This is a high-level introduction to the critical factors that need to be considered when assessing the early concept and feasibility costing of new timber or timber hybrid buildings.
Be sure to review the NZ Wood Design Guide for more detailed and in-depth guidance.
Why timber
Timber can give you more bang for your buck
Getting cost-clever with timber buildings can give you more bang for your buck – economically as well as environmentally. Key to this is doing the costing early on for all aspects of each stage of a timber building’s creation – from the blueprint to move-in.
When assessing the early concept and feasibility costing of new timber or timber hybrid buildings, there are several critical factors that need to be considered. The NZ Wood Design Guides - Chapter 4.1: Costing Timber Buildings is a great resource for getting an in-depth understanding of how to do this effectively.
As the capacity of the New Zealand engineered wood supply chain increases and the number and scale of timber, and hybrid construction projects escalates, mass timber costs will normalise with economies of scale.
Early, holistic costing can help balance timber’s books
When assessing a mass timber or hybrid timber build opportunity, it’s important to consider all the factors that impact on total outturn costs as well as design and construction benefits before taking the options to the design table for project decision-making purposes.
Identifying cost parity or a low-cost premium relative to structural designs in alternative materials such as steel and reinforced concrete is generally achievable. However, this typically requires that the concept design is optimised for a mass timber option and that a comparison of total outturn costs is done – not just a simplified element-for-element substitutional cost comparison.
For example, past Quantity Surveyor experience has demonstrated that, compared to a traditional Structural Steel frame, Lightweight Timber wall frames with LVL (laminated veneer lumber) joist floors are comparable in cost.
Let’s look at what needs to be considered when assessing the early costings of creating timber or hybrid buildings.
1. Figure out the foundation financials first
At the concept stage, foundation designs typically have not been done. Hence, early QS estimates tend to be based on database information relating to “similar” previous builds. The basis of the assumptions is stated in the estimates, but these are likely to be on the conservative side and reflective of higher mass reinforced concrete of steel superstructures.
To unlock the benefits of any timber structure, it needs to be recognised that timber is approximately one-fifth the weight of concrete, so against any precast or reinforced concrete solution, the foundation loading is drastically reduced, and the foundation design and associated costing should respond accordingly.
This benefit needs Structural Engineering input to unlock and, traditionally, only gets developed later in the Design process after initial feasibility has been provided. To demonstrate a more accurate costing of the timber option, this beneficial difference needs to be estimated earlier.
Actual project costs have shown that where ground conditions are the poorest, such as TC2 or TC3 conditions (Technical Category 2 or 3), Engineered Timber quickly becomes the preferred lowest cost-effective solution.
The cases below are good examples of the cost advantages that can be achieved with the use of engineered timber where poor soil conditions exist. They also demonstrate the need to consider the potential benefits of the application of mass timber at the earliest possible stage so that this can be reflected in the estimated total outturn costs earlier.
A Christchurch project, a two storey backpackers, was initially not financially viable due to the costs associated with the proposed piling requirement in the foundations. This led the designers to explore lighter superstructures and or prefabricated options. The final solution involved much shallower timber piles, cross-laminated timber (CLT) floor and wall panels, resulting in acceptable budget costs of $2,200/m2 in 2013/2014 excluding the site-specific foundations.
Another Christchurch project, a four level retirement village, used a preloading technique of gravel, piled 3m to 4m high, until acceptable ground settlement was reached. Following removal of most of the preload gravel, a thick 300mm thick reinforced concrete ground floor was cast without requiring the use of deep concrete piling. This outcome was made possible due to the lighter weight superstructure consisting of three levels of CLT panel structural frame design.
2. Remember to factor in fire safety
There can be additional costs related to ensuring the clauses in the Building Code relating to fire safety are met.
Many of the related costs are necessary and apply to other building materials, not just timber. Some of the fire safety costs associated with meeting the required fire resistance ratings in the Building Code may include but are not limited to:
In some situations, engineered timber sections or panels may need to be larger than required for structural purposes so that the required fire resistance rating can be met. It is not uncommon for timber designs to be evaluated in an equitable way, whereby initial cost estimates are based on structural performance only. This is in common with other structural material solutions being compared. Fire requirements are typically considered under a separate section.
In situations where encapsulation is being considered as the solution to provide the required fire-resistance rating, then no increase in section sizes over and above that required for structural performance is typically necessary.
3. Sidestep the extra scaffolding to save
Using CLT panels in floors and or walls can enable reduced hire time for scaffolding. In any panelised solution, the methodology used can result in significant savings as the scaffolding costs are time-related for the hire duration. With CLT panels or other panelised approaches, it may be possible to reduce the scaffolding time.
Edge protection can, in many cases, be installed on the floor panels at ground level, then the complete panel and temporary edge protection lifted into position. The additional time-saving advantage of this is that the rapidly installed timber engineered floors can be immediately available for use by the tradespeople. This contrasts with concrete floors that require time to cure and for temporary propping to remain in place typically for 28 days.
Subsequent trades can also use internal CLT stairs without requiring external scaffolding. The use of CLT floors and roofs eliminates the need for fall netting. Overall, this has the potential to result in cost savings of up to 20% or more of the overall scaffolding costs.
Where CLT panels are of visual grade and designed to be exposed, there is potential for savings to consider in the omission of plasterboard linings and paint finishes. The gain associated with the omission of plasterboard linings needs to be offset against potentially higher cost visual grade timber and the possible need to coat with a clear flame retardant finish.
Reduced labour on-site as fewer carpenters and joiners are required with panelised systems, which results in significant cost savings in labour resources.
Shorter mechanical and electrical scheduled programme time as the trades are able to fix directly to exposed timber substrates and commence service fixing immediately after the panel walls and floors are fitted.
Pre-fabricated CLT wall panels can have openings for doors and windows machined into them prior to delivery to the site. This enables windows and doors to be sourced early with sizings taken directly from drawings. This is made possible by the very accurate offsite CNC machining of panels, and this reduces lead-in time to order windows, which traditionally can only be commenced following site measurements. This has huge programming advantages, in time and cost savings, which should be included in total outturn cost estimates.
Many factors have the potential to reduce programme time and in turn, provide potential savings in reduced P&G costs, compared to traditional construction in steel and reinforced concrete. The speed of mass timber construction is a critical benefit and advantage to the project's critical path and can potentially provide 15% P&G cost savings.
The benefit to the client of being able to occupy, rent or sell their building sooner, greatly increases financial return on their investment. This project financing benefit should be factored into early comparative costings with other material solutions as it’s a direct cost reduction to the client.
Generally, a smaller crane capacity is required due to the timber weight being far less than precast concrete and steel members and panels.
Less wastage is experienced with prefabricated products, and with fewer skips required, it reduces wastage on site and reduces wastage removal costs.
Note that panel size optimisation can result in significant savings, which may be achieved in the fine-tuning of the design. Designers need to get familiar with each supplier’s manufactured sizes and then optimise the usage to avoid wastage. Architects may be keen to cut elaborate features, and whilst this is possible, the implication of cutting time and factory logistics can result in increased costs.
