After surveying the land and triple checking all the measurements we are ready to start breaking ground on the tiny house foundation. This means we are going to use an auger to drill the holes where we made our markings on the in the previous article.
How do I auger pier holes?
I would recommend renting an auger with a 12” drill bit that is attached to a rig. Augers can be very dangerous devices when operated freehand. When the auger is attached to a rolling rig it stabilizes the drilling bit making the job much safer. Make sure the auger drill bit is 4 feet long to get enough penetration into the earth. The auger can be wheeled into position and pushed down into the earth to drill the holes.
Be Careful
Augering dirt is a hard job so be prepared physically for that. Line the drill bit over the 12” circular marking you have made and push it down into the earth. Try to drill straight down. After drilling the holes double check everything with a laser level. Make sure the holes line up lengthwise and widthwise.
How do I set up piers for pouring tiny house foundation?
When drilling the holes you will notice they will be different depths depending when you hit bedrock. Go as far as you can with the drill bit into the earth. Your yard will also have an unflat topography that will be at different heights. Your goal is the have a row of piers that are in a line and are level to place the beams on. You will want to cut each pier to be the right height so the structure is level.
Prepare the hole
First tamp down the soil at the bottom of each pier hole. Make sure it is firmly compacted. Next fill the bottom of each hole with 2-4” of crushed granite. Compact the granite down thoroughly with a tamping tool. This will allow drainage under the pier and firm up the earth under each pier.
Place the pier forms
Place a pier tube in each augured hole. You may need to shave a little more to get the tube in the hole. Use a level to make sure every pier tube is sitting plumb and level in each hole. You want to identify where the highest elevation tube is and measure from that tube. I would assume the piece of land you are building on isn’t completely flat and level. This is the part where a laser level will really come in handy.
How did I cut the pier tubes level?
The highest elevation pier is the one that sits higher topographically from the other piers. This pier is important to find it will determine how low to the earth you want your house to be at its lowest point. Your yard may appear flat but theres a gradual sinking or rise that needs to be accounted for.
Choose how high off the ground you want your lowest to the ground pier to be. For me I chose 1.5 ft because I wanted adequate clearance and to avoid moisture near the ground at miniumum 1.5ft clearance. This would ensure other piers at lower elevations would be above 1.5ft
Cut the highest elevation tube first
On the highest elevation pier I measured 1.5 ft off the ground as the point where the pier tube would be cut. I then cut that tube to 1.5 and put it back in the hole. I placed the laser level on the top of cut tube and projected a level laser beam across the other tubes.
Work off this tube
You will work off this one tube for the whole tiny house foundation. It helps to do this at dusk so you can see the laser beam. Go around and mark all the pier tubes with a pencil at the laser beam. Draw the mark all round the circumference of the tube. After all the pier tubes are marked, place laser level at other marked points in the grid to double check everything.
What tool can I use to cut the pier tube?
After double and triple check your markings start cutting the other tubes. You can cut the tubes with a sawzaw or jigsaw blade. Double check as you go that the tubes are level, plum and all the cuts are level with each other. Make measurements between the two rows to make sure everything is level.
When you are done run a string line across all the tubes ensuring they are straight and level with each other. Keep checking and tweaking things until they are completely in line.
After all the tubes are level and straight pack down the dirt around each pier tube. Make sure the pier tube is really secure in the hole. Use the tamper to ensure the tubes are packed and won’t move.
How do I place rebar in tiny house foundation?
For a 12” pier form I recommend 3 pieces of rebar that run the length of the tube. I use #4 rebar to make sure the pier is strong enough. Push the rebar into the crushed gravel layer and into the ground. When you pour the concrete into the pier it will cure around the rebar pieces.
Place strong tie, wire and rebar
Simpson strong ties will go into the concrete piers to create a slot to place the beams on. I took steel wire and slid a piece into the hole on the simpson strong tie. I then took the wire and tied either end around two of the rebar pieces in the pier tube. This will attach the strong tie to the rebar when the concrete is poured. For now just let the strong time that is tied to the rebar hang over the end of the pier tube.
When you have added rebar, wire and simpson strong ties to every pier tube its now time to pour the concrete.
How much cement do I need?
| 1 Foot Deep | 2 Foot Deep | 3 Foot Deep | 4 Foot Deep | |
| 8″ Pier form | 1 bag | 2 bags | 3 bags | 4 bags |
| 12″ Pier form | 2 bags | 4 bags | 6 bags | 7 bags |
How do I pour concrete for tiny house foundation?
I mixed all my concrete with a shovel and a hose in a wheelbarrow. I only had 10 piers to pour so this method made sense but for more piers it may save time to rent a cement mixer. The advantage of having a pier and beam foundation is that it will require less concrete and less equipment to complete. Wear your respirator, gloves and long sleeves anytime you are mixing concrete.
Mixing and pouring
Each pier hole will require between 4-7 50lb bags of concrete. Slice into each bag with your shovel. Pour 4 bags in the barrow and dowse the concrete powder with water from the hose. Mix slowly and thoroughly until the contents of your barrow is the consistency of porridge. Tip the spout of barrow over into the pier tube filling the tube and surrounding the rebar.
Simpson strong tie
After the pier tube is filled to the brim place your simpson strong tie into the top of the wet concrete. Make sure to submerge the metal wire and place the strong tie in the center of the tube. Pouring concrete for a tiny house foundation is hard work!
How do I check my work?
Go down the row filling each tube with concrete and placing the simpson strong on each pier. As you work your way down the row of piers make sure you run a string line through every strong tie they are lined up straight. You can put a level on the string line to make sure the ties are creating a level surface. Raise the tie in the wet concrete to adjust up or down.
Next Row
Start down the next row of the tiny house foundation doing the same process. As you set the strong ties in the concrete run a string line widthwise to ensure both rows of piers are level. Top off any piers that need more or are sinking after pouring the concrete. Double check everything before the concrete starts to set.
Let it Set
Next step is to wait to let the concrete set. Wait 24 hours before touching the concrete. After this you want to water the piers with a hose twice a day for 5 days. Keeping the concrete wet helps continue the chemical process allows the concrete to become harder and more durable.
How do I install structural beams in a tiny house foundation?
After the concrete sets you should have piers in a row that are level. The 4×6 beams will slide right into place of the strong ties. The way I did it is that each 12 foot beam would have 2.5 piers underneath them. The two beams would butt together resting on the middle pier. If you have followed the steps properly the beam should slide right in place and provide a level surface to all other beams.
Bolted down
The beams will be bolted to the strong ties creating a strong and secure surface to frame the rest of the house on. Find the spots on the strong tie where there is a hole to insert a bolt. Mark this hole on the beam with a pen. Find a drill bit it that fits in the hole your strong tie allows. Drill into the beam creating holes for the bolts to slide into on the beam.
Estimating piers for tiny house foundation
For me I had 10 piers and 2 holes per strong tie so I needed to drill 20 holes. After drilling all the holes slide bolts through the strong ties, through the beam and out the other side of the strong tie. Tighten the nut on the bolt to connect the beam to the strong tie. Go around tightening all the bolts to the piers.
You now have a strong secure surface to frame on. The 4×6 posts will provide support for the next layers of framing.
If you haven’t invested in a nail gun yet now is the time to buy a compressor and pneumatic framing nailer. Don’t forget a hose, filter and box of nails to complete the kit.
How do you frame the floor joists for a tiny house foundation?
A floor joist is a grid platform that will allow adding subfloor on top of. The rim joist is the joist at the end that holds the cross joists. You want to start with the rim joists that will be running lengthwise, each rim joist will be 12 feet long. Start nailing 8 foot 2x10s perpendicular to the rim joist. I chose to have my rim joists be 24” on center so the 12 foot rim joist would get 6 joists nailed perpendicular starting at the edges and spacing 24” on center.
Pull the floor joists to center
Once you have nailed one side you can pull the rim joist and other joist pieces across the 2 beams to get centered. Do this for both 12 foot rim joists on the same side. Pull the other 12 joist across the beams to straighten and center it out with the other 12 foot rim joist. Use a squaring tool to make sure joists are running perfectly 90 degrees from the rim joist. Then take 12 foot rim joists on the other side and nail them to the ends of the 8 foot floor joists.
Adding rim joist on other side
Now you can nail the 1st 12 foot rim joist to the other side. I also used joist plates that reinforced the 2 sections together on either sides. If everything is correct you now have a grid floor joist. Now just line up the joist with the beams so its completely level and balanced on the beams. I toe nailed the floor joist into the 4×6 beams and used brackets to reinforce the connection.
Blocking
I also used blocking in between each floor joist to increase the strength. Blocking is placed halfway between each rim joist in the middle of the floor joist. Don’t be afraid to use nails. The more nails the strong the connection in the wood.
How did I lay the subflooring?
I chose to use 1/2 inch OSB subflooring for my subfloors. The peices come in 4’x8’ diameters so the they will line up with the 24” on center spacing. I laid the OSB sheets the opposite direction of the floor joists. The floor joists are running widthwise so I had the OSB sheets run lengthwise. Since the sheets are 4 foot wide and the house is 8 foot wide the sheets ran in 2 rows perfectly without any extra cutting.
Staggered seams
Make sure the seams don’t line up. Start one row with a half cut sheet and the other row with a whole sheet and the seems will run staggered the entire way down the row. Before you nail down a board apply a line of wood glue onto the floor joists. This will ensure the floor wont creek if any nails come loose.
Conclusion: Tiny House Foundation
If there are any gaps in the subflooring you can fill the gap with caulking. After nailing in all the subfloor pieces I now had a large flat and level surface of plywood to continue framing the house. It’s all up from here. The tiny house foundation will be the structure for the rest of the build after.
Next Step: How to Frame a Tiny House
More resources: https://homeinspectiongeeks.com/what-is-a-pier-and-beam-foundation/