Expert: Dan Griffin - 3/14/2009

Question
QUESTION: Hello, We want to add a concrete patio (23ft by 12ft) to our backyard attached to the house and set the posts (for the later patio cover) in the same concrete. How many posts do we need? What size? Where should we place them? Is it a good idea to drill into the foundation to join the patio to the house so that when the house shifts so will the patio? We are not personally drilling into the concrete (contractor) but we don't know if we should have the posts add into the same concrete now or add the posts on top of the patio later. We live in the Houston area.

ANSWER: Cheryle, here is some advice.  Through the years I have known so many people who want a patio.  I have always suggested installing a footing and planning on installing a cover.  The answer has almost always been, oh, no way, we just want a patio.  We will never cover it, and we sure won't enclose it.  Within 4or5 years or less, they then ask about the cover, and then how to screen it in, then how to make it a Florida room, then a den or spare bedroom or whatever.

It is refreshing to have someone at least think about the cover.  I still suggest you consider a frost depth footing tied to the house footing.  If and when you decide to close it in as habitable space, code will require the footing and it is more difficult to add after the fact.  A frost depth footing will also provide adequate support and fastening for any type roof you decide to use.  I would plan on surface mounting the columns so that you don't need to design the cover now.  Placing columns or block outs now would lock you into a single choice.  When it is time, I would consider a metal building type roof, though they can be noisy in the rain.  The whole thing can be covered with only a very simple column in the two outside corners.  Wood and shingles will require more columns and heavier beams.

Another problem to consider. . .  As a patio, the concrete should be pitched a minimum of 1/4" per foot to shed water.  When you decide to close it in for the spare bedroom, you will want to level the slab.  I can't think of anything to ease that problem at this point, it can be dealt with several ways once you head that direction.  HVAC is also a huge issue once you decide to close it in.

I know, I know ! ! ! !  You're never going to close it in, it's juat a patio.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your quick response. What is a frost depth footing? We will use wood and shingles. Are we going to need any columns against our house or just at the outer end of the patio? Surface mounting the columns...How much concrete is needed underneath the mounting to support the columns and the wood/shingle cover?

Answer
You will need to contact your local building department for your local frost depth.  Houston is 18" or less.  I would plan a 12" thick bearing pad about 16x16 for the columns if you are not doing the footing.

If your frost depth is 12", ask your contractor what he would charge to give you a 12" deep thickened edge slab with 2 #5 bars.  Then ask what he would charge for just a 4" thick slab.  The difference is the extra cost for the frost depth footing.  Good patio, sidewalk,  and slab work uses a 6" thickened edge with one #5 bar anyway.

Make sure that they saw or strike a joint at the mid point of the slab, so no concrete is over 12' without a joint.This needs to be done the same day the concrete is poured, not the next day.  Make sure they plan to cure the concrete.

You really need to design the deck before you start if you don't know how you are tying the deck into the house.  The best choice would be to cut back the existing roof and tie the patio deck back onto the house wall and making the patio roof tie directly into the house roof.  To get the proper pitch for a shingle roof, you may need to extend the patio roof far up the existing roof line.  Composition shingles require minimum 3 in 12 pitch - wood requires more, not sure on Timberline, etc.  If your patio extends 12', it will need 36" of fall minimum.  If your existing soffit is 8=9 feet tall, the porch eave would be 5-6' tall with its carrying beam lower than that.  We know this couldn't work, so the patio roof would need to go higher up the house roof.  The number of posts and the depth of the beam are subject to the weight of the roof you apply.  I would plan on 5 columns on the outside eave line to keep the carrier beam minimal and the columns small.

Shingles will create nigtmare gable sides on the roof.  YOu really should consider a standing seam roof or one of the flat roof materials.