Precast Stairs Made Easy
Precast stairs are a value stream that is certainly in high demand in the precast market. There is a constant need for precast stair flights on most projects – commercial buildings, parkades, high rise residential and the list goes on… Stairs are an excellent complementary product for producers of flooring systems like hollow core and double tees, as well as total precast. Despite this, there seems to be a stigma about the complexity of precasting these elements, and as such, dedicated production systems for stairs are surprisingly uncommon.
Why? Some suggest it is likely due to the exacting tolerances and finishes required. Others say it is due to the perceived complexity and labor associated with forming stairs. Whatever the contributing factors may be, in many instances producers seem to shy away from producing the stairs and subcontract out the production to ‘specialist precasters’, shipping from hundreds of miles away or even resort to building their own plywood forms. Both of these scenarios result in higher costs and reduced profitability.
With the right production system and methodology, however, producing stairs can be an absolute breeze.
Adjustable steel molds have proven to be an economical and profitable investment for stair production over the long term. When selecting a forming system or mold, here are the main points to keep in mind:
1. Stairs are highly repeatableDue to accessibility compliance requirements set forth by regulatory boards such as Americans with disabilities Act ADA and International Building Codes (IBC), the range of dimensions commonly used for stairs is very limited. See here.
This factor works well for the precaster because it both standardizes and limits the range of dimensions required for stair forms.
Furthermore, it is normal that on a particular project, floor heights are standard as well, meaning that the precaster can settle into a standardized production run that requires little adjustment.
Adjusting the molds can take time and must be precise, therefore it is good practice to understand what range of dimensions will be commonly produced and select a system that supports that range of adjustability with minimal effort.
2. Production planning is key
If mold adjustment are needed this means a longer turnaround time for production. In such cases it is better to plan some redundancy into the production system to ensure steady output. This is why it is advisable to purchase at least two molds so that production can continue even if one mold is offline for adjustment. Flights with integrated landings typically require more effort and therefore more time to prepare. For ease of production and other considerations such as tolerances and structural efficiency, it is usually advisable to precast the landings separately in a dedicated mold.
3. Size matters
All stair molds are not equal and typically vary based on the size of the elements required. There are two main types of molds:
Vertical Molds (Stairs are Cast On-Edge)
Horizontal Molds (Stairs are Cast either Treads Facing Up or Down)
Although riser and tread ranges are quite limited, stair widths can vary somewhat, depending on the application. For stairwells, flights are normally between 3′-0″ and 5′-0″ wide. For vertical molds this works well and the mold is a workable depth for preparation and finishing for most stairs. It should be noted that vertical stair molds require a minimal amount of labor per casting cycle – between 4 and 6 hours. This is due to the minimal labor required once the mold is closed and ready to cast.
For flights wider than 5′-0″ it is more practical to use a horizontal mold where the width of the element is much less of a concern. Further, crane clearances are less of a concern in this orientation. Because of this orientation, these molds are typically wider and take up more space in the plant and require about 25-30% more labour because of the additional finishing required compared to vertical molds.
4. Finishing is value-added
Precast stairs are typically supplied as a pre-finished product which adds value for both the producer and end-user. Therefore the mold can play a significant part in achieving the required level of finish on the project. Normal finishes are fair-faced with or without tread or nosing details, which are easily achieved with liners or magnetic formers. The best finishes are achieved in steel molds with integrated vibration systems to eliminate entrapped air. Where architectural grade finishes are required, horizontal molds with the treads face down typically provide a superior finish. Vertical molds are also capable of high-quality finishes but will require particular attention to the vibration method to ensure that entrapped air is eliminated. For adjustable molds, particular attention must be paid to gasketing of shutters (to ensure that paste does not leak past shutters, leading to broken edges at release).
5. Training is Critical
Proper training is essential to ensure maximum return on investment. Like most new processes, although precasting stairs is not complex, skilled trades can have a challenging time using stair molds for the first time. It is therefore recommended that when buying into a new production system, allowance is made for training the production crew who will ultimately use the system. This will accelerate the production team through the learning curve and significantly improve initial results. Training is usually conducted within 1-2 business days depending on the type of production system.