drainage for a gazebo slab foundation

[lubos1984]

Hi guys,

Always appreciate feedback from the experts. This site has really been helpful in hearing your advice.

A client asked me to design a hexagonal gazebo with a concrete slab foundation for him. When i came for a site visit i noticed that he started some preliminary work already including building the subbase for the slab. i noticed that one end of the concrete slab closest to the house had a vertical drop of about 5.5" below existing grade (lawn grass). At the other end of the slab the grade has a vertical drop of about 4" below the slab.

With the existing grade at one end higher then the top of slab I was thinking to slope the concrete for surface drainage and also include a subdrain along the perimeter of the gazebo to divert any underground water away from the slab. However, i was wondering if the subdrain would be effective in this case since the surface runoff would flow over the slab prior to infiltrating into the ground to be collected in subdrain. The grade already drops away from the house.

See attached pic

 

[lubos1984]

another pic of design options..

 

[oldestguy]

I'd not depend n a subdrain to collect surface water, assuming the porous backfill will be the entry place for water. Grass, leaves, etc will eventually plug it. You would need open grates instead. Ideally that inlet should be outside the higher part of the slab. With pompous backfill to a perforated pipe, consider the step grading from entry to collection to avoid the backfill plugging.

 

[EireChch]

OG - how will grass leaves etc. block a sub drain, its below the soil? Wrap the perforated pipe in a filter sock.

But anyways, a perimeter sub drain is overkill in my opinion. This is a DIY job.

Just put a cross fall on the slab, and be done. If its raining youre not going to be using the Gazebo..

 

[lubos1984]

Thanks OG and Eire. This gazebo is located in a cold climate so was thinking about frost heave. I want to get water away from there as fast as possible. The grade does slope away from the house. Maybe put some river stone in the higher elevation side to allow the water to flow into the sub drain faster ? Thanks again for the insightful comments

 

[oldestguy]

If this soil is frost susceptible, (high silt content, etc.), it will heave anyhow even with no obvious water source.

 

[lubos1984]

thanks. I have a 6" drainage layer under the slab to hopefully avoid that.