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Sydney Sandstone and Brick Structures

There was a time, many years ago, when it was thought water was the most deadly enemy of any historic masonry building. That naive notion died within the first hour on our first rehabilitation project. The most deadly enemy is man and what he does in the name of maintenance and rehabilitation.

(We will ignore, at this time, those who think “Old Buildings” should be demolished in the name of “progress”
Most of our historic Sydney structures, even some of the most elegant, have experienced the cycle of decline and neglect, eventually winding up in the hands of people incapable of maintaining the buildings, whom feel cheaper is better, or simply refuse to spend any money on the building unless forced to. Whatever the reason, the maintenance, if any, was probably more damaging than helpful. However, through sheer luck and the inevitable cycles of time, many of these buildings survive long enough to wind up in the hands of people who feel they are worth rehabilitating. But that doesn’t mean they are saved. It takes more than good intentions.martin place 2

The first law of Rehabilitation: Do No Harm.
Twenty-five years ago, when I first began to work in this area of architecture, there was probably no one who could write the things you will read here. We didn’t know!
Whether through ignorance or for the love of money, all manner of incorrect, incompetent, inappropriate or destructive measures have been inflicted upon older masonry buildings, all to frequently by those of us with the best of intentions. This article is intended to give just enough of what we have learned in the last quarter century to make you dangerous….. dangerous, that is, to those who choose to remain ignorant or just want to separate you from your money.
What do you need to know to be dangerous?
The second law of Rehabilitation: It is always less expensive to do it right the first time.
Treat the cause, not the symptom: Most of the problems associated with historic masonry buildings don’t begin with the masonry itself. They are the result of the failure of some other component of the structure. Ninety-nine times out of a hundred, the problems we see are related to water. This may be roof leaks, flashing failures; failures in the gutter / down pipe system or failure to remove water at ground level. Unless these problems are corrected first, any treatment of the symptom is doomed to failure.
Myths and the Charlatans
Waterproofing will solve the problem of wet walls: Without getting into the whys and wherefores of historic Sydney sandstone bricks, let it just be said that water absorption is a characteristic of the material. Because of the methods of sandstone brick construction a hundred years ago, it was not a problem then, and it is not a problem now. But Waterproofing Is!
Historic brick and mortar breathe – a lot. Apply a waterproofing to keep the water out and you trap it inside the wall just as well. Unlike modern construction, historic brick walls really are brick all the way through. Water entering the wall, whether from the outside as liquid, from the inside as water vapor, or from the top as in a roof leak, will migrate to the exterior and escape unless you do something to prevent it…… like waterproofing. Brick wall construction is a system that has worked for as long as man has been making bricks, and that’s a lot longer than we have been making waterproofing.
Another rule: Don’t trap water in the wall.
That leads us to the next problem, painted brick. If your painter or paint salesman tries to convince you that oil and latex paints breathe, stick his head in a plastic bag and ask him to breathe. That is how your painted brick wall will feel. Compared to older brick, paint is like a plastic bag. If you think paint will solve the problem of a wet wall, the only thing you will have is a wet wall with pealing paint, and a much smaller bank account.
Another problem that you need to be aware of involves landscaping. Yes, landscaping is a beautiful part of our historic environment. Just be sure that there is sufficient room for air to circulate. Taking that one small step further, ivy and other plants growing up a brick wall look beautiful, but they can be damaging to your brick structure. As beautiful as it may be, it really is best to remove them.
Until about 1930, the mortar used in masonry construction was composed almost entirely of lime putty and sand. It was relatively soft and worked very well with the softer, more porous bricks of the time. Unfortunately, over time the exposed area of lime putty mortar can be eroded and pointing, refilling the mortar joint with new mortar, may be required.
Here again enters modern man with his miracle product, silicone! “Let’s just squirt these joints full of silicone, particularly in the areas under bad gutters where water is a constant problem. After all, silicone will keep out the water; it is easier to do than pointing and certainly much cheaper than fixing the gutters.
First, lime putty mortar breathes even more than the brick. When you seal the mortar joint with silicone you once again create a barrier to water escaping from the wall. Instead, fix the real problem that is allowing the water to get into the wall. (Rain falling on a wall is seldom the problem.) Then, if necessary, have the wall repointed.220px-Formal_use_of_Sydney_sandstone_in_Neoclassical_style


 Second, the person who believes this is cheap does not understand proper silicone application methods and is doomed to another failure before he even begins. Two of the most important things to know when silicone a joint are: the depth of the joint must never be greater than the width of the joint; and the silicone must adhere to both sides of the joint but never the back of the joint. To silicone properly you must first cut the joint to a depth about one and one half times as deep as the joint is wide and install a backer rod to prevent the silicone from sticking to the rear of the joint. If you don’t, your cheap silicone joint lasts about a year before it pulls itself loose from one side of the joint or cracks in the middle. Now you have a deteriorated mortar joint full of useless silicone that looks ugly, leaks and will be an expensive pain to remove when someone finally decides to do the job correctly.
Efflorescence: This is the white, powdery substance that is occasionally seen on brick exteriors. It really is not harmful in itself, but can be a symptom of harmful things that are happening. Efflorescence is nothing more than naturally occurring salts that dissolve in water, are carried to the surface and are left behind when the water evaporates. It can be removed by brushing with a stiff bristle brush followed by a light detergent wash. If the wall has not been pointed and the joints have not been sealed, this usually indicates that too much water is getting into your wall. Before you spend your time cleaning, look for the cause and correct it. If you don’t, the efflorescence will come right back.
Lime putty mortar breathes more than the brick, so most of the water in the wall will migrate to the mortar and evaporate at the joints. However, an improper pointing job or sealing of the joints can stop this, forcing the water and the salt it carries into the brick. If the water evaporates before it reaches the surface, the salts (the same thing that causes efflorescence) are deposited inside the brick where they expand and exert pressure on the surface of the brick. This, along with freezing temperatures, will cause pock marks to develop in the face of the brick and chipping to occur adjacent to the mortar joints.
Repointing or simply pointing is more complex and not even understood by many brick masons. No, that is not a comment on the quality of contemporary brick masons. It is only recognition that there has been a significant change in the way we do things and many brick masons have never had reason to become experienced in historic means and methods. (By the way, the term “Tuck Pointing”, although widely used even by those of us who should know better, refers to something totally different than what we are discussing here.
Here is a short discussion of materials:martin place


 Sydney sandstone:
Sydney sandstone is the common name for Sydney Basin Hawkesbury Sandstone, historically known as Yellowblock, is a sedimentary rock named after the Hawkesbury River north of Sydney, where this sandstone is particularly common.
It forms the bedrock for much of the region of Sydney, Australia. Well known for its durable quality, it is the reason many Aboriginal rock carvings and drawings in the area still exist. As a highly favoured building material, especially preferred during the city’s early years—from the late 1790s to the 1890s—its use, particularly in public buildings, gives the city its distinctive appearance.
The stone is notable for its geological characteristics; its relationship to Sydney’s vegetation and topography; the history of the quarries that worked it; and the quality of the buildings and sculptures constructed from it. This bedrock gives the city some of its ‘personality’ by dint of its meteorological, horticultural, aesthetic and historical impact. One author describes Sydney’s sandstone as “a kind of base note, an ever-present reminder of its Georgian beginnings and more ancient past.”
Brick:
Older brick may look similar to contemporary brick, but as a result of the way it was made, it is generally much softer and absorbs much more water. Also the face of the brick is considerably harder than the interior.
Like all other materials, bricks will expand and contract with temperature and humidity. This movement is very small when you are looking at an individual brick, but adds up to something significant when you build a wall. The point is that brick walls move. You may not see it, but unless this is considered when the wall is constructed (or pointed), you will see the resulting cracks.
Mortar:
Prior to 1930, mortar was usually composed of lime putty and sand. This mortar was relatively soft and remained slightly pliable throughout its life. This mortar “cures” or hardens by a chemical process that requires the presence of carbon dioxide, not by drying out. Because of the slightly pliable nature of the mortar, it can move with the expansion and contraction of the brick and small cracks can actually repair themselves. The presence of a small amount of water in a wall is required for curing and actually beneficial in maintaining the flexibility of the mortar.
Contemporary mortar contains hydraulic cement as well as lime and sand. Hydraulic cement is the major component of concrete and cures in the presence of water (hence the name hydraulic). This mortar is considerably harder than lime putty mortar, has no flexibility once it cures, is very impervious to water when compared to lime putty mortar and actually shrinks slightly as it cures. If there is insufficient water, this mortar will not cure properly, losing adhesion and becoming slightly powdery. When building a wall with this type of mortar, control joints (joints that allow for movement) must be installed at regular intervals, both horizontally and vertically, to accommodate the movement.

The wall:
Important information: The mortar must always be softer than the masonry unit (brick or stone) to avoid damage.
A brick structure of 100 years ago would normally have walls composed entirely of brick. The outer layer or wythe (pronounced “with”) of brick (the “face” brick) was harder, with a face more impervious to water. The inner Wythes were normally composed of softer bricks called common brick. The mortar was lime putty mortar. The thickness of the walls was determined by the load the wall would carry and the stability requirements. Houses would normally be three or four wythe thick, while taller buildings could have walls considerably thicker. It was very common to reduce the thickness of walls as one progressed higher in a structure because the load was reduced, and this also provided a shelf upon which to set the floor joists. The use of lime putty mortar greatly reduced or eliminated the need for control joints because of its flexibility.
Contemporary “brick walls” are usually composed of a structural frame covered with a skin or veneer of brick. The brick no longer has a structural function. It is there to look good, stop water from getting to the structure and reduce maintenance. The bricks and mortar are much harder, they are both very impervious to water and the walls must have control joints to prevent cracking. As you can see, the methods of construction are very different.
Repointing or Pointing (remember, not Tuck Pointing):
This is the process of removing some of the mortar from a joint and replacing it with new mortar. This is usually done when a considerable amount of the mortar has been lost from the face of a joint or because cracking has occurred. Just because a joint appears to be soft does not mean it needs pointing.
First, the mortar didn’t just decide to fall out or crack. There is a reason the wall needs pointing. Fix that first. Then find yourself a mason that is experienced in historic work. If you know more than he does, you are already in trouble.
Normally an entire wall does not need pointing; only those areas that are bad. If your mason says the entire wall is deteriorated, get a second opinion. I recommend using a knowledgeable consultant or architectural conservator because he has nothing to lose by telling you the truth.220px-Qvbsyd


 Next, what kind of mortar is to be used? There are two things to be considered here and both are related to the composition of the mortar. First, what is the mixture; how much lime, how much sand, is there any cement in the mixture? Second, what type of sand was used? This is actually a major element of the colour of the finish. Unless the project is large or very important (with a budget to match), lab testing of the mortar may be too expensive. Normally the mason used local sand, so an examination of the original mortar will probably give you all the information you need about the sand. Then I recommend making several test samples using slightly different mortar mixes. Cure them properly for several days and compare them to your existing mortar. The new mortar should be softer than the original and slightly darker because it will lighten with age. One of the most common historic mixes was 1 part lime putty to three parts sand. Sometimes powdered limestone, ground sea shells, brick dust and other seemingly strange elements were added. Starting in the late 1800s, cement was occasionally added in small amounts. Another mix that seems to be common in rehabilitation work today is two parts lime putty, two parts Type N masonry cement, and eight parts sand.
Of major importance, and frequently overlooked, is how the mortar will be removed from the joints. This is one of the most time consuming and definitely the dirtiest part of the job. If your mason says he is going to grind the joints, find another mason. The best way to remove the mortar is by hand with a chisel or brick rake. On occasion I will allow the mason to use a thin diamond blade to cut a small line down the centre of the bed joint (horizontal joint). This is called a relief cut and allows the mortar on either side to be removed easily. However, you only do it in the horizontal joints. In the head joints (the vertical joints) the blade will strike the brick at either end of the joint before it cuts to full depth. This will notch the brick on the top and bottom of the joint. On occasion the joints are so narrow there is no power tool that can be used without widening the joint or damaging the masonry. Frequently the mason will try to tell you that the joints will look better if he can widen them and that it is the only way to get mortar in the joint.
Why is this critical? First, the interior of the brick is softer than the face. Once the mason hits the brick with the grinder, your first line of protection against water and dirt is gone – forever. Secondly, the character of the wall is forever changed. Remember, you are paying for an historic building and that building has a certain character that is forever destroyed when the joints are damaged.
Placing the Mortar: Of critical importance, before the placing of the mortar begins, the wall must be thoroughly wet to avoid drawing the moisture from the mortar. The mortar is then packed into the joints with a narrow trowel in thicknesses of 1/4″. Attempting to fill the joint with a thicker amount of mortar makes it difficult to properly compact the mortar into the joint and leaves voids where water can accumulate and pop the joint in freezing weather.
Pointing mortar should be mixed in amounts that can be used within an hour. If the mortar becomes stiff, it should be discarded. Do not add more water.
Striking the joint: Striking is the method of finishing the mortar joint and compacting the surface to make it shed liquid water better. The best way to determine what was originally done is to find an area of the wall that has been protected from the elements and see how it was struck. The joints are going to be struck in some fashion, why not the original? Under no circumstances should it be “rod struck” (a round rod is used and produces a concave joint that looks like this parenthesis).
Cleaning: If the mason is careful, little cleaning is required. After the joint is struck, the use of a soft bristle mason’s brush will remove most of the surface mortar. If this doesn’t quite do the job, larger tags of mortar can be removed with a wooden paddle. Additional cleaning with a stiff bristle brush and water can be done after the joints are sufficiently set to avoid damage. Muriatic acid should never be used on historic masonry. Some masonry cleaners are available that will help clean without doing extensive damage to the new mortar joints. Several masons have suggested the use of vinegar as a safe, natural, bio-degradable solution for removing the white film that may remain. If a chemical solution is going to be used, including vinegar or detergent, the wall must always be thoroughly wetted prior to the process. Otherwise the cleaning agent will be drawn into the brick.
Curing: Because older bricks absorb water readily, they will quickly draw the moisture from the mortar. Proper curing requires that the wall be kept wet for three days or more. Depending on the temperature, humidity, wind and sun conditions, this may require hosing the wall down every hour or two during the day.
A lot of masons will tell you this is all unnecessary. The use of contemporary mortars will do a better job for less, the mortar sets up so fast that wetting the wall is not necessary and no one is going to care how the joints are struck. So, to make you more knowledgeable than him, here is what you need to understand.
1. Contemporary mortar is impervious to water when compared to lime putty mortar. We are now dealing with the same problem we discussed earlier about caulking and efflorescence.
2. Contemporary mortar does not move and your wall does. Cracking will definitely occur. Frequently this will be adjacent to a window or door where the wall is weakest and now you have a structural problem.
3. Contemporary mortar (even Type N which is 50% lime putty) is very hard compared to lime putty mortar. When the wall moves the lime putty mortar will have some give and the contemporary mortar will not. This places extreme pressure on the outer edge of the brick (where the pointing has occurred) and can actually spall (split) the face right off the brick.
4. Contemporary mortar, even more than lime putty mortar, requires water to cure. Just because it feels hard in a short time does not mean it has cured. It will take seven days to reach 90% of its eventual strength it is cured properly.
5. Contemporary mortar is not the same colour as your lime putty mortar and this will be obvious.
Remember, you are the one who must live with this and pay for this, not your mason.
Now there is only one more critical element to discuss, the value of the preservation work
When we reach this point, the second law of rehabilitation changes to read: It is more valuable when done correctly.

220px-19th_century_Sydney_sandstone_wall

 

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