Breathable Stone Repair

As natural resources go, stone unarguably ranks among the most important in the UK. For centuries in many parts here, high-quality stone, thanks to its durability and versatility, was one of the most widely used building materials. Thus, in many communities, civic, ecclesiastical and industrial buildings, as well as houses, monuments and bridges, were made of stone.

From this, one can see that stone structures have over the years, played an indispensable role in defining the identity of many parts of the UK. One can go as far as arguing that concerning the cultural heritage of the UK, stone buildings remain one of the UK’s most treasured and iconic symbols. Sure, we have football and Earl Grey, both precious in their own right. But those two, just don’t come close to the regal beauty and architectural splendour you find in majestic stone houses. From Buckingham Palace to St Paul’s Cathedral and from Big Ben to Westminster Abbey, you simply cannot help but marvel at these icons that serve as reminders of our glorious past.

Types Of Building Stone

Now we must know that not all stones were created equal as some are more suited to building than others. To this end, we shall be discussing the three most widely used stones in construction. They are:

• Sandstone

Sandstone is the softest building stone used. It is one of the more popular variants of sedimentary rock and has distinct “bedding” caused by its layered formation. This means that when using sandstone, it needs to be laid with the correct alignment of its bedding, which is usually horizontal. It is relatively soft and therefore, easy to carve. It, however, decays quickly, and its porous nature means it must be kept dry at all times and allowed to dry whenever it gets wet. Famous sandstone buildings in the UK are Harlech Castle, the Sheriff’s Court buildings, Central Glasgow and Castle Gates Library.

• Granite

Granite is a durable hard stone which is usually pale or silvery grey. Unlike sandstone, granite is less porous and does not absorb water easily. This advantage, however, is negated somewhat by the fact that rain penetrates cracks and runs directly into the building instead of getting absorbed. Famous granite structures in the UK are Tower Bridge and Bell Rock Lighthouse.

• Whinstone

The term whinstone is used to describe a range of dark coloured hard, dense rocks like basalt. Due to its hardness, it is not used for detail as much as sandstone is.

Now due to its widespread use in architectural detail and its peculiar weakness (its porous nature), the focus of this piece shall be mainly on the best methods of repairing sandstone.

Causes Of Damage To Sandstone

As we have already mentioned, sandstone is particularly susceptible to damage. While weathering agents like wind, rain, frost and general atmospheric pollution are usually the main culprits in this regard, there are a few other causes which shall be discussed under this heading.

• Excessive Moisture

Due to the porous nature of sandstone, moisture from leaks in gutters, downpipes, flashings or falling mastic around windows usually permeates the stone surface. If left unchecked, this causes stone decay over time.

• Inappropriate Repairs

As surprising as this may sound, there are some repairs which worsen the condition of stonework. This usually happens when the wrong stone repair products are used. More on this shall be discussed later.

• Human Contact

Due to an increasing interest in historic structures, human contact on stonework is gradually becoming one of the reasons for stone repair. Examples are stone floors worn away by foot traffic, graffiti and damage from souvenir hunters.

• Plant Growth

The horizontal layers of sandstone make it highly susceptible to damage from plants. Small windblown seeds lodge in open joints of the stone, and corresponding root growth causes severe damage

• Rusting Cramps

When cramps used to hold stones together are not rustproof, they rust when exposed to wetting and eventually break the stone.

With this bit being tidied up, it is time to get to the real business of the day,

Sandstone Repair: What You Thought You Knew

The advent of the internet has made information on almost any subject available to anyone. This, in turn, has seen a rise in the number of people performing tasks previously meant for professionals only. A perfect example of this is the time honoured art of masonry, which has seen an increase in the number of amateur stone workers (or DIYers as our friends from across the pond call them).

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For instance, a simple Google search on how to repair sandstone would instantly provide you with information usually not privy to those outside professional circles. Your search results would probably be something like “Step 1: Clear the damaged region. Step 2: Mix the cement…”. When you come across information like this, we have only one piece of advice for you. Flee. You’ll learn why in a minute.

Cement Based Mortar For Stone Repair: To Be Or Not To Be?

Sometime around the 19th century, Portland cement was developed. The novelty of this discovery, coupled with the fact that it dried faster and was easier to work with, saw it replace traditional binders. Today, many firms still make stone repair products from cement but based on our professional experience (and research), this is far from ideal.

The reason for this is simple. Cement, due to its hardness and lack of permeability is what experts refer to as non-breathable mortar. The term non-breathable is used in masonry circles to refer to material which traps moisture and salts within the stone. During winter months (or whenever it rains in fact) the trapped water is subjected to repeated freezing and thawing, causing underlying damage to the sandstone. Eventually, the cement repair fails, leaving the sandstone in a worse off state than before. It is also important to point out that cement repairs on sandstone ruin the beauty of the stone and more often than not, end up looking even worse than the original weathered stonework.

Now with cement based stone repair products out of the way for good, we find ourselves faced with one big question? What sandstone repair products are the most effective in repairing damaged stone?

Breathable Stone Repair: The Perfect Solution

At this point, you may probably be wondering “if not cement, what else then? What are the suitable sandstone repair products one can use?” Lime based products are perhaps some of the most effective in repairing damaged stone. Before the advent of Portland cement in the 19th century, natural lime was traditionally used as a binder in mortars and plasters. And today, its popularity seems to have returned as more and more people become aware of its benefits. There are two different types of lime used in stone repair – hydraulic lime and non-hydraulic lime. Hydraulic lime usually appears in the form of bagged powder and is therefore much easier for builders as it is similar to working with cement. On the other hand, non-hydraulic lime is in putty form and thus, softer, slower to set and a bit trickier to use.

Benefits Of Lime-Based Stone Repair

When it comes to repairing damaged sandstone, few things are as effective as lime-based products. Some of the benefits are:

• It is breathable

Unlike cement, lime-based mortar is breathable. This means it is vapour permeable. So rather than retain water which eventually damages the stonework, it absorbs and evaporates moisture from surrounding masonry – thus, protecting the stonework from salt and frost damage. This quality alone shows the superiority of lime products in making sandstone repair mortar.

• Flexibility

The mortar used in repairs is meant to be softer than the surrounding masonry. This is necessary to allow for the expansion of trapped water when it freezes. Now as sandstone is notably soft, regular cement is the worst material one can use in making repairs. Sandstone repair fillers made from lime are, however, known for their low modulus of elasticity. They are incredibly flexible and allow for movement and thermal expansion.

• It is sympathetic

As we have already mentioned, lime-based mortar is by far softer than modern cement. This is another advantage as softer mortar will not wear away the surrounding masonry over time. In addition to this, you can easily reclaim bricks and stones whenever they get dismantled.

Aside from large scale commercial repairs, several home remedies exist to repair minor cracks in stone. These remedies are specifically suited to sandstone and are thus, recommended above your usual Google search results.

One of such ways is by sandstone repair resin or epoxy. These usually come with pigments to match the colour of the stone in question. Mix the pigments with the epoxy and carefully inject the mixture into the crack and leave to harden for 24 hours. Stone repair resin is particularly useful as you don’t need prior experience in stonework before knowing how to apply it.

In Conclusion

Regular cement is in its rights, a brilliant product. However, about repairing stonework, it pales quite embarrassingly, next to breathable repairs made from lime based products. This is because apart from the unique advantages already mentioned above, breathable lime-based repairs have the added benefit of being eco-friendly. They produce less carbon dioxide than cement and even go on to re-absorb same, thus leading to a reduced carbon footprint.

They also as a bonus, retain the natural beauty of the stone and go on to reveal the colour and characteristics of the aggregate used in the mixing process.