What is MDH?



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Over the last two or three decades, a number of specific issues have emerged in the construction of medium-density housing and mid-rise buildings. These issues include lack of movement control in concrete masonry, inadequate cover for reinforcing steel, steel corrosion and weathertightness defects.

Movement control in concrete masonry

If shrinkage is not allowed for in concrete masonry, there is a high likelihood of cracking, which will let water in.

Thermal movement as a result of temperature changes occurs in all materials – some more than others. Some materials change as a result of variations in atmospheric moisture levels. A number of materials irreversibly shrink as they cure.

To accommodate this, we need to allow for the movement that will occur.

One of the materials that shrinks over time is concrete masonry. If this shrinkage is not allowed for, there is a high likelihood of cracking, which will let in water. This may affect the reinforcing (it will rust) or lead to interior damage of materials that  are less moisture resistant.

Concrete masonry materials are subject to movement from:

  • concrete drying shrinkage (irreversible)
  • temperature changes
  • moisture absorption and release
  • earthquakes
  • building settlement and/or loading
  • settlement of supporting soils.

They can be subject to a non-reversible shrinkage of up to 6 mm over a 10 m wall dimension as the wall structure cures. Drying shrinkage of concrete masonry can be significant, depending on the amount of reinforcement and the restraint provided by other building elements such as the foundations or intersecting walls.

Movement control joints

Issues reported with movement control joints include:

  • omission of joints
  • joints that are provided but filled with mortar
  • joints that are too narrow (or too wide)
  • no backing rod to regulate sealant depth
  • not priming before sealant application
  • poor sealant application techniques
  • no maintenance of joints.

 Specifications and drawings covering the concrete masonry installation must clearly show:

  • reinforcement requirements including minimum concrete cover
  • shrinkage control joint locations and sections
  • profile of sealant-filled joints.

 Locations where joints are required include:

  • at 6 m maximum centres generally
  • at internal corners within 600 mm of the corner on both walls
  • for external corners:
    • within 600 mm of the corner of one wall where the dimension to the next shrinkage control joint on the adjacent wall is not more than 5.2 m
    • within 3.2 m of the corner on adjacent walls
  • at changes in wall thickness
  • at wall height changes of more than 600 mm.

Shrinkage control joint locations should be shown on the building elevations by the designer and not be left to the mason on site to determine.

Specific detail requirements are:

  • creating a 10 mm joint width to fit in with mortar joint widths
  • stopping horizontal reinforcing each side of the joint
  • installing an 800 mm long lapping bar across the joint with one end able to allow longitudinal movement
  • ensuring bond beams are continuous across a shrinkage control joint
  • installing a vertical bar and filled cell on each side of the joint.

Full and half blocks must be used to create the vertical joint. Using closed end blocks (slotted for the reinforcing) creates a neat straight joint and prevents grout bridging the joint. Alternatively, the gap can by filled with a 10 mm compressible filler board to prevent grout spread, which may be removed after the grout has cured.

The completed joint is then sealed with a flexible sealant over a backing rod.

Joint details and sealant selection must be shown on the building consent documentation.

Reinforcing steel cover

When reinforcing steel is too close to the surface, it begins rusting and expanding, causing the surrounding concrete to break away.

Steel reinforcing bars are used in concrete and concrete masonry to provide resistance to tensile loads and for concrete slabs and panels to help limit the risk of shrinkage cracking as the material cures.

The steel relies on being sufficiently deep (called cover) within the concrete to protect it from moisture and corrosion. Where it is too close to the surface or the concrete is poor quality, it will begin rusting. As steel rusts, it expands, and this can result in concrete spalling – where pieces of concrete are broken away by the expanding steel. As the concrete breaks away or cracks, more moisture can get in and the deterioration will accelerate. The corrosion will also reduce the tensile strength of the steel as the steel deteriorates.

How much cover?

How much cover depends on:

  • what the steel is embedded in – concrete or with the grouted cells of concrete masonry
  • concrete or grout strength
  • what the surface of the concrete is exposed to – soil or air
  • the corrosion or environmental zone
  • the type of steel used – mild steel, galvanised, stainless steel
  • intended life of the structure
  • cement type.

Cover is measured from the outer edge of the reinforcing to the outside or exposed face or surface. Cover specified is for mild steel bars and makes no allowance for any properly applied and maintained coating system.

For footings and foundation walls, these are the minimum cover requirements in NZS 3604:2011 Timber-framed buildings and NZS 4229:2013 Concrete masonry buildings not requiring specific engineering design:

  • For concrete masonry:
    • 45 mm in exposure zone B
    • 50 mm in exposure zone C
    • 60 mm in exposure zone D.
  • For concrete:
    • 75 mm for concrete placed directly on or against the ground (can be reduced to 50 mm where there is a DPM between the concrete and the ground (NZS 4229:2013)
    • 50 mm when placed against formwork – for NZS 3604:2011 buildings, the concrete strength must be appropriate for the exposure zone
    • 30 mm for the top of an exposed slab protected from the weather
    • 50 mm for any slab surface exposed to the weather.

Tables 3.6 and 3.7 of NZS 3101.1&2:2006 Concrete structures standard outline the requirements.

For concrete masonry structures built to NZS 4229:2013, the cover requirements from the external face of uncoated masonry are:

  • for exposure zone B – 45 mm with 17.5 MPa grout
  • for exposure zone C – 50 mm with 20 MPa grout
  • for exposure zone D – 60 mm with 25 MPa grout.

 For interior locations, the cover requirement is 45 mm with 17.5 MPa grout.

Cover requirements for raw concrete masonry structures are given in Table 4.1 of NZS 4230:2004 Design of reinforced concrete masonry structures.

Steel corrosion

Mild steel exposed to atmospheric moisture will corrode unless it is adequately detailed, has the right level of protection for the location and is well maintained.

Deterioration of mild steel can be worse where:

  • construction details or steel profiles trap moisture or dirt such as an exposed flange that is near horizontal or a junction between two members
  • the applied corrosion protection has been damaged during erection or afterwards
  • galvanised or painted surfaces are modified on site such as cutting or grinding to make it fit or welding on an extension
  • the steel is difficult to access for inspection and cleaning/maintenance
  • the wrong or inadequate coating system for the location and environment has been used
  • poor application of applied coatings – not following the supplier’s recommended process for number of coats and sequence of application
  • poor preparation of the original steel before coating – this means that the coating won’t stick well to the steel.


The following standards are relevant to preventing steel corrosion:

  • AS/NZS 2312:2014 Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings
  • SNZ TS 4304:2018 Durability requirements for steel structures and components
  • NZS 3404:1997 Steel structures standard
  • New Zealand Steelwork Corrosion and Coatings Guide (published by HERA to be used in conjunction with AS/NZS 2312:2014).

Specifying steel

When specifying steel, the following must be identified before deciding on the appropriate level of corrosion protection:

  • Access for inspection and maintenance.
  • Environmental conditions now and in the future.
  • Detailing – avoiding any flat surfaces or debris traps.
  • Whether all work can be done in factory conditions or whether it is likely the steel will need to be modified and therefore recoated on site.

Options for mild or carbon steel protection:

  • Hot-dip galvanising after fabrication – note that any modification after galvanising will destroy the coating. Galvanising may be used with an additional coating system, typically described as a duplex coating system. Hot-dip galvanising without additional paint application will often be insufficient corrosion protection in many situations in New Zealand.
  • Specialised corrosion-resistant paint (such as epoxy, polyurethane, zinc-silicate, urethane-acrylic, chlorinated-rubber) system – a number of specific coats applied in the correct order to the prepared steel.
  • Polyester powder coating.
  • Steel primer and acrylic or alkyd finish coat(s) for dry interior locations where there is no risk of moisture affecting the steel.
  • Metallisation plus paint anti-corrosion protection consisting of thermal spraying of zinc/aluminium, sealing of the pores and application of a finishing layer with liquid paint.
  • Thermally (hot) sprayed coatings of zinc, aluminium and zinc-aluminium alloys to give long-term corrosion protection to steel structures in aggressive environments.

Zinc spray protection is less durable than hot-dip galvanising and is vulnerable to insufficient surface preparation. It is not recommended for exposed steelwork.


Good details:

  • avoid dirt and moisture traps
  • where possible, use an RHS section rather than a channel or PFC
  • separate materials where there is a risk of galvanic corrosion
  • avoid crevices or joints that also trap dirt and moisture
  • have rounded rather than sharp edges to steel sections to allow a full coating build to corners.

Protected lightweight steel framing

Buildings that are built with zinc-coated mild-steel frames will meet durability requirements provided the steel is located within a dry enclosed environment and is not exposed to the exterior environment or water entry when is use.

Weathertightness defects

BRANZ inspections of MDH developments identified significant problems with weathertightness.

Across a number of new MDH developments, issues were found with loose wall underlay and flashing tape. This risks the transfer of moisture across the cavity and therefore water ingress.

In existing MDH developments completed and occupied between 2010 and 2017, BRANZ inspections found multiple instances of compromised weathertightness caused by poor flashing details. Defects included:

  • insufficient clearance between the cladding and head flashings
  • minimal clearance between the cladding and service penetration flashings
  • minimal gap between the horizontal flashings and the cladding
  • vents not flashed to the cladding.