Parking Garages

 Parking garages by nature have slopes in and out of the structures.  These slopes have sheet flows that must be captured.  The logical and effective way is to employ a trench drain.  While this is a fairly low speed vehicular traffic application, the trench drain must be properly sized and located to ensure that the trench drain will work properly and last a lifetime.


Sizing the drain

  • The flow can be coming off the street or a parking lot, or it can be coming from dripping cars inside the parking structure.  If the flow is coming from inside the parking structure, a 4" trench drain is all that is typically required.  If the flow is coming from the parking lot or street the flow calculations must be run to determine the required flow rate of the trench drain system.
  • If the slope is steep going into the trench drain it may be necessary to increase the size of the grate to the next size up to ensure that the water does not bypass the trench drain system.

Choosing the channel

  • This is an exterior application that has no chemical concerns.  Traffic and thermal cycles are the major concerns.  Depth of the channels and installation can be a concern on the upper decks especially when pre-stressed concrete beams are used.  If installing on an upper deck ask for a clamping collar to ease installation.
  • A material with similar thermal coefficients of expansion to the concrete encapsulating the channel is always a good idea due to large swings in temperature from night to day and season to season.  This is especially critical on the elevated levels as the ground does not insulate the drain at all.  Choose steel, fiberglass, polymer concrete, or concrete trench materials for best long term life.  
  • Make sure the trench drain has a heavy duty frame to handle the dynamic loads of vehicular traffic turning and breaking on the trench drain system.
  • The drain on the upper level needs to be sealed in order for the upper level trench to not leak to lower levels.  A watertight weld is best for metal drains while a two part epoxy should be used for sealing fiberglass, polymer concrete, or concrete trenches.

Selecting the grate material

  • The trenches will need traffic rated heavy duty grates.  In some instances the drain may also be in an area where pedestrian traffic is likely.  Slotted grates are most commonly used, but ADA compliant grates can be used where pedestrian traffic is likely.
  • For trenches that will have vehicular traffic, a H-20 rated grate that meets AASHTO 309 standards is recommended.  The speed is slow but the constant and turning and braking on the drains warrants a heavy duty grate.  The material here should be a reinforced steel grate or a ductile iron grate.  More decorative grates are possible at a higher cost.
  • Ensure that the grates are properly locked down so that they don't bounce out of the trench with turning traffic.  Locks are also recommended in many downtown areas to reduce theft.  Vandal resistant bolts can be included upon request.

Designing the layout

  • The layout for a parking garage is typically simple.  These drains almost always slope to one end where they transition to the outfall pipe.
  • Try to locate the trench drain so that it does not receive traffic turning or breaking on the grates.  This will reduce stress on the system and decrease the likelihood of future problems.