Shrinkage Cracking and Surface Defects in Exterior Plaster

Shrinkage Cracking & Surface Defects In Exterior Plaster “On 2 Storey Buildings”

Often in winter conditions floor joists have a moisture content in excess of 28%. When dry and at the operating conditions this moisture content will be below 14%. The shrinkage resulting from this drying out can cause walls to shrink in excess of 30mm on double storey buildings. If the plaster has nowhere to go it will fold or crush. This information bulletin provides alternative acceptable procedures designed to minimise instances of shrinkage cracking and surface defects that can develop as the building dries out to its operating state.

Shrinkage where the house becomes smaller and twisting of the timber as it dries due to the alignment of the grain of the timber. Not all timbers twist.

This information bulletin is more relevant for winter construction but can be a useful general guide if wet timber is to be used in summer.

CHECKLIST FOR GOOD TRADE PRACTICES

YES

Use dry framing timber and keep it dry during construction or wait for wet framing to dry. Remember to specify treated timber beyond the threshold of rot.

Keep the floor joists and perimeter beams 10mm back from the framing line. This prevents minor twisting affecting the plaster.

Replace severely twisted studs, lintels, beams or joists before plastering begins.

Preferably use Hybeam, Twin-a-plate, Posi-Strut or Long Reach joists during winter months.

The Hitex sheets should be fixed as early as possible and allow the moisture content of the joists to be below 22% before plastering begins. This will minimise the risks of shrinkage cracking and surface defects.

Provide adequate ventilation to keep the humidity down as the building dries out to its operating state.

Use low sheen paints as they don’t accentuate uneven surfaces.

Specify accurately the level of surface finish required before plastering starts. The smoother the finish the more care needs to be taken with out of line or twisted timber. It’s a good idea to request a sample wall to be done early on in the project.

Hitex is a system supplier conversant with building issues.

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	Quality of Exterior Plaster finishes are extremely sensitive to the moisture content of the timber framing when the Plaster is installed.
	The higher the moisture content at the time of plastering the higher the incidence of shrinkage cracking and surface defects.
	In an ideal environment plaster doesn’t contain anything to cause these defects.
	The most significant area of shrinkage is a wet floor joist. This accounts for 75% of total shrinkage.
	Fast track low budget winter construction of 2 storey houses, apartments and flats have the highest probability of experiencing these problems.
	The most serious problem (shrinkage) is not normally seen for 3-6 months, well after the house has been handed over and is occupied.
	The problem has been with us for a long time but we now have alternative products to minimise the effects of building shrinkage.
	Builders who demand progress on jobs where wet framing has been chosen and not given time to dry will need to take on the risk of subsequent defects.
	Using kiln-dried timber will only stop these problems if it’s kept dry during construction.
	Control joints at the floor line does help.
	The type of timber used is changing to faster growing softer woods which means the problem is actually becoming more extensive.

	Shrinkage causes fold, crush and radial cracking mainly around openings or changes in the building line.
	Twisting joists and studs cause bulging outwards of the plaster. The area around the joist or stud bows inwards to compensate causing a rippling effect along the wall.

	Hitex sheets can be fixed to the framing but plastering must not commence until the moisture content of the joists is below 22%.
	Hitex recommend the use of the following alternatives for joists and beams.

	Joists and Perimeter Joists are to be installed 10mm inside the line of the framing to allow for lengthening of and twisting of timber.
	Any twisted studs, lintels, beams and joists are to be replaced prior to fixing Hitex sheets.
	Base lines of Hitex are not to be concreted in as this prevents the Hitex from compressing downwards to relieve stresses.