Major causes of rain penetration
Rain penetration in buildings can result from a number of causes:
- Choice of materials.
- Method of use of materials.
- Design detailing.
Correct selection of materials
If brickwork is to be left unrendered then it is essential that only bricks suitable for this application are used. All face bricks manufactured under controlled conditions, will have satisfactory rain penetration properties. These bricks are fired to a temperature where the permeability and durability of the product render them suitable for all normal exposure conditions.
The use of stock bricks in unrendered application is strongly discouraged as these bricks will not perform adequately in terms of either durability or rain penetration resistance.
Manufacturers will supply, facebricks (FBX, FBS or FBA bricks), which meet the durability requirements for unrendered or bagwashed application. These bricks should be used in place of stock bricks for these exposure conditions. It is essential, however that the manufacturer be informed of the intended application at the time of ordering.
2. Mortar Sand
The permeability of mortar plays an important role in rain penetration resistance and the use of poorly graded sand is not recommended for face brick work.
The South African Bureau of Standards, in conjunction with the National Building Institute and the Portland Cement Institute have established that sands which comply with the grading requirements of SABS 1090 will give mortars with excellent rain resistance qualities. Such sands are readily available in most areas of South Africa. If there is any doubt about the suitability of a sand for mortar use, the Bureau of Standards or the Portland Cement Institute will test the sand for you.
The presence of excess quantities of clay in mortar sand (greater than 10%) will cause excessive shrinkage of the mortar with resultant joint cracks of up to 1mm wide. This type of shrinkage cracking is not confined only to the strong mortars but can also take place in mortars as weak as 1:6 or 1:7. Mortar sands containing excessive clay are often used because of their plasticity. Poor rain penetration resistance of such mortars, however, does not justify their use. If plasticity improvers are needed then lime can be added to the mortar mix. More information on the properties of lime mortars is discussed below.
3. Mortar Mixes
It is a well researched and established fact that the use of lime in mortars will result in brickwork jointing of superior properties:
- Workability is considerably improved with benefits in joint quality, bricklayer productivity, and the appearance of the finished wall.
- Water retentivity is improved with resultant minimization of separation cracks.
- The free lime in the cured mortar is slightly soluble and will migrate to areas of rain penetration where it will crystallize and seal up leaks.
These benefits are recognised by all major building authorities as evidenced by the recommendations contained in the following documents.
- SABS 021 – The Waterproofing of Buildings;
- British standards BS 121 – Code of Practice for Walling;
- The Standard Building Regulations Chapter 8;
- The Technical Guide to Good House Construction published by the NBRI and the Association of Building Societies of SA;
- The SABS Code of Practice for the Structural use of Masonry;
- SABS 285 – Calcium Silicate Bricks.
The recommendations for mortars in brickwork are shown in table 1 below.
||Below DPC, parapets and where structural requirements dictate
||General Face Brick work.
General Internal face work and all stock brickwork
|* Proportions by loose dry volume
The most common cause of rain penetration in brickwork structures is the incomplete filling of perpend joints. The complete filing of all mortar joints, therefore, must rank very high in the design of brickwork for adequate rain resistance.
The practice of furrowing the mortar bed should be discouraged as it can lead to incompletely filled bed joints.
Excessive tapping of bricks to bring them into line with strings, results in the migration of water to the interface between brick and mortar with the resultant formation of separation cracks in the cured brickwork.
The use of bricks of high suction rate can result in excessively fast removal of water from the mortar bed. This is particularly true if lime has not been used in the mortar mix. The rapid removal of water can result in the formation of separation cracking. A bricklayer can control suction rate by wetting bricks prior to laying.
Incorrect use of materials
Of most significance in this category, is the incorrect use of damp-proof course materials. This is a subject which cannot be adequately covered in a manual such as this and the reader is referred to:
"Damp-proof Course Detailing” by John Duell and Fred Lawson, published by the Architectural Press in London in 1977 and obtainable from the RIBA Bookshop in London.
There are however certain principals which when followed will result in adequate rain resistance of walls.
There are 3 prime functions of a damp-proof course and these are:
- To provide a barrier against rising damp;
- To resist water penetration from above,
- To resist horizontal water penetration in performing the above two functions.
Some common detailing and construction omissions are:
- Not taking the DPC right through the mortar joint
This omission leads to a path for rising damp to penetrate past the DPC. This is important in areas where there is a high water table, particularly if the ground waters are saline.
The DPC stops short of the edge of the brickwork. Rising damp can penetrate past the DPC into the brickwork above.
- Not sandwiching the DPC between wet mortar
The usual method of using the DPC is to lay it down on the concrete slab or brickwork without providing a seal between the materials. The absence of a seal, except in highly loaded situations, will lead to an open capillary path for horizontal rain penetration. The most effective sealant is mortar and DPC should be detailed as a sandwich between wet mortar as shown in the details below.
- Not Lapping and sealing the DPC
In a horizontal sense, along the run of walling, joints in the DPC should be lapped by at least 150 mm and should be sealed if the joint is to effectively resist horizontal rain penetration.
Sealing of polythene DPC is difficult and care should be taken to ensure that the sealant used is not reactive towards polythene.
Bitumen based DPC’s can be sealed by heating with a blow torch and then bedding together, or by using a Bitumustic sealer.
It is not commonly realised that certain types of DPC materials have adequate mortar adhesion characteristics to allow them to be used for protection of parapets and free standing walls. It is essential, however, that the sandwich construction method be used otherwise lateral stability is dangerously impared.
Parapet wall or freestanding garden wall.
Parapet wall detail
Movement joint goes through the coping
DPC material with mortar adhesion characteristics sandwiched between wet mortar.
DPC as above.
Brickwork expansion into movement joint results in compressive lifting of coping.
- Class 1 mortar 1:0,25:3
- Cavity construction essential.
- Coping brick preferably 290mm wide to overhang the parapet for drip purposes.
- Coping perp joints must be flush to avoid water streaking on the wall.
In both cases flush jointed perps in coping avoids rain water chanelling down the face of wall.
- Polythene DPC/s are not recommended in these applications.
- If a brickwork coping is used consider the use of a preformed metal flashing to throw the water away from the wall.
- Parapets should always be detailed in cavity wall construction.
- Freestanding walls and parapets need frequent movement joints. The Standard Building Regulations require a movement joint every 7,5 m in parapet construction and the same figure should be used as a guide in free standing walls.
- Movement joints must run right through the coping.
Good design detailing, particularly consideration of water channeling, DPC type and position, overhang protection, positioning of movement joints and the choice between solid and cavity construction, can make a big difference to the rain resistance of walls. The reader is referred to 3 ducuments for futher examples of correct detailing:
- SABS 021 – The Waterproofing of Buildings;
- Everyday Details – Cecil C. Handiside published by Architectural Press and available from the RIBA Bookshop in London;
- Damp-proof Course Detailing by J. Duell and F. Lawson, published by Architectural Press and available from the RIBA Bookshop.
Some aspects of detailing and DPC positioning have been discussed in the section on the correct use of materials. The reader is referred to that section for more information.
Of particular importance is the choice between solid and cavity wall construction. In areas having a high driving rain index such as Natal coast, the Eastern and Western Cape coastal region, it is advisable for brickwork to be detailed in cavity wall construction. Research in South Africa, as shown, that cavity wall construction is the only truly effective way of preventing water penetration in these areas.
The use of cavity wall construction, however, should not be overlooked in other parts of South Africa, as this method of construction has greatly superior thermal insulation characteristics and can contribute to reduction in energy costs for both heating and cooling buildings.
Movement joint positioning is also important because not only do cracked walls, resulting from the imposition of unexpected loads leak, but they are difficult to repair.
It is worth repeating here, that in the construction of parapet and freestanding walls, the movement joint must be taken right through the coping to avoid damaging this first line of defence against rain penetration.
Rainfall provinces and variability for Republic of South Africaed rain fall tests.
The National Building Research Institute has developed a method of simulating driving rain and testing the suitability of construction methods or details for rain penetration resistance.
Customers in doubt about the suitability of a brick or construction detail for a specific geographic area are invited to contact the CBA in Midrand, where test results may be available to solve the problem.