We normally don't think much about front porch foundations because usually they are not visible. But when it comes to foundations, what you can't see can hurt you.
I'm sure you've passed a house or two where the porch was literally falling off or sorely in need of porch repairs. There could be several reasons for this but a poorly built foundation may be the major culprit.
Foundations for porches aren't that much different than foundations for homes. A foundation has to safely bear not only the weight of the porch substructure but also the roof, railings, columns, and more as well.
Consider the following:
In addition, regardless of the slope, always build to ensure water runs away from the structure. Soil structure is also important and will dictate how deep your footings need to be.
For steep slopes, you will need either piers (columns of reinforced concrete) that sit on concrete slabs deep into the ground or perhaps even an engineered foundation.
These usually consist of short columns of either concrete block or poured concrete (piers) that rest on concrete pads called footings. Often these are poured as one unit. Typically, wood posts are then anchored to the piers which in turn support beams that are attached to the posts.
IMPORTANT: Although the process and structure of building your porch's foundation is similar to that of a backyard deck, the foundation will be carrying significantly more weight and must comply with your local building codes. Therefore, it is extremely important to understand what is involved to ensure your porch will be safe and maintain its structural integrity for years to come.
Footings carry the weight of the porch directly onto the soil. Depending on your specific geographical location, footings must be a minimum distance below grade to extend beyond the frost line and be sufficient in size to spread the load. The load includes the weight of your structure (known as the dead load) and people, furniture, and snow (known as the live load).
Don't let this intimidate you. In most cases, your local building codes department will be able to tell you the minimum requirements for footings which incorporate the load factors for your area.
When building footings consider the total weight they will carry. If you are planning to install heavy objects like hot tubs, stone fireplaces, etc., be sure to address this with your codes department or a construction engineer before you pour your footings.
Dimensions of piers, footing depth along with footing size, are determined based on geographical areas. I've found that 16 inch square footings (or up to 18 inch diameter footings) that are 8 inches thick to be fairly common. Check with your local building codes department.
Footing Details: Two typical footing and pier construction techniques:
Concrete footing forms like the Quikcrete® concrete form featured below are ideal for pouring concrete footings for porch foundations. They usually come in 4 foot lengths are come in 8, 10, or 12 inches in diameter.
Like decks, piers are an essential part of porch foundations. Piers are short concrete columns on top of footings that support posts. Piers, which are normally below grade, are meant to transfer porch loads to the footings.
In the photo below, footings and piers are poured simultaneously. Note a piece of sonotube is used to form the pier.
Using a precast concrete pier (example at right) as part of your porch foundation is sometimes acceptable in geographical locations where there is little to no frost line.
Here's a novel way to install almost "instant" piers for your front porch or deck. No muss, no fuss, and you can start building the same day.
Learn about diamond pier foundation systems.
Typically, posts are usually treated 4x4s, 4x6s, or 6x6s. As a general rule, concrete and wood don't mix so I almost always try to avoid mixing the two: I avoid placing my posts directly into or onto concrete.
Rather, I use adjustable mounting hardware to keep the two apart while simultaneously ensuring a solid connection.
Concrete front porch foundations usually consist of concrete poured at grade over a base material like stone or gravel.
Porch foundations from slabs don't have to be just at ground level. Concrete slabs can be poured at any height and offer many different finishing options.
If building a porch a few feet off the ground, concrete blocks are typically used to create the form. It is then filled with stone to within four inches of the top. Concrete is then poured on top.
If the porch is considerably higher, steel channels and steel sheathing panels are installed to height. Concrete is then poured over the steel sheathing.
See our concrete flooring section to see exactly how its done.
These usually consist of a continuous poured concrete footing supporting foundation walls that support the structure. They are often used where water is an issue or on steep slopes.
Unless your porch is free-standing, you will need to attach the ledger board to your home. Like piers, ledger boards transfer the load from the structure directly onto the home's wall to which it is attached.
Just as important, they help displace the lateral load making for a substantially more stable porch.
As with deck ledger boards, there are a few planning basics that are important to note about front porch foundation ledger boards:
Local building codes normally dictate your ledger board requirements. Many porch and deck failures are the result of poor ledger board installations. Therefore, in many locations, ledger boards must not only be attached to your home as indicated above but also supported by posts and footings as shown below.
Or, in some cases, porches or decks must be built as stand-alone structures as indicated below.
So be sure to check with your local building codes department before you start construction.
Dave wrote a 14 page tutorial with tips for building a square porch or deck foundation.
When building a quality foundation, squaring it is important and this will make the rest of your project go more smoothly.
Traditional front porch floors are constructed a bit differently than decks. Whereas decking materials usually run horizontal to the home, porch flooring traditionally runs perpendicular. Therefore, it is important to note that one or more additional beams are required.
As a general rule joists that are spaced 16 inches on center can extend 1 1/2 times in feet their width in inches. Thus:
a 2x8 can span to 12 feet (8 plus 4" (half of the width) = 12 feet)
a 2x10 can span 15 feet
a 2x12 can expand out to 18 feet.
However, this can vary by geographical area and sometimes by the type (or species) of wood you are using. Be sure to check with your local codes department.
I've included graphics below to illustrate how a porch foundation is typically constructed. Depending on the length of your porch, you may need to add an additional beam to carry the joists.
You must see our porch flooring section for additional details regarding hanging joists and adding your porch decking materials.
If you are considering building or replacing your existing front porch you owe it to yourself to check out
this video courtesy of Vintage Woodworks - great must-know information!
Or watch it right here:
Putting up a porch foundation? Take a look at our porch and deck foundation pictorial to see how builders constructed a back porch foundation in six hours.
Your porch's #1 enemy isn't birds nesting, though that may be a nuisance to some. The best thing you can do to protect your porch is to prevent water from reaching it.
Before you go, take a look at these tips to help you protect your wood porch foundation.