Mark McPherson, AIA NCARB
Design principal, McPherson Architecture
With more than 300 sunny days each year, Colorado has more days of sunshine than San Diego or Miami Beach. With that in mind, what better amenity can a building owner provide tenants than outdoor space to enjoy our great Colorado weather?
Tenants consider building amenities to be an extension of their own space, and outdoor meeting, collaboration and recreational spaces are highly sought after by tenants. A roof deck can help recruit and retain tenants and, in a highly competitive job market, building amenities can help tenants recruit staff.
In downtown Denver, where land is at a premium and outdoor space is limited, roof decks may improve the value of an existing building asset. The incremental cost of structuring a new building to support an occupied roof deck can be cost effective. For existing buildings, a roof deck can present challenges in design and engineering, but with careful analysis, those challenges can be transformed into opportunity.
Design Considerations for Existing Buildings
Structure. The existing roof structures of most buildings were designed to support snow loads, but not the live load of an occupied roof deck amenity. Often, structural analysis will find that building columns have additional load capacity, but not the roof slab itself. In that instance, a secondary structure of steel beams can be added to bear on existing columns and to support a new deck above the existing roof.
A new structural framework is supported by existing building columns.
Occupancy classification. While analyzing the structural capacities of your building, it’s important to begin a building code analysis. Roof decks are considered “assembly” spaces from a building code standpoint, and the exit requirements are calculated using a dense occupancy factor of 15. (Raised architectural planters can be excluded from the square footage calculation, and special gaming areas like a bocce ball court can be calculated at the less dense occupancy load factor of 50.)
The occupant load of a roof deck will let you determine whether existing exits are sized for required stair width, location and number of exits. Generally, the design will need two means of egress (exit stairs), unless the total occupancy exceeds 500, and then three exits are required.
Toilet room requirements. Plumbing fixture requirements as outlined in the International Building Code also must be met for the exterior spaces of an occupied roof deck. If excess fixtures exist in existing toilet rooms on the same level as the deck, or one level up or down, these can be counted toward the total required number of fixtures.
Building construction type. Most high-rise buildings in downtown business districts are construction Type 1A (noncombustible) and the buildings are sprinklered. However, noncombustible construction means that all materials used in the roof deck construction must be noncombustible, too. Deck materials like Redwood or Ipe Wood are not allowed in noncombustible construction, and, in cities like Denver, the building department has a memorandum that bans any type of combustible wood product, even if treated and formulated to meet Class A requirements.
There are deck systems available with concrete or porcelain paver tiles, which can provide the required noncombustible material, and the porcelain pavers may look like wood in both grain and texture.
Drainage. Proper drainage of the roof deck amenity is important, and a raised paver system with tiles supported by pedestals allow drainage to be unimpeded. The pavers can be removed for maintenance below and the system design allows for drainage between paver tiles.
Lighting. Lighting extends the roof deck use and provides safety. Lighting should illuminate the walking surfaces and be used to highlight plantings and special architectural features like gazebos and pergolas. A photometric study will be required in the construction drawings to demonstrate that the light is not contributing to light pollution.
Emergency lighting, fire strobes, exit lights and other emergency systems are required for roof decks. Owners also should consider installing security cameras tied to the building security station. Wireless internet access also may be provided to allow tenants to work outdoors.
Plant maintenance and irrigation. Plan for a drip irrigation system for your landscape areas and select perennial plants and shrubs for the bulk of your green spaces. Be aware that there are building code requirements for plant maintenance, but most commercial maintenance companies do a fine job of meeting the requirements.
Perform a sun study to illustrate the path of the sun during each season. Shaded structures such as gazebos and pergolas can become outdoor rooms and define meeting and collaboration areas.
Fire pits. Fire pits are allowed in most building code jurisdictions, and a tempered glass fire screen will prevent people from touching the flame. The fire pit should have an emergency shut-off switch within sight of the unit and a timed switch for users to control the unit and save energy.
A roof deck can be a great amenity for an existing office building and, with careful analysis and planning, it can add value to the asset and improve the marketability of the property. Users will make use of the deck from early spring to late fall and use it as an extension of their own tenant space. It will make the building more marketable and help tenants to recruit staff.
