I'm not an expert construction worker, but I worked with my father and uncle (both of whom have built a few houses) and my mother, to construct a foundation for a cabin. This documents my learning experience for one way to build a foundation.
The main picture here shows us on the foundation of the cabin, with boards laid out where we planned to put the walls. My dad is standing in the future kitchen, my uncle is lying on the future bed (boots in the future mud room), and I am in the back warming my hands on the future fireplace.
My apologies in advance for the limited information in the descriptions – I neglected this instructable for several years in the Drafts folder, so my memory is not terribly specific! A good use for this instructable is as an overall guide for the pieces and order of the process. You will need to supplement with more specific information in order to do this yourself. But the benefit to my slow post is that I can verify the foundation is sound! We have now (over the course of a few years) built a very functional and rather beautiful house on top, just as imagined.
Step 1: Siting
When choosing a site for your house, there are a few things to be conscious of:
- Is the land level? (It doesn't need to be perfect, but it's easier if it's close)
- Does the house fit?
- How much wind will the house receive? How much light?
Pick your dimensions, then measure out a rectangle on the ground. Place markers at the corners (ours were the pink buoys in the picture). Now, very carefully measure the diagonals. This requires some math: Obviously, the diagonals need to be equal (for a rectangular foundation); you need to use the Pythagorean Theorem to get the lengths for the diagonals.
Step 2: Corner Posts
At each of the corners, you will need a very solid post. It should be broad, strong, flat on top, and highly rot-resistant. We used a tarred log that washed up on the beach. Cedar also has good properties for this purpose.
Dig a hole, deep enough to bury ~2/3 of your posts, at each corner.
When you get close to depth, lay boards across from post to post and use a level to ensure that the tops will all sit evenly.
Put a bit of sand/gravel in the bottom of the hole to make sure there is drainage (so water doesn't sit on the grain of the wood and cause rot), place the logs, re-check level, and pour concrete to fix the corner posts permanently in place.
Step 3: All of the Other Posts
You'll need posts throughout your foundation. At a minimum, these need to be close enough that the ends (and ideally the centers) of your floor joists (two steps forward) always have a post to sit on. We used 4x4 posts of treated wood, and followed the same dig/measure/pour concrete process as in the previous step.
Step 4: Lay Bearers
Place bearers on top of the posts. These should be very solid lengths of treated wood (I think we used either 2x6 or 4x4s). Affix to posts.
Step 5: Floor Joists
Line up floor joists on top of the bearers. There is a specific, standard spacing– you will need to cut lengths of wood to that standard spacing and use them to line up the joists appropriately. Toenail the joists onto the bearers.
Step 6: Decking
More treated wood, now 1x6 to line the top! Cut to length, lay it down, screw it down onto the joists.
Step 7: Check Your Screws
Since this is a foundation, make sure there are no screw-heads sticking up to be in the way when you build the rest of the house. Do a final pass to make sure everything is flush.
Step 8: Foundation!
Foundations are pretty cool by themselves (this one made a lovely deck). We built up from there.
Photos in order (reference the first photo for floorplan: laying down in the bedroom, boots by the front door, kitchen where my dad is standing):
1. Foundation, on the day it was finished
2. Raising the first wall
3. Walls up, looking inside
4. Just inside the front door, looking at the bedroom and admiring the new propane lighting system (roof is on!)
5. From the outside: we added a sleeping loft with a window! It's above the kitchen window.
6. The sleeping loft, finished
7. The side facing the water has three picture windows. Here is the interior wall going on.
8. Putting in the kitchen
9. Our house, and the bay