Sandstone is a rock that was formed by sedimentaery deposits over a huge period of time and unfortunately these deposits will gradually waste away. The traditional method of cutting out and replacing damaged sandstone blocks can be very expensive.
Most of us would expect to achieve the following- ---
1. Slow down the deterioration process.
2. Stop the progression of major structural damage.
3. Complete good quality repairs at a reasonable cost..
4. Keep the building face as close to original as possible.
5. Leave the building looking good and well maintained.
How do I fix the cause of the damaged sandstone ?
Firstly you need to understand what has caused your sanstone to spall and deteriorate. More often than not it will be due to water. It could be the natural fall of rain hitting the sandstone surface , or perhaps an over flowing gutter or worn pointing allowing water ingress and spalling in the event of frost.. If there are concentrated areas of damage, then the source of the water ingress has to be found and in turn fixed.
Another cause of damaged sandstone is poor quality maintenance and repairs.
Under certain conditions using ordinary cement to point or patch repair can cause damage. Most sandstone buildings were built during a time when mortars contained either all lime and no cement or mostly lime and very little cement. Cement mortar traps moisture and salts right below the hard pointing and creates problems. Look for bad patches -- patches that didn't breathe, trapped moisture or trapped salts and caused deterioration around that area in the sandstone.
It is also possible that the damage is merely the result of the natural elements brought along in our weather.
When surveying the damaged are you may find roof problems, leaking gutters and downspouts. I myself would normally make recommendations to the owners and the root cause results in some roof repairs needing done.You have to stop the source of water iingress... you have to correct those problems before you even think about repairing the sandstone.
I would strongly advise that scaffolding or an aerial platform is used to enable the person surveying the building to get a good thorough look around as it is near impossible to see from ground level. This person should be competent in both stone masonry and working at height regulations.
There are 4 major repair approaches:
1. Major replacement with new sandstone... either replace the entire block or take out part and do what are called Dutchman repairs. Problems with major stone replacements are first that you have to find matching stone. In many cases the quarries are closed, the stone is not available. You may be able to find salvage stone but you have to be very careful. You have to find perfect matches. If you're off just a little bit, all those repair areas are going to really stand out. You need to find workmen who are able to do this type of thing-- to actually take the stone, cut the stone, dress the stone and install the stone. That is not easy and it is also very expensive.
Major replacement of stone is not reversible. Once you've taken the stone out and thrown it away, it's gone forever. It's also very intrusive. You're going to end up taking large pieces of the building out. You have to very careful that you're not disrupting the structure of the building. Finally it's also very expensive. Sometimes, if you have a big budget and you have stone you can get, this can be a viable alternative.
2. Retool the surfaces. Take away some delaminated material and retool the surface to make it look like what it was originally. It's not always feasible to do this, in fact in most cases it's probably not feasible to do this. The amount of deterioration may vary and you may have to remove tremendous amounts of stone to retool the surface... It is a very intrusive process. You're going to end up removing stone from surfaces that are in perfectly good shape to bring everything back to a similar plane, and I object to removing any good stone. It is not reversible, you remove that stone and it's gone forever. The deterioration processes that were going on will continue, whether you retool the surface and move it back a little or whether you leave it where it was. It also requires consummate skill and therefore it is expensive.
3. Scaling off the loose pieces. It's inexpensive, you don't need skilled labor to do it. It increases the safety factor. It can be combined with patching. Scaling happens inevitably. If there's a piece that's hanging there and it's going to come off sooner or later..we're just making it a little bit sooner.
4. Patching. Patching can be effective at a reasonable cost. In general, patching is reversible. If the patch does not hold up well, 20 years from now somebody can take that patch out and redo it. It involves minimal intrusion into the building fabric. You can usually avoid removing more stone than is badly damaged.
In general I've been very surprised at how effective patching can be at slowing deterioration. In some of the buildings that I've been involved with over a period of years where it looked as though the deterioration was occurring at a very rapid rate, and that it was going to continue at a rapid rate even with the patching, we founds that by patching areas where the water was getting in, where there was soft stone or high weathering, that we prevented a lot of moisture from entering the rest of the stone and places that I would have expected to see deteriorate, didn't deteriorate. To get those bad areas repaired to prevent water from entering apparently is important. If done properly, or reasonably well, it can also enhance the appearance of the building. And generally, if you do things right, you probably will not make things worse. And that's one of the prime criteria in dealing with old masonry.
The patching material you use must match the colour and texture of the existing material. It must adhere well, it must be breathable, it must have similar thermal expansion characteristics, and it must weather well. You don't want to do a patch that's going to deteriorate in 5 years. That's not cost effective for the client. I would advise keeping patching as minimal as possible. Do those things that really must be done for structural purposes and to prevent water intrusion and for appearance sake, but don't go crazy. It's very easy to look at a sandstone building and say "we can patch this, this, this and this, almost everything there". Try to keep it minimal. Minimal intrusion.
Helpful Hints for Patching
1. Use a hydraulic lime repair mortar which should ideally be made up of lime, coloured dye, sand and fine stone particles. Avoid feathered edges. If you feather edge a repair the feather edges will deteriorate rapidly. I like to cut, as opposed to chisel, the areas where the repair is going to be done. Chiseling tends to fracture the surface, you don't get a nice even cut and you're also banging hard on a very fragile material and you can cause more damage with that. A little 4" high speed grinder with a thin diamond wheel can be really effective at cutting out the area that you're going to repair and you can even undercut the repair, that is cut back at a slope from the surface so that you dovetail the area. A diamond grinder is quick, it's easy, it doesn't damage the surface at all.
2. Avoid thin repairs.
3. Colour matching. I have a philosophy that no colour match will be exact. So if you can't be exact and you're going to be wrong, be wrong several times. Let me explain that.
If you use one colour for all the patches on a building, when someone stands back they're going to be able to pick out all those areas, because the color is just a little bit off. But if you use several different colors, maybe one that's real close, maybe one that's just a little bit too dark, and you start intermixing those -- do a repair here with the darker one, do a repair here with the one that you think is just right, intermix them and patch a stone here -- when someone looks at the building, even though each repair may not be exact, the overall effect will be that you won't pick out that there are a whole bunch of repaired areas. My other rule about colour is that it's always better to be a little bit too dark than too light. If you're too light, the repairs will stand out really obviously, but if it's a little too dark, most people's minds go "Oh, there's a little variation in the stone or maybe there's a little dirt on it". You don't pick it out as an obvious bad repair.
4. The surface texture is critical. If you get the colour right, the next thing you have to do is
get the surface texture right. It doesn't have to be real exact, but if the surface of the stone is rough in the area that you're patching, don't do a really smooth repair. It'll stand out like a sore thumb.
The accuracy of the shape is important is important, but surprisingly, for overall appearance, from what I've seen, the shape of the repair, how closely you match the profile of the original stone, is probably less important than a good colour match and a good texture match. You can be off a little bit, you can have relative amateurs doing some of these repairs and they don't get it exactly right. They're repairing a window sill, it's got a little dip in it, repairing an ornament and it's not quite right, but that is much less important in terms of the overall appearance than getting the colour and the texture correct.
A Guide to doing a small patch repair
1- Cut out all the damaged parts of the sandstone block ensuring that all areas are about 6mm deep.
2- You will need to drill holes and use plastic rawl plugs and rust proof screws to attach a galvanised mesh to the repair area.
3- Before attaching the mesh brush away any powdery residue.
4- After you have securely attached the mesh (below surface level) you will need to spray the repair area with water (to allow better adhesion).
5- Plaster on your colour matched hydraulic lime repair mortar making sure that it is pressed into all the contours.
6- When the mortar has dried a little you can spray on a little water and achieve a smoother finish by going over again with your trowel.
7- Then using a spirit level and chisel , scrape out the edges of your block to match the original.
8- Texture your new block with a stipple tool or hand texture by slashing with a chisel.
Be very carefull that the hydraulic lime repair mortar does not dry out too qiuckly or it will crack and weaken. You may need to keep the area moist by periodically spraying a fine mist of water.
* If you try to repair sandstone in coastal areas with sand and cement, the tri-calcium aluminate and sulphate will react with the chloride in the sea air to break down the sandstone *