The infamous 4x4 vs. 6x6 question and I’m looking for an engineering answer and not what looks good or "in my day … " or f-it and use metal.
Putting in fence posts and the fence will span 15 feet with a gate. Posts will be 10 feet long and at least 40% will be buried. Support will be gravel and sand NOT concrete. Adding to this my fence will have footers. That is they will be attached at ground level by lumber of the same size as the post so out of the ground the post structure would look like connected H’s. I will be using pressure-treated cedar for all ground contact.
Slats will be alternating with a 3/4 in gap between so wind will hit the trailing slat and be redirected. Here is what it’ll look like.
A 6x6 nominal has just under 2.5 times larger cross-section than a 4x4 cross-section. The question is with modern lumber and my design would using 6x6 make a significant structural difference over 4x4s such as the difference between failure or not in a 50 mph wind. Also, would the added cross-section significantly reduce warping if I keep my fence sealed every two years?
Just to add, I know a 10’ 6x6 is prohibitively expensive but if it does significantly improve the life of the fence (20 yrs vs 5 yrs) then it would be worth it.
I mean, yes, a 6x6 is going to make a significant structural difference, but so would a 2 foot across redwood trunk, but in both cases it seems like overkill for a simply 6 foot tall privacy fence. Of course, you may like the look of 6x6s, but you’re asking for structure and not aesthetics. If you want the structure to last longer, I’d recommend more posts to prevent sag rather than wider posts. For the same cost, you could put 2 4x4 posts per 8 foot length and you’d have a pretty solid structure. I guess you could always compromise with 4x6 posts as well. Anyway, just the opinion of a DIY-er and not a contractor.
Also as an aside, you should probably concrete them in, not just use gravel if you’re concerned about longevity. I just checked a local code and their recommendation for 80 mph winds is 5 feet between 4x4 posts or 8 feet between 4x6 posts. Also, a minimum of 10 inch diameter concrete pier around the post with 3 inches of wood sticking out the bottom into sand.
The specs aren’t real clear - will there be a gate opening 15 feet wide (two 7.5 foot gates)? If so you certainly need 6x6s and concrete. You really can’t make a gate support too stiff. The H structure is good but better yet would be if they are X braced.
I have installed a lot of gates and none of mine ever ended up needing wheels on them.
In this case the sand keeping it upright. Using bigger poles will not add significant strength. Using concrete or more poles will have much more effect.
You need to find out the local building codes which will dictate how your fence should be constructed.
Climate is very important. You need to know if there is a frost line and how deep you need to bury your posts to prevent them from moving. What type of soil are you setting them in? What are the ground conditions and surrounding area. i.e. ground water, drainage, trees/roots, etc… Is there likely to be high winds in your area?
There will most likely be a height restriction (and minimum height) so your post length will be dictated by these measurements.
What will be your frequency of the posts? What is the planned width of your gate?
I’m picturing 4 posts with a 36" gate with 2 x6’ panels.
It’ll be a 3 foot gate with two 6’ spans. Frost line is about 2.5 to 3 feet so the post will be below that at 4’. Soil leans towards clay. Going to use gravel instead of concrete because of the moisture.
But now I have a question. The Librarian mentioned the sand would be the weak link. Should I just use gravel as the fill?
Yes, make sure your wood is rated for ground contact and use end-cut preservative to seal your ends.
You’re fighting two battles, keeping water from penetrating the wood while keeping the preservatives from drying out and/or being leached away.
Capping the top of the post with a store-bought cap or even a replaceable wood square will help maintain the wood preservatives in the upper section.
Your weakest link will usually be where the rails are secured to the post, use fence rail hangers with galvanized joist hanger nails.
