Peaking occurs basically for different reasons which in some instances are beyond the control of the builder or subcontractor. They are generally manifest 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. Even "kiln dried" timber can shrink, warp, bow and twist causing the gypsum panels in a partition or ceiling to be put under stress, either by compression or tension, depending on the temperature or humidity conditions. These stresses are relieved when the panels bend outwards in the region of the joint.

Once this bending takes place, the system takes a set and very rarely returns to normal. It can progressively become 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.

Due to timber shrinking most in the direction of the growth rings, less across the growth rings and very little along the grain longitudinally, it can show irregular peaking in ceilings and walls and at the same time produce nail pops.

Potential Causes of Peaking

The worst time for peaking is during the spring months and especially when there has been an extremely wet period followed by a sudden rise in temperature.

Peaking is a universal problem, which can only be limited by ensuring correct moisture content of substrates, good ventilation and strict accordance with manufacturers fixing instructions.

Peaks can also show up once the house has been painted, particularly with gloss or semigloss paints. Often these are not actually peaks but rather are slight humps over the joint produced by the joint stopping. It is the illusionary effect of light or shadow that cause the hump to look like a fine peak over the joint. Strong sidelight in particular tends to highlight this phenomenon.

Other main causes are:

• Water trapped in the Bedding Compound behind the tape. Bedding Compound is plaster and sets while still being wet, in the same way as concrete. The water endeavours to escape by diffusing through to the atmosphere. When a second coat of Bedding Compound is applied too early, it prevents this happening, causing extended drying times which can, in turn, cause peaking.

• A similar situation arises if extensive gaps (i.e. over 3mm) are left between the sheets. Gaps between sheets should be filled first with Bedding Compound and allowed to completely dry before application of tape and subsequent coats of Bedding Compound.

• Creep in timber due to the timber being wet and then subject to excessive temperatures (i.e. in plenum and roof spaces).

• No gaps being left at the base of the wall sheets. A 10mm gap is essential to allow for 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.

• Where there is an expansion joint required.

• With concrete floor slabs it is essential to have good ventilation to prevent released water vapour from diffusing to roof structures and causing timber movement.

• In all cases of peaking, it is essential that the house be left as long as possible (i.e. reach its ambient condition) before undertaking remedial work, usually six to nine months, or through the cycle of the four seasons.

• Unless the peak is actually a hump incurred during the stopping, let the peak develop fully before undertaking repairs - usually six months is sufficient. Make repairs under hot and dry conditions.

• Smooth peak down to reinforcing tape without cutting through the tape. Fill the concave areas on either side of the peak with a light fill of compound. After this is dry, float another very thin film of compound over the entire area.

• Examine the area with strong slidelighting to make certain that peak has been concealed. If not, use additional feathering coats of compound. Redecorate. Peaking can recur, but is usually less severe. Continuous wetting will aggravate the condition.

• Remember that gloss or semi-gloss paint will highlight surface irregularities, particularly under the influence of glancing light.