulated for use on plasterboard. Each coat of air drying joint compound must be completely dry prior to the application of any following coats (approx. 24 hours). Allow "setting" type compounds to fully harden prior to the application of following coats.
- If there are gaps of 3mm or more between sheets, stop these gaps with setting type compound and allow to dry before applying the normal layers of jointing compound. Provide adequate ventilation within the house as the compounds dry, particularly if a concrete floor slab is used.
- Provide a 10mm gap at the base of walls to allow for any shrinkage of framing members.
Drywall Fastner Pops
A nail or screw pop is a protrusion of the fastener or the compound covering the fastener through the surface of the plasterboard.
Fastener popping may occur with any type of material secured to timber with fasteners.
Causes of fastener popping
- Timber shrinkage and/or twisting
- Improper application procedures
It may not be possible to check the actual moisture content, but using timber that has been stored under cover or using kiln-dried timber is best. Manufacturers recommend a timber moisture content not exceeding 16% maximum (range 12% - 16% in NZ) prior to lining. Using premium grade timber rather than poorer grades, which are more susceptible to movement, is also good practice.
Best practice is the use of the fastener and adhesive system.
Whenever the back of the plasterboard is not held tight and secure against the face of the framing member by the head of the fastener, a potential fastener pop exists. Figure 1 shows plasterboard held tight and secure against the framing member. Figure 2 shows a fastener pop as a result of moisture movement in timber. Figure 3 shows the board not held tight and secure against the framing.
Fixing through Adhesive
If fasteners are applied through adhesive a pop can occur as the adhesive dries and pulls the board closer to the framing member.
Ensure fasteners are fixed a minimum of 200 mm from adhesive.
Repairing Fastner Pops
When to Repair?
Fastener pops that occur before or during decoration can be repaired immediately.
Moisture movement in timber, is the likely cause of fastener pops occurring after decoration.
Do not repair until the timber has stabilised, ie. reached an equilibrium.
How to Repair?
An appropriate fastener should be fixed about 50 mm from the popped fastener applying sufficient pressure against the plasterboard to assure firm contact with the framing. Remove loose compound, apply new compound as required.
Will Pops Recur?
If timber has endured a full heating season, the chances are that the timber has reached its equilibrium moisture content and has stabilised. Seasonal fluctuations in moisture content are unlikely to cause future fastener pops.
Another problem which can affect plasterboard is the use of radiant heating elements on the upper surface of the ceiling panel.
At best this is a highly inefficient and costly heating method.
At worst the heat necessary to provide a functional heating effect can damage the plaster core and under these circumstances the plasterboard manufacturer's warranty would be at risk. In NZ there is no warranty for the plasterboard where radiant heating ceiling systems are installed.
The best solution is to use an alternative heating system
Lighting design is very much a matter of cause and effect. By understanding the role that fittings play and the consequences of their position it is possible to minimise adverse effects on quite normal standards of workmanship.
When specifiers, builders and home owners are considering the type of finish they require from plasterboard, it is important to understand how the overall appearance is likely to be affected by glancing light and the choice of decoration.
Although a flat finish is the common requirement this is not always possible with present building technology. No matter how flat a surface may appear, there are always undulations and deviations from flatness. What is possible is the appearance of flatness, though this is dependent on the level of finish and the way the lining is perceived.
LIGHTING VARIATIONS AND CONDITIONS
Where possible ceiling and wall joints should run in the direction of the light source e.g. at right angles to windows or large openings. Avoid butt joints wherever possible.
Finished joints are subjected to a variety of lighting conditions. Light intensities are constantly changing throughout the day, depending on the position of the sun, sky conditions, window, door or the location of light source, reflections from surrounding buildings, ground etc, subjecting the ceilings to diffused light from all directions.
At night, the intensity varies again.
Glancing light is the light that shines obliquely across the surface of the wall or ceiling. The worst instances of glancing light occur with single unshaded light bulbs fixed directly to the ceilings or walls or in rooms with windows up to ceiling level or windows adjacent to walls.
Because of the angle of the light, illumination of one side of the most minute undulation creates a shadow on the other side. This highlights perceived imperfections or the different light reflection characteristics of surface texture variations (joints, patches) which under more diffused light would not be visible.
NATURAL GLANCING LIGHT
The best way to present circumstances that accentuate glancing light effect is by illustrating common conditions. Windows that extend up to (or close to) the corner of the room, allow natural light to fall upon the side wall at a very shallow angle. The angle of light will, vary during the day.
Accordingly, the plasterboard joint will be more apparent at certain times.
A similar situation to that described above occurs with windows that extend to the ceiling surface. Obviously, dawn or dusk will produce the most pronounced 'glancing light' but reflection from an outside surface can extend the duration of the effect considerably.
Another situation is a raked ceiling coupled with high level windows facing critical light or louvered windows. In this case, the problem is invariably complicated by reflections from adjacent roof surfaces.
Where the physical circumstance producing glancing light effect already exists, the situation can be improved by appropriate soft furnishings, pelmets, curtaining, blinds or redecorating with light matt finishes.
ARTIFICIAL LIGHTING - CEILINGS
The most common way of providing artificial lighting is by a surface mounted ceiling fitting in the centre of the ceiling. This also provides the worst characteristic for producing glancing light effect. The light source is close to the ceiling surface and therefore the angle of incidence onto the ceiling is very shallow. Placing the fitting centrally in a room means that the shallow angle of light occurs in all directions.
Clearly, from the point of view of reducing apparent defects the ideal lighting technique is to use a series of fittings hung as low as possible below the ceiling. The angle of light striking the surface is as great as possible and the multiple light sources cancel out each other's shadows.
ARTIFICIAL LIGHTING - WALLS
The same effect applies to wall fittings. Wall mounted lights may well show up a ceiling in a kindly manner, because they are some distance below it. They will, however, tend to accentuate minor imperfections not normally seen in the walls. Similarly, high output light sources are more severe in their effect because they create deeper shadows. The whiter the light, the stronger the contrast, the greater the perceived imperfection.
Soft low wattage diffused lighting provides the best result.
The most severe case occurs when a surface mounted fluorescent batten fitting is mounted in the centre of a surface. It produces high output white light at a shallow angle of incidence.
The positioning of feature lighting using small spots or floodlights should be well planned. The fittings should allow for a generous angle of incidence to the featured surface if they are to minimise perceived defects.
In general terms, recessed fittings and ceiling surfaces are compatible. However they should be kept well away from walls. This is assuming that they are deeply recessed and do not have a flush mounted diffuser, which transmits light to the ceiling surface.