Some weathertightness problems can occur in painted, single-skin concrete masonry buildings. So, what should designers and builders look for when designing and building waterproof concrete masonry to help ensure weathertightness?
A good place to start is remembering that a concrete masonry wall, by itself, is porous. The permeability varies depending on what the units are made of – for example lightweight pumice aggregate is especially porous.
Care needs to be taken with masonry because secondary drainage and drying principles, including drainage cavities that apply to framed buildings, aren’t present in concrete masonry. Concrete masonry may be more naturally durable than timber, but the main ‘tools in the armoury’ for keeping water out of a masonry wall are good building design and effective surface coatings.
Building design that addresses the nature of materials and the environment they are used in can significantly improve the odds against leaking, as will careful detailing, such as around windows and doors. The principle of drainage cavities around windows and doors does apply, but the final back-stop is effective masonry sealing, especially in reveals of openings before joinery units are installed.
The mortar joints between concrete masonry units pose a particular weathertightness problem. As new mortar dries and shrinks, cracks develop between the mortar and the masonry units. Correct tooling of the mortar, which re-compacts the mortar after the masonry unit is laid, helps reduce the risk of cracking. This, plus a coating system that is correctly selected, applied and maintained, is especially important for ensuring weathertightness.
Therefore, the most effective means of weatherproofing concrete masonry will include combinations of:
- 1. building designs that deflect water away from the masonry, such as with eaves overhangs, upper floor overhangs, and rainscreens
- 2. details that deflect water away from critical joints, for example, flashings, drip edges and mouldings
- 3. good masonry design specification and construction
- 4. coating systems specifically formulated and applied to seal the surfaces of the masonry.
Buildings with good eaves all around will naturally be at less risk of leaking. Two principal ‘drivers’ for leaks are wind pressure and gravity. Reduce the water ‘on top of a wall’ through the use of eaves, and many leaks caused by gravity are eliminated. Reduce the amount of water running over the face of a wall or joint, and wind pressure can draw less water into the joint.
Horizontal surfaces, such as sills and horizontal ledges, should always be sloped to shed water, and drip edges should be formed at all overhang projections such as along window heads. One particular problem is that tooled concave mortar joints can often provide water pathways past a joint. The use of sealants can have a limited (and short-term) effect, with the best solutions coming from flashings set into sealant in a rebated saw-cut.
Buildings with parapets (and associated box gutters) are more at risk of leaking because weather-tightness is more reliant on getting everything ‘right’ and it staying ‘right’. They lack any of the ‘forgiving’ qualities that designs that shed water more easily provide.
The concrete masonry units, mortar, grout-mix, and workmanship are outlined in NZS 4210, Masonry Construction: Materials and Workmanship and NZS 4229 Concrete Masonry Buildings Not Requiring Specific Engineering Design. Following these Standards can provide a means of compliance with aspects of the Building Code and are a good prerequisite for effective weathertightness results. The quality of materials and work-manship, including recommendations such as filling all block cells, correct vibration, and using correctly formulated grout mix, will ensure greater stability of the wall on which the performance of water-resistant coatings depends.
It is important to choose a proprietary coating system designed for sealing concrete and concrete masonry. It is recommended that waterproof coatings applied directly onto concrete masonry are water-borne dispersion coatings giving 180 – 250 micron dry film thickness. Coatings can also be in the form of coated cement or polymer-modified cement plaster, insulation material over-coated with polymer modified cement plaster, or applied waterproof membranes. Clear coatings are not generally recommended because of difficulties in achieving lasting weatherproofing performance.
An often-overlooked requirement is the sealing of hidden surfaces, such as reveals of windows and doors, or hidden wall surfaces immediately above eaves lines. While these surfaces may not be directly exposed, sealing them is important for the effectiveness of the waterproof joint. The sealing may only need to be the first one or two coatings of the coating system, depending on the coating manufacturer’s recommendation.
The weatherproofing of concrete masonry is important for complying with Building Code Clause E2 External Moisture, which deals with the weathertightness of buildings. The Standards NZS 4229 and 4210 give guidance on how to achieve this. In addition, readers are referred to two other significant publications that deal specifically with the weathertightness of masonry. These are:
- Concrete Masonry – a guide to Weathertight construction, available free of charge from the New Zealand Concrete Masonry Association at www.nzcma.org.nz
- Weathertight Solutions – Volume 4, available through BRANZ at www.branz.co.nz
- article courtesy of DBH – CodeWords April 2010
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