There are many options when looking to put an addition onto an existing home. Finishing of basements, conversions of attics, expanding the footprint outward laterally, or on tight urban lots, expanding upward with a second- or third-storey addition. This latter option poses unique opportunities and challenges compared to the others. Key considerations include structure of the existing dwelling, wall composition, ceiling heights, mechanical supply and roof type.
WEIGHT EXPECTATIONS
When adding a new living area above an existing one, we are adding load. A roof load (dead load) is vastly different from the load applied when we add living space, as we are adding both dead load and live load to the existing foundation and footings. Dead load is the weight of the structure itself (think lumber and sheathing), while live load constitutes the weight of the occupants and their finishes, furnishings and personal effects. Typically, for a house, dead load is 20 pounds per square foot, while live load is about 40 pounds per square foot (heavier in kitchens). As such, a bungalow that is turning into a two-storey home is almost doubling the weight it exerts atop its foundation and footings. The good thing is, for the most part, footings and foundations of bungalows were built to the same size and thus, same structural abilities as their two-storey neighbours. This often means you can top up an additional storey without having to do any foundation restructuring. In the case of expanding up from a two-storey to a three-storey, the loading changes dramatically and the verification of existing footings and the potential of additional support may be required.
OPTIMIZE THERMAL PERFORMANCE
Since load travels from top to bottom, from roof to footings through walls and floors, the composition of existing wall types can come into play when planning and designing a top-up. Solid masonry walls are great for adding to, as they function much the same as a foundation does. Wood-frame walls, or other similar wall types, require lateral bonding (such as a floor package) before being extended up, as they are more susceptible to racking. This can be limiting if you are trying to raise existing ceiling heights before adding the new floor level (such as changing from an eight- to a nine- or 10-ft. ceiling).
The difference between existing wall compositions and new wall compositions is that it can impact thermal performance between existing floor levels and the new one. This can result in different heat loss/gains, as well as varying moisture transfer properties, which can impact efficiencies, comfort and health of the occupants. An original, un-insulated, solid masonry wall will perform very differently than a new-wood stud wall with an insulated cavity or skin. Including some “house as a system” design principles can help address these potential pitfalls, or at least manage expectations. Building codes tend to change over time, and what may have been possible when the original walls were built may no longer be so today, therefore planning wall alignments and compositions can be a critical step to the success of any vertical expansion, both from a form and function perspective.
SPLIT-SYSTEM HVAC SOLUTION
Heating and cooling, as one builds up, is a key consideration. Warm air naturally rises and cool air naturally falls. As such, lower levels will often be cooler than upper levels. Forced air type systems are most affected by convection, and with mechanical systems typically located in basements, getting adequate airflow to upper levels can be challenging. Stratification of temperatures increases as you go up to two storeys, and increases even more with the addition of a third. For this reason, we strongly suggest some type of split-system for HVAC to combat physics and provide optimal control and comfort.
The most difficult thing to achieve is proper cooling of the upper levels in the summer. High heat from the sun upon rooftops and windows can add considerable solar gain in the areas most often reserved for bedrooms, which can result in uncomfortable sleeping conditions. Creating top-down cooling systems will combat the heated area as the cool air will naturally fall to the lower levels. Split-systems with newer micro-furnace technology can also offer more complete zoning, allowing people to control temperatures per floor, resulting in a more efficient and reactive system.
TOP INSULATION
In Toronto, third floor roof types (currently) must be sloped in nature. This means it is likely the living area is within a portion of the roof space. Cathedral or flat roofs are more challenging to insulate to the same levels as conventional and often end up with about half the R-value (as is required by code). For this reason, the insulation used within this roof should be maximized with the use of two-pound spray foam where possible, as it provides the highest R-value per inch, and is a perfect air seal. If you think of a toque, most of the heat gain/loss in a home occurs through the roof–so the thicker the toque, the better the performance and the greater the comfort.
When planning your own top-up addition, remember there is real value in working with a professional to design and build the space. The process is as important as the final product here, not just the price. We recommend you start your search at the relevant professional associations to explore your options, including the OAA (Architects), AATO (Architecture Technologists), and BILD/RenoMark—the home of the professional builder and renovator, to find the true industry professionals to help guide you to success.