Using form tubes for posthole foundations is the most time-intensive method, and also the sturdiest. The fence can be secured solidly to the ground. It can be installed on uneven terrain and can withstand heavy winds. Both wood and metal posts are suitable.
1. Determine the location
1.1. Put in stakes and run a string around the perimeter of the proposed fence; you can also use spray paint.
1.2. Mark locations for the 4″ × 4″ treated-wood posts. Spacing, centre to centre, may be at 4′, 6′ or 8′ intervals depending on the size of the fence panels.
1.3. Use a posthole auger to dig holes 8″ to 10″ in diameter and 3′ 6″ to 4′ deep for optimum stability and to prevent the posts from loosening when frost occurs.
2. Set the foundation
2.1. Lay a bed of 0-¾ crushed stone 6″ deep at the bottom of every hole.
2.2. Insert the form tube (sonotube).
2.3. Secure the tube by screwing it to an outer frame.
2.4. Make sure the distance between form tube centres is indeed 4′, 6′ or 8′, depending on the size of the fence panels.
2.5. Insert the post and steel reinforcing bar into the tube.
2.6. Pour the premixed concrete and water into the portable mixer.
2.7. Close the lid tightly and mix for 30 seconds.
2.8. When the concrete is ready, pour it into the tubes.
2.9. Maintain the post in a vertical position, then use a post level or carpenter’s level to check that it’s plumb.
2.10. Make the concrete mound-shaped around the post; water will drain from around the post, keeping it dry.
2.11. Fill the cavity around the tube with stone dust, one inch above ground level.
2.12. Lightly water the surface in order to compact the stone dust.
2.13. Posts may be higher than the desired height; you can trim them when the fence installation is complete.