Old House Porch Remodeling: Building A Concrete Footing For A Porch Or Deck

By Bruce W. Maki, Editor

This article describes the procedure for making a simple square concrete footing. While the structure being supported is a porch for an old house, this footing method would work equally well for a deck. This footing is quite large (16" by 16") and was used to support a 6x6 post. Smaller posts could use a much smaller footing, perhaps 8" square or 12" square.

When this photo was taken we had just completed a repair job on the stone foundation below the porch. The porch framing had rotted and was removed a few years earlier. We used a pair of 2x6's to prop up the porch roof.

The previous owners had used these concrete blocks to support the corner of the porch deck. We continued to use the blocks for temporary support while the house foundation was repaired. But at this point the blocks were invited to leave.

Right below the cement blocks was this large (but shallow) footing. This hunk of concrete was about 24 inches in diameter. This footing was a waste of time and material. It doesn't need to be so big, but it needed to be deeper in the ground.

Here in Northern Michigan footings normally must be at least 48 inches deep to ensure that frost never gets below them and heaves them upward.

We dug the hole about 5 feet deep. First we used a shovel to excavate about 2 feet down, and then we used a pair of post-hole diggers to complete the task.

The soil on the bottom of the hole was leveled and tamped with a 4x4 post. We made a simple footing form out of 2x8 boards. The footing will be 16" x 16", and almost 8 " thick.

Technical Stuff:

This footing size is far bigger than the minimum required. We estimated the load on the porch column to be no more than 2,000 pounds. For the roof, we estimated 500 pounds of dead load (the wood and shingles) and 500 pounds of live load (one-fourth of the 5' x 13' roof area, multiplied by the 30 pound-per-square-foot snow load for this area). Then we assumed the same load for the porch floor.

This 16-inch-square footing has 1.78 square feet of bearing area. Even the lowest capacity soil (soft clay or loam) can withstand 2000 pounds per square foot (PSF). But the soil here is a sandy clay, which can support 4000 PSF or possibly more. So our footing would be able to support over 7,000 pounds safely. True, it's over-built, but the extra cost is trivial (a 60-pound bag of concrete mix).

We used a long stick to accurately position the old porch column perfectly plumb. The red numbers on the stick are from some other job and don't mean anything.

In a wheelbarrow I mixed 2 and 1/2 bags of ready-mix concrete.

Estimating Concrete Quantities:

Cured concrete weighs between 120 and 140 pounds per cubic foot. Each 60 pound bag makes about 1/2 of a cubic foot of concrete. I calculated the footing volume to be 1.2 cubic feet, so I needed a little more than 2 bags. 

I placed a couple of inches of concrete in the bottom of the  footing, and then pushed in two pieces of reinforcing bar (rebar) in an X-formation. These pieces of rebar are set about an inch above the bottom.

When the concrete form was filled, I pushed in two more pieces of rebar, just below the surface by an inch or so. Some builders simply place one set of rebar in the center of the footing (when measured vertically), which is adequate for many purposes, but does not offer the strength advantage of rebar placed near the top and bottom.

This is an anchor bolt. The L-shape keeps it locked in the concrete.

I pushed the anchor bolt (with the nut and washer removed) into the center point of the footing.

This is a concrete edging tool. Very inexpensive, less than $5. The edger is meant for creating smooth rounded edges on sidewalks and driveway slabs.

I used the edger to form a neat corner on the footing. I have never seen professional concrete contractors do this to footings. So why did we do it? Because a rounded corner is less prone to chipping and cracking. This took about 5 seconds. I had the tool nearby. What the heck?

This is a premium galvanized steel 6" post base. The hole in the center is for the anchor bolt. The post rests above the concrete footing, which keeps it drier, thus prolonging it's life.

While the concrete was still wet, I put the post base in place, so it would be level and slightly embedded in the concrete. I did not install the washer and nut until the next day, when the concrete had hardened.

Later, when we did the porch framing, we installed the 6x6 treated post.  We probably could have used a 4x4 post, but we wanted to use heavy-duty materials to ensure a long-lasting repair to this century-old house. This 6x6 cost $20 versus about $5 for a 4x4 post.

We back-filled the post hole. We poured gravel (the homeowner had purchased a dump-truck load earlier) around the footing and the post.

As we filled the hole, we maintained a layer of gravel adjacent to the post. This will help to keep water away from the wood and should make it last longer. We filled the hole about 6 inches at a time and packed the dirt and gravel with a block of wood before adding more soil.

Why are we so concerned about longevity of this treated lumber post? Because treated lumber does not last forever. I have seen treated lumber begin to decay after less than 20 years of exposure to the elements. Anything that can be done to reduce the amount of water next to the post will surely reduce the chances of the preservative being washed away.

We have seen countless charming old houses with terribly sagging porches, all because the builders in those days didn't take porches seriously, and didn't have pressure-treated lumber at their disposal. We are restoring/remodeling this house with the intention of making it stand up for at least another century, preferably two.

Another method to prolong the life of the treated wood post is to place a barrier of landscape filter fabric around the outside of the hole, and fill the inside of this tubular "sock" with gravel. The filter fabric will prevent fine particles of sand and silt from filling up the voids around the gravel, thus ensuring good drainage for many years to come.

Footings For Another Pair Of Posts:

We also dug holes for a pair of posts that will support the handrail newel posts. Since these posts will bear little load, we simply dug 4-foot-deep holes and poured some concrete in them.

We attached anchors to the 4x4 treated posts.

Each post was set into the wet concrete, and then clamped to the 2x6 rim joist that defines the edge of the porch.

Continue to the porch deck framing.

Copyright © 2001, 2005  HammerZone.com

Written March 8, 2001
Revised January 5, 2005