Post and beam shed foundations are made by using wooden posts to support the foundation beams around the perimeter of the shed. The floor joists are then connected to these beams.
This type of foundation is great for building a shed on uneven or sloping ground, as the posts are of varying lengths to ensure that the floor joists are level, whatever the ground below may be. The posts are supported by pouring concrete into the hole around them or perhaps tamped gravel.
There are several ways to construct these popular post and beam shed foundations. Besides pouring concrete or gravel into the holes around the posts, some people prefer to use 8 x 8 or 12 x 12 concrete blocks.
The first block is set into the ground at least partially, while another block or two is mortared on top of the first block. The post is set into the center hole of the blocks, then concrete is poured into the openings of the blocks around the post until the blocks are full.
The post and beam shed foundation is one that is usually accepted by building authorities, even those in colder climates where frost heave is a problem. It’s a sturdy foundation type and fulfills the requirements of most building codes and HOA regulations. Besides enabling building on sloping ground, the concrete around the posts will withstand any soil erosion that may happen over the years.
I have answered the most common questions on building a post and beam foundation for your shed first, be sure to scroll down to get answers before you begin construction. I have also added a complete step-by-step guide for installing a post and beam foundation.
How Many Posts Do I Need for a Shed Foundation?
This will depend on the size of the post and beam shed you plan to build. Commonly, posts are spaced about eight to ten feet apart. If you plan to store heavy items such as lawn tractors or you plan to use the shed as workspace housing some heavy woodworking equipment, you might consider having more posts and spacing them more closely.
Also, the size of the shed will dictate the spacing. For instance, an 8 x 12 shed usually features three posts on the long sides, which means that they are six feet apart. The size of your beams is important. For instance, a 2 x8 usually can span a maximum of eight feet without support.
You’ll want to also take your desired joist spacing into consideration. It makes for a sturdier construction if you plan to have some of your joist’s rest above your posts. Some people space the joists at 20 to 24 inches apart, but the farther the distance between joists, the less sturdy the floor will be. Again, your planned use for the shed figures into consideration of your joist spacing. A common joist spacing for a shed storing heavy items is 16 inches. Get more information on joist spacing in my article What Shed Floor Joist Spacing Should I Use?
How Deep Should Post Footings Be for a Shed?
First of all, you need to check with your local building code authority to find out what specifications they might have. If your area has regulations as to putting footings below the frost line, the code enforcement office can tell you how deep they need to be. They may also have regulations on the diameter of your holes or the size of your posts.
If you don’t have any regulations, it’s up to you. Just remember that the deeper and larger the footings, the more solid they will be. It’s much wiser to perhaps go a bit overboard on the foundation than to find out a few years down the road that your shed is having stability problems.
What Size Material Do I Need for the Posts?
A post and beam shed foundation usually rests on posts ranging from 4 x 4 inches in size to 8 x 8. The larger size is often formed of two posts together. Remember that your floor joists will be connected to these beams, so you want this part of the foundation to be able to do the job.
What Size Material Do I Need for the Beams?
The beams that will form the perimeter of your shed foundation are typically 2 x 8s. Many people prefer to use two together for added strength.
There is another way of forming the foundation, involving using 4 x 4 or 6 x 6 skids inset a bit from the outside of the frame. These skids are what is supported by the posts. If you are considering this type of foundation, be sure to check with your building code office first, as some places won’t approve this type of construction.
Do I Need Pressure-Treated Wood for the Posts?
It’s always a good idea to use pressure-treated wood for the posts. If you want to use gravel to support your posts in the holes, you must use pressure-treated wood posts. No matter how firmly your gravel is tamped down, there will be spaces for water to seep through.
Also remember that termites tend to build tunnels out of the mud to hide them while they travel over something like your concrete to get to what they really want, namely your untreated wooden posts. The posts will be subject to some weather, such as heavy rains splashing up onto the wood, and humidity.
It just doesn’t make sense to use untreated wood for the posts. Do check with your supplier as to the proper treatment for the use you plan to make of the posts, as there are different levels of treatment.
Do I Need Pressure-Treated Wood for the Beams?
Since your beams will be subject to humidity and perhaps rain splashes and freezing temperatures that the interior wood of your shed will not be, it’s always a good idea to use pressure-treated wood for the beams as well as the posts. Any part of your foundation is not a good place to skimp or cut corners.
One caveat is that you need to buy posts with the proper amount of pressure treatment. Your lumberyard or home improvement center personnel should be able to guide you to the proper pressure treatment for what you want. Pressure-treated lumber is always tagged with the level of pressure treatment applied.
Do I Need to Set the Posts in Concrete?
There are people who prefer to backfill the postholes with gravel. Some prefer just to backfill the holes with soil or to insert the posts into metal coffee or paint cans filled with concrete sunk deep into a hole, filling up the rest with soil. This, however, is not the best idea.
Even pressure-treating wood does not ensure that it won’t deteriorate over time, so putting the posts right into the soil won’t ensure that your shed foundation is long-lasting. Being in constant contact with soil is never good for any kind of wood. Besides, the soil by itself is just not capable of resisting the applied vertical loads that come from the posts. This is why most posts are set into concrete. If your not sure if you should use concrete piers over treated wood read my article How Do You Build a Concrete Pier Foundation for a Shed? for more information.
Do I Need a Concrete Pad to Set Posts On?
You don’t need a concrete pad at all in post and beam shed construction. What you do need are concrete supports of some kind, either made of poured concrete or filled concrete blocks, sunk partially into the soil at the required depth to hold your posts. A concrete pad won’t give you the necessary load-bearing ability and stability that the concrete-filled holes will give those posts and really isn’t needed at all.
How to Build a Post and Beam Shed Foundation [step by step]
Step 1] Permits and setbacks. This is really where your project starts. If you are in an area with building codes (and most people are) you will need to go to your building authority or code enforcement office before starting any soil preparation or anything else.
You should have detailed plans to show the building authorities, and you probably will need to submit a copy of those plans if you need a building permit. Even if you don’t need a permit to build, if you plan to run any utilities such as electricity to your shed, you will need a permit.
Zoning also comes into play. You should also have a diagram showing the location of your shed along with distances to property lines, fences and other buildings. You should also show the location of any above-ground or below-ground utilities such as utility lines or gas lines. There are usually regulations governing all these things. An inspector may need to come out to actually see the proposed building site.
If you are installing a pre-built shed, you still have to provide a foundation and may need a permit for that. In addition, it doesn’t hurt to show the code enforcement officials what kind of shed you want to buy.
In some places, there are height limitations placed on shed roofs. Other areas subject to high winds may require certain types of foundations or even tie-downs. Areas prone to frost heave have their own regulations. It’s better to get everything checked out and settled before starting anything.
If you live in an area governed by an HOA, you’ll have to get their approval as well. They may have regulations or requirements that are different than the code enforcement office. For instance, you may have a building site approved by the zoning officials toward one side of your lot, but the HOA may tell you that your shed shouldn’t be visible from the street and must be located behind the house.
Getting the proper permits and approvals is vital before starting any construction. If you don’t, you can be hit with some surprisingly high fines for violations. Some are flat amounts per violation, while others are daily fines added up until the violations are fixed. If you put your shed in the wrong place, you may even be required to move it, which may require tearing it down and starting over. Get more information in my article Do You Require a Building Permit For a Storage Shed?
Step 2] Prepare a plan on paper and show the layout for the posts. This is important to show the building authorities and the HOA board. Specify the measurements and distances on your plans and the depth of your posts. Even if you won’t need a building permit, you’ll probably need approval for the location of your shed and the foundation type.
Code enforcement will want to make sure that you are building the proper foundation for your shed so that it won’t fall apart and perhaps blow into the street or some other yard.
Step 3] Get the ground ready. Stake out the location for your shed and level the soil in the space. It would be smart to dig out a few inches so that you can lay a few inches of tamped gravel underneath your shed. This will help moisture evaporate and rain flow out from under your shed. You certainly don’t want to find that puddles are standing under your shed.
Some gravel outside the perimeter of the shed extending out a foot or two will help steer water running off your roof away from the shed. You may find that the runoff will start to dig its own trench under the edge of the roof if it is allowed to keep hitting bare soil. If your ground is uneven or slopping where you want to build the shed check out my article How to Build a Shed Base on Uneven Ground.
Step 4] Determine the required depth of posts. Most building authorities require sinking the posts at least a foot below the frost line in your locality. You may end up having to dig holes at least four feet deep. In addition, you may be required to secure them with concrete rather than gravel.
Step 5] Mark out the holes. Plan out your proposed posts and mark their placement. You may want to put stakes in a little outside of the actual hole placements and mark the ground with the hole centers. The best post settings are those that use concrete forms in the holes so that a few inches of concrete rises above the ground. This keeps the wooden posts from coming in contact with rainwater.
Step 6] Dig the holes. For shallower holes, you can use a post-hole digger. For deeper holes, you may want to rent a hydraulic auger. This will especially help if you have stony ground or heavy clay soils, which are like concrete to dig through when they are dry. You’ll want to make the holes about two inches larger than the post at the corners.
In other words, the corners should be at least an inch from the edge of the hole, while the flatter parts will be farther away. If you plan to use tamped gravel in the holes instead of concrete, you’ll want even wider holes to get enough gravel in them to keep the posts steady.
Whether you plan to put the posts into the ground and pour in concrete or gravel, you’ll want to put about four to six inches of gravel as a base first. Make sure it is tamped down firmly. This creates a sturdy base for your posts and concrete.
Step 7] Install the Pressure Treated Posts. This is where a helper or two is absolutely necessary. You’ll want to insert the posts so that the tops are even, and also, they need to be kept absolutely vertical while you are pouring in concrete or gravel and tamping it down.
There’s no way somebody can hold a heavy post completely straight while shoveling in gravel and tamping it down, much less pouring in concrete. Your helper will need to hold the post until the fill is finished. You may even want to plan for bracing to keep the post straight if it’s set into concrete.
Step 8] Mix and pour the concrete or gravel. Unless your holes are not very deep, you’ll want to rent a concrete mixer rather than doing it yourself in your wheelbarrow. The mixer will keep rotating the mix, keeping it from setting up while you are working and keeping you from feeling rushed. Ask your supplier for the best concrete mix for your job. There are different types, some even with reinforcing fibers mixed in.
Figure out how much water your concrete batch will need and start by pouring half of it into the mixer. Add your concrete slowly, mixing it as you go. Add water gradually. You may not need all the water you measured, or you may find that you need a little more. The proper consistency for concrete is something like thick oatmeal. Once it’s mixed correctly, give it three to five minutes more before pouring it.
If you’re using gravel in the holes instead of concrete, pour it in two inches at a time, then tamp it down all around. It’s pretty impossible to pour it all in at once and properly get it all tamped down. You want as little airspace as possible around your posts. Any airspace will let in water which can eventually destabilize the soil, your gravel, and perhaps your post.
A post and beam shed is one of the most common shed foundations. It’s almost as sturdy as putting the shed on a concrete base. It’s also the best way to deal with sloping or uneven ground at your shed site. Check out my article What’s The Best Shed Foundation Option For You? to get more information on other options for a shed foundation.
Sure, it’s work digging those deep holes and handling the posts, but in the long run, it’s much less work than preparing for pouring a concrete pad. Look into the various ways a post and beam foundation can be built and what types are allowed in your area.