There are three main reasons why you might want to set your building on a gravel pad.
- When a building is set on block pillars (called piers), the shed is supported by piers spaced apart every few feet. For most applications, that is completely adequate. However, if you have a building that will have a large load (you are storing a car or a chest of gold), it will be to your advantage to have the skids in full contact on gravel rather than on intermittent pressure points on top of piers.
- If the ground is significantly out of level, and you want to level it out, gravel may be your leveling agent of choice.
- If you want the building sitting close to the ground, a properly installed gravel pad can lower the building a few inches.
Retaining Wall vs. Free Form
If you choose to go with a gravel pad, you have two options for building it.
- Build a retaining wall–essentially a large wooden box–and put the gravel inside that.
- Build a “free-form” pad.
Before we go through the steps to build each of these, lets see what materials you’ll need.
Gravel Pad Materials
Gravel. Use gravel (technically called “stone”) that will compact, such as 21A or 21B. There is very little difference, but David Frazier at Frazier Quarry recommends 21A (also known as “1-inch crusher run”). This stone has fine material mixed in with it, which fills up the air pockets and allows the stone to compact down so that it becomes dense and will not settle after the building is set.
Clean stone such as 57s or 58s, which have no fine material, will tend to shift more. Thus, setting a building onto a pad of 57s or 58s runs more of a risk of moving the stones around. Also, once the building is down, the stones may continue to shift, allowing the building’s skids to sink into the stones. (If you need to figure the amount of stone to order, Frazier Quarry also has a handy little project wizard.)
Timbers. If you build a retaining wall, you will need to use pressure treated 4×4 or 6×6 timbers, or creosote treated railroad ties. Do not try to use boards, such as 2×6’s, standing on edge. They will not be strong enough.
Pins. You can use rebar to pin the first course of timbers into the ground. If you need more than one course of timbers, use galvanized spikes that are long enough to go through the top timber well into the lower one.
- Stake four corners where you want your gravel pad. Set your stakes at least 1.5 feet out from every side of the building. Be sure your stakes are set square! (Hint: when you measure diagonally from corner to corner, the distance should be the same for both diagonals.) When you are finished, the length and width of your staked area should be a minimum of 3 feet longer and 3 feet wider than the footprint of your building.
- Using a level, find the lowest corner of your staked area. Starting from there, dig a trench to lay the timbers in. (Since you have allowed 1.5 feet of margin on each side of your building, the timbers will lie inside–not outside–the stakes. You will still end up with at least a foot of gravel away from each side of the building.) Use a level to make sure that the trench is perfectly level all the way around.
- Lay in the first course of timbers. Secure them by drilling holes down through them, then pounding rebar or rods through the timbers into the ground.
- As necessary, lay in additional courses of timbers. Be sure to offset the corners in log-cabin fashion. Using galvanized spikes, nail down through the corners (which when offsetting the corners, holds the structure together). Now that the corners are secure, spike all four sides every 2-3 ft, driving the spikes into the timber below.
- Now that your box is built, you have some excavating to do inside it. Dig out dirt to at least 4 inches below the top of the box. The dirt inside the box does not need to be level; your rock will make the pad level. Just make sure that the dirt is never higher than 4 inches below the top of the box.
- Before you fill with gravel, double-check to be sure everything is accurate so far. 1) The box should be set square. 2) The timbers should be perfectly level. 3) Each course of timbers should be spiked to the next course down every 2-3 ft. all around the perimeter of the box. 4) The soil inside the box should be below the top of the timbers at least 4 inches.
- Fill the box with gravel, laying down no more than 4 inches at a time. (Too much gravel at a time will not compact adequately.) Now, compact the gravel using a vibratory compacter (available from any power tool rental) or a hand tamper. Make sure the gravel is compacted fully, so that it won’t continue to settle after the building is on it.
- Make sure the gravel is completely level. Do this by using a “screed board,” such as a 2×4, to span the width of the box from one side to the other. Resting the screed on top of the sides of your box, scrape across the entire width of the box, filling in the dips and leveling off the high spots. Do not try to do this using only your eye and a rake! You will inevitably have high and low spots, which will leave the skids of the building wobbling on your pad.
- Alternate tamping and leveling off with the screed board, until the gravel is perfectly level with the sides of the box. Your new gravel pad is finished!
This gravel pad is recommended only on level ground. If you use it on a slope, you run the risk of erosion, which will cause your building to settle. (If you do choose to do free form even on a slope, be sure to allow a minimum of 18 inches of margin all around the building to allow for some erosion. Along the sides, the slope of the gravel from the top of the pad down to the ground should be at least 2 to 1: for every two feet of height of compactable gravel, the bottom of the gravel should extend 1 foot past the top. But again, because of the possibility of erosion, we don’t recommend building a free form gravel base on a slope.)
- On level ground, stake out your gravel pad site, leaving at least 1 foot on every side of your building. If you have a 12×20 building, your stakes should be 14×22. Be sure you set the stakes square!
- Remove the top 4 inches of grass and soil.
- Now you need to make temporary forms from which to screed (level) the gravel. Using long 2x4s and stakes, set the 2x4s on edge along the length of the site (end forms are optional, though not really necessary). Set the 2x4s against the sides of the excavated area. Use the stakes to establish the correct height (approximately level with the ground), making sure the 2×4’s are perfectly level.
- Fill the site with gravel.
- Using another 2×4 to span across the forms, level off the gravel. Compact the gravel using a vibratory compactor or a hand tamper. Continue to alternate screeding and compacting until the pad is fully compacted and perfectly level, without any dips or high spots.
- Remove the temporary sides, and fill in any gap with gravel.
- Don’t build a gravel pad, and then build it out of level. You’ll still end up with a block set. (One person carefully prepared a gravel bed on the side of a hill, with landscape timbers 3 high, and filled it up with gravel. Unfortunately, they made it the same slope as the hill. All that work, and the delivery guys still had to do a block set to set the building level!)
- It’s just not adequate to dump a truck load of gravel and “smear it around” with a rake. This is fine if you want a block set, but if you want the building directly on the gravel pad, it will have to be completely level.
- Don’t put too much gravel down at a time. Put down no more than a 4″ lift (or layer), then compact it. If you put down too thick a lift, the gravel will continue to settle after the building is on it.
- Don’t use pea gravel or any other type of gravel that will not compact properly.
- Finally, we hope this post will help you build your gravel pad, but it is not an engineered set of specifications. Be sure to do your own homework, and particularly be sure that you build to your local county or city code.