Expert: Ted Barnhill (Principal, Design45 LLC) - 4/9/2010

Question
Hi Ted,
A question from neighboring Wisconsin about a garage plan I am working on. The garage will be two story, the lower mostly in ground, built into a side hill.  An upper level will be at ground level allowing easy access to that level from the yard.  The upper level will be restricted to lawn equipment and perhaps a shop area..not motor vehicles. I am considering using a portion of the upper level as a covered, (but not enclosed)deck so there would be some exposure to rain and snow.  What are your thoughts about waterproofing the deck floor so water does not seep into the garage area below?  There would obviously be people on the deck from time to time and I am concerned about foot traffic causing problems with waterproofing efforts.  Maybe there is a product or decking system I can consider?  Thanks for your help and you're welcome for Favre!!  :)  John

Answer
John,

Yes, thanks for Favre; as a non-native Minnesotan I can watch him without mixed feelings.

You are right to be looking at your proposed deck/balcony over finished space as a tricky area.  These types of roofs can be built to stand up to foot traffic and more without leaking, but must be done properly.

No matter what roofing membrane and floor surface you choose, the substrate must be sloped 1/4" per foot to keep water moving off the deck.  This is critical for long-term durability, especially through our freeze-thaw cycles.  Achieving this slope isn't difficult but raises issues of door threshold height and thickness of roof structure, so plan it out carefully.

The best system I know for this situation is a tiled surface in a Schluter assembly.  See (http://www.schluter.com/1322.aspx) for details.

A more affordable deck can be achieved with loose-laid wood decking over an EPDM (sometimes informally called rubber) roof.  The wood deck can be pre-made wood squares in a parquet fashion, or 5/4" deck boards on wood sleepers.  Be careful to use an EPDM slipsheet or pads between the deck and roof membrane to avoid wear/punctures over time.

Another great option is 2'x2' concrete pavers on corner pads/pedestals, again over an EPDM sloped roof.  This can be done fairly economically.

By the way, you might consider engineering your upper garage for motor vehicles.  A common solution in a case like yours is to build the lower level walls with CMU or cast-in-place concrete, then cap it with precast concrete planks, which easily span the garage and carry vehicle loads.  For little additional expense this may provide you more function.

Ted Barnhill
Design 45, LLC