Floor systems is the topic of today’s post. It’s actually more complicated than it would appear. In a standard stick frame home, floor joists are laid down on a knee wall or a wall top plate, sheathing is put down and voila! A person has their floor in place. But it’s not quite so simple when the walls are concrete. After considerable research and thought, I settled on one method of ICF floor system for our off-grid home.
First, let me explain the dilemma we faced. We planned on having a reinforced concrete wall right up to the roof with a total height of 15 feet. Within that height of 15 feet, we needed to have a first floor set somewhere in the middle. How does one attach a floor to a concrete wall?
Floor Basics 101
Back in Maine when I built our first home, the floor consisted of a rectangular wooden frame set on concrete pillars with a carrying beam running lengthwise down the middle of the rectangle. Floor joists rested on the perimeter of the rectangle and were supported in the middle by the long carrying beam in the center of the rectangle. I spaced the floor joists every 16 inches. Simple!
Our wilderness home in Saskatchewan utilized the same concept except instead of the wooden rectangle sitting on concrete posts, we built a short pressure treated knee wall that sat on a pressure treated plank laid on a sand base. Once again a carrying beam ran down the middle. We laid floor joists every 16 inches. Then we built our first floor walls, put in another bunch of floor joists and we had our second story floor done.
Some Info on Lumber
That carrying beam in the middle is necessary because a piece of lumber can only span a certain distance. Our distance was too far for one piece of lumber so we did it in 2 pieces. Without getting too technical, there are many tables online or in books to help in determining how far a piece of lumber can safely span. Some of the factors that determine the safe span are the species of wood, moisture content, the size of the piece of lumber, and distance to span. As an example, let’s suppose I have 2 posts set 10 feet apart. Below is a pot of boiling oil. I want to put a piece of wood from one post to the other so I can safely walk across that pot of boiling oil. Do I want to walk across a 2 X 2 inch piece of balsa wood or would I feel safer with a 12 X 12 inch piece of oak?
Keep in mind, two other points. The orientation of a piece of lumber is very important. A 2 X 4 laying flat is not as strong as if the 2 X 4 were up on its edge. Another way to look at it this. If the 2 X 4 were flat, any weight on the center will bend it down considerably. Put the same piece of lumber on edge and you will have a hard time bending it down. Also keep in mind, there is rough sawn lumber which is a full 2 inches by 4 inches and then there is the standard planed 2 X 4 which is really only 1 ½ inches X 3 ½ inches. Quite a bit of difference and strength.
3 ICF Anchor Floor Systems
For the ICF house we are currently building, there are 3 systems that I considered for the floor. I could go with a ledger board and anchor bolts, Nudura plates embedded in concrete or Simpson plates embedded in concrete. Each method of anchoring floor joists have their place.
The Ledger board is a 2 X 8 or 2 X 10 that lays flat against the wall where you wish to have your floor. It is held in position or anchored by anchor bolts that go through the ledger board. A rough hole is cut in the foam ICF form and the anchor bolts are placed in the holes. When the concrete is poured, all the anchor bolts are locked in the concrete which in turn, supports the ledger board. Spacing of the anchor bolts should be determined by the local building code and the anchor bolts should be staggered above and below the centerline of the ledger board. In other words, I wouldn’t drill holes right on the center line of the ledger board. It would be stronger to drill one hole above the centerline and then the next hole below the centerline. Keep repeating that pattern. Then standard metal floor joist hangers can be used on the ledger board to hold the floor joists in place.
The next 2 systems are similar to each other. Both systems rely on cutting slots in the ICF form in order to push a metal plate through that will ultimately be surrounded with concrete and locked into the wall. I ultimately chose the Nudura System which relies on 2 parallel metal plates. With a chalk line, I marked the location where I wanted the floor to be. I cut two slots through the ICF foam and pushed the plates through. It’s a tight fit and the metal plates stay put. There are tabs on the plate which act as stops. When those tabs are positioned against the ICF form, it is in the correct position and ready for the concrete to be poured.
There is another metal plate that wraps around the end of each individual floor joist. This is applied prior to setting the joists in place.
Once I was ready to install the joists I temporarily screwed in a 1 X 4 slightly below my chalk line to help support the end of the joist. Another chalk line above was the definitive location where I wanted the top of each floor joist to reside. Since there are slight variations in lumber this was the best way for me to assure the top of each of my floor joists was in line with the next. Always remember when setting a floor joist to eye it lengthwise and put the crown up. After I was sure any crown was up, I slid the end of the joist between the plates and using special screws that come with the system, I fastened the joist to the hanger. The screws go through the metal plates into the joist and that is how joists are attached to the concrete wall.
I should also mention, the floor system serves another very important function. One cannot back fill around the outside of the house until the floor is in place. The floor acts as an internal brace to keep the walls from pushing in. It helps to stabilize the building.
Part of the engineering that needs to be done before the project is ever started is knowing what allowable spans are within code and where support beams, columns and concrete footings will be needed. Additionally, any stairwell needs to be laid out in advance.
For example, I knew that I had a span of 44 feet. No way was that possible with a standard piece of lumber. I divided that span into more manageable sections, four 11 foot chunks. I had concrete footings in the proper location for support columns. I also needed to have beam pockets in the walls at the precise location to accept several 6” X 10” X 36 ft. carrying beams. Imagine trying to build that solo. But I did!
The trick was to build my beam in stages with temporary supports. It was a tedious process but I was able to construct my carrying beam one piece at a time and overlap the seams with another piece of lumber which made the whole beam more stable. I also glued those plies together to make the whole beam stronger.
In regards to the steps, I needed to lay out the steps before building the floor since I needed to locate the stairwell opening in the proper place along with the correct headroom. The only way I could figure to do it properly was to take my framing square and actually mark out right on the ICF wall where the landing and steps would go. I used a black magic marker to mark the wall so I could visually see where the steps would be. Then I framed the floor joists and headers to accommodate the stairs. Of course, if our stair well wasn’t located along the outside wall, I would have had to devise a different method for laying out stairs and determining proper headroom.
When it came time to sheath my floor, I used tongue and groove (T&G) OSB (oriented strand board) made for flooring. Each sheet was glued down with PL Premium on every joist and then screwed down with decking screws. It is solid and that will minimize any future floor squeaks. Now I will be able to sneak around the house undetected by Johanna to snag a piece of chocolate cake!
Until next time, keep the dream alive! We wish you a great day!
Ron and Johanna
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