Our Top Tips for Working with Architectural Cast Stone
Cast stone is a great material for many building purposes. It’s cheaper and more versatile than other natural stone options and a mason is required to have extensive knowledge of architectural cast stone to create structures that will last. Precast stone can be used for a variety of different things including coping stones, window sills and decorative stones. It can be moulded into many different shapes and the production of this type of stone is complex. It is essential that the mason correctly handles the product during production and while using it to avoid damage and keep the cast stone looking great.
If you are learning how to work with cast stone, we’ve created this blog with our top tips to help you out.
What is architectural cast stone?
Cast stone is a refined architectural concrete building unit manufactured to simulate natural cut stone and is used in masonry applications. It can be used as an architectural feature, trim, ornament or facing for buildings and structures. You may find different types of cast stone made from white or grey cement, sand, crushed stone or natural gravels and mineral colouring pigments to create the ideal colour and appearance. It is still able to exceed most natural cut building stones such as limestone, brownstone, sandstone, bluestone, granite, slate, keystone, travertine and other natural stones.
Within the first year, cast stone is particularly fragile and in the first 90 days, it needs to be handled and treated properly while it cures. Cast stone handling, storage and installation must follow an extensive checklist. Once you have received the stone, it needs to be stored appropriately before it is used. For example, storage of the cast stone must be above ground on non-staining planks or pallets away from heavy construction traffic. If you plan to store the cast stone on the job site or in a warehouse for an extensive time it should be covered with non-staining tarps. Make sure the cast stone is stored safely, away from areas that could cause it damage or it could be easily knocked into. Once you are ready to use the cast stone, you should only remove the pieces you require and leave the rest safely covered.
Masons must follow the following technical requirements such as:
- Ensure climate conditions are compatible with the mortars
- Setting with full mortar joints
- Filling all dowel holes and anchor slots
- Installing flashing and damp proofing
- Using only non-corrosive anchors
- Ensuring that weep holds are installed over windows
Cast stone with projecting profiles such as columns, pilasters, entry jambs and window sills need to be protected with non-staining materials during the construction process. Corners and edges are particularly vulnerable, you should be careful not to knock into any other items and orders supplied on pallets should have polystyrene protection between each unit. Any other additional requirements are listed in the TMS 404-504-604 Standards for Architectural Cast Stone Design, Fabrication and Installation.
Once you begin the fitting process, cast stone should be handled with clean hands and avoid contact with mortar as it can easily stain the face of the stone. When fitting sills, they should be bedded at either end of each unit and pointed along the bottom edge. You should ensure that the cast stone is laid flat on even ground and never stacked or stored on top of another unit after it has been removed from its packaging.
Using pointing mix/mortar
All stone-to-stone joints will need colour-matched pointing mortar to reduce the visual appearance of joints between stone units and to create the illusion of one large piece of stone rather than multiple units together. The reason cast stone units are split into smaller pieces is to make them manageable. If they were too big it would make them too heavy and you would require machinery to lift them into place.
You’ll receive the matching pointing mix in two separate packs, one is an aggregate mix and pigmentation while the other is white cement. All you need to do is mix them at the ratio of 4 parts aggregate to 1 part cement. This kind of martyr can go off much faster than normal mortar mix, so we suggest only mixing a little at a time - just as much as you need. You’ll also need less water than a typical sand and cement mortar, so bear that in mind.
Begin with a small tub or bucket and mix the 4:1 ratio before adding your water.
Add the water gradually and mix with a small finger trowel.
Mix until the mixture is a workable paste and is slightly firm
Add tape to either side of the joint and avoid staining the surrounding stone.
Use the trowel to achieve a smooth flush finish.
Remove the masking tape to reveal your jointing.
Sealing, protecting and cleaning precast stone
Many precast stone items don’t require additional sealing, painting or protection as it has an extremely low water absorption rate. This means it won’t face frost damage or discolouration. However, it will weather over time, just like natural stone but it is dependent on the environment and how much light is received. Northerly-facing elevations and precast stones that are located next to busy roads and particular trees will weather much quicker than south-facing elevations.
To clean precast stone, use a mild detergent and hot water to remove most of the contaminants from the surface. You can pressure wash the stone, but only after the first year. However, take extreme care when using a pressure washer nearby as it can blow pointed joints and push water into the building, particularly around windows.
You can find sealants for the cast stone that will help protect it from natural weathering and reduce the need to clean stone, especially lighter colours such as Portland. You will need to check it is suitable to use by carrying out a patch test and make sure you are happy with the desirable finish. Some may leave behind a gloss-like texture which you may not be happy with.
Touch-ups and repairs
From time to time, you may experience damage on cast stone which may require a touch-up or repair. You’ll need to get this material from the cast stone supplier and the mason will include touch-ups as part of the ordinary pointing and wash-down operations before final inspection.
Repairing cast stone requires dressing and applying a fresh material to achieve the desired finish and shape and covering the repair with a wet rag to avoid water evaporating before the cement is hydrated. You should avoid repairs in cold weather or on hot sunny days. It won’t be possible to immediately colour match the repair to the original cast stone and it will appear lighter or darker based on whether dry tamp or wet cast stone was used. The matching process should take three months or maybe even a year or longer due to the curing stage. If your repair colour matches after a few days, it won’t stay that colour and in the long run, won’t match the original stone.
Shropshire Brick & Stone - Reconstituted Stone Suppliers
Do you need quality cast stone suppliers to provide you with architectural cast stone? Shropshire Brick & Stone are cast stone specialists with an extensive range and knowledge of cast stone construction materials. We design and manufacture a wide range of cast stone products including coping stones, post stones, window sills and decorative items. For more information, get in touch with our team today.