Expert: Roger Zona - 5/16/2010

Question
QUESTION: Greetings;
We need to pour a 60x15 concrete slab under the deck in our backyard. It runs alongside the house foundation, and I would like to understand whether or not it should be attached (anchored) to the house foundation or not.   What would be the pros and cons?  I would appreciate your advice.

Thank you!
Habib

ANSWER: Hello Habib,

You didn't say where you are located so I will make some assumptions.  You said the slab would be under the deck.  Is the deck existing or yet to be constructed?

OK - here goes......you live in an area that has a winter and therefore freezing conditions.  This will cause the slab to move and therefore should not be attached to the house.  It may abut the foundation and there should be an expansion joint material between the slab and the house.  Also, I assume the slab is to be reinforced with wire mesh.  The slab should have control joints every 15 feet - 3 joints - since the slab will most likely move and crack in cold weather.  If you plan on constructing the deck on top of the slab, I assume you are going to suspend the deck above the slab.  If this is the case, wherever there is a column or post, you need a footing that is below the frost line.  I also assume the deck is attached to the house.

Is the concrete under the slab to be structural in nature and support the deck entirely or is its purpose to stop the growth of weeds under the deck?  If that is the case, then the slab need only be approx. 2" thick and without mesh reinforcing.  It's called a "slurry coat" and is used to stop growth of weeds, plants and insects.  It is a good idea to poison the soil before pouring a slab or slurry coat.  

If you live in a temperate zone such as Florida, there is less movement but all the other conditions apply.  Deck posts or columns don't need a footing but the posts should be pressure treated wood and concrete poured around the post in a hole.  The posts should extend approx. 2 feet into the ground.

If you can send more specific details, including location of your home, I can get more specific also.

I hope this is helpful

Roger
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---------- FOLLOW-UP ----------

QUESTION: Dear Roger,

Thank you very much for your time and effort in responding to my inquiry.  My apologies for not being too clear in my question.  We live in Arlington, State of Washington, area code 98223.   I am planning to pour a 5” inch thick slab, with number 4 rebar (0.5 inch).  The deck already exists, and is attached to the house, supported by three 8x4 pressurized posts, with each post sitting on a heavy duty column of concrete.  The deck height is 11 feet with an area of 22x18 feet.  The plan is to pour a slab down alongside the foundation of the house, which happens to be partly, under the deck, plus an added open area in the backyard.

Best regards,
Habib

Answer
Dear Habib,

I checked weather.com and see that you do get freezing temperatures in the winter.  Also, the deck is really elevated.  Pouring a concrete slab should be much easier.

5 inches thick is a little overkill.  4" is adequate using a 2500# concrete mixture.  The reinforcing should be a mat with rods 2' on center each way (bars running both directions and wired together) and #4 rebar is very good.  However, you should still provide expansion material between the slab edge and the house and, with an edging trowel, cut control joints every 15 feet.

You might want to consider having a thickened edge - say 8" high by 8" deep - along the entire exposed edge.  If you do, run 2 rebars (one above the other and separated 2") along the entire perimeter.  Then if you ever decide to screen enclose the grade patio, the slab will support the added weight.  

Be sure you remove all the sod and root material from the slab area and compact the earth very well.  Use a compaction roller or a "sheeps foot" vibrating compactor.  You can probably rent these machines.  Also I would still poison the soil to prevent termites if you have them in your area.  Don't forget, you should slope the slab slightly to allow water runoff.  I think a slope of approx. 1/8" to the foot (2" from the house to the edge) will be adequate. Use a finishing trowel to keep a smooth surface.  You can also apply concrete stain to the finished slab for a nicer color.  Don't walk on the slab for the first 3 days and wait until the 10th day to move on any furniture.  It takes at least three weeks for the concrete to reach full strength.

The job should take approx. 12 yards of concrete.  (Here in Florida that's about $1800 worth.)

I think that about covers the project.  Good luck.  I'm sure you will get much enjoyment using your new patio.

Roger
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