For a pergola with about 4 to 6 feet between posts and under 3 feet from the ground, 4×4 posts will be sufficient. However, experienced contractors generally recommend 6×6 posts as 4×4 posts will be unable to carry as heavy a load as most porches require, and may warp and bend over time, causing a safety concern.
The wonder of a beautiful porch is often an extension of the homeowner’s dream but building the framework for a sturdy pergola can serve as a hurdle for some first-timers planning to DIY. The first decision to understand to be able to make all other decisions correctly about your new porch is a simple one: 4×4 or 6×6 posts? Does it make a difference? What changes between one and the other?
There are a few reasons to choose 6×6 posts over 4×4 ones, more than just safety concerns, some of which we’ll talk about.
Related Reading: Difference Between Pergolas, Ramadas, And Gazebos
Load Bearing: 4×4 versus 6×6
So, one of the main things to consider before building a pergola is how much weight it is expected to support. Obviously, you don’t want your relaxing evening out on the porch interrupted by your pergola tumbling down upon you, so it’s worth doing some calculations before you start building. How much weight a post can bear is impacted by several factors such as:
- Wood Grade
- Load Type
- Whether or not it’s Pressure Treated
- How high off the ground it is
But a 3 foot, #2 grade 4×4 can bear approximately 17,426 pounds, and its 6×6 equivalent would be able to carry 20,834 pounds, making it a 16% improvement. That’s not a huge amount, however, once you get above 3 feet, it begins to make a much bigger difference.
An 8-foot 4×4 post will be able to carry 6,468 pounds, whilst an 8-foot 6×6 post could carry 18,032. That’s a 64% difference. At 14 feet, it gets even worse, with a 4×4 post supporting 2,339 pounds and a 6×6 post 10,550 pounds. 78%.
The conclusion is that naturally, you should do the lumber calculations for your own deck yourself just to make sure, but as a general rule, for posts shorter than 3 feet, it won’t make much of a difference, but above 3 feet, use a 6×6 post.
Needless to say that whilst a properly treated 4×4 post can withstand rotting for up to 40 years, a 6×6 post, with its greater volume, does much better in this regard too.
Notching: 4×4 versus 6×6
Another factor that puts a 6×6 post over a 4×4 is the possibility of notching them. For those who don’t know, notching is when you cut a little groove into the posts of a structure so that other structural pieces of wood can be fitted in snugly. This may be done for aesthetic reasons or by being able to add items such as rim joists.
But be aware that it creates a weak point in the post. Naturally, the thicker post will survive this weak point better than a thinner one, which means that a 6×6 post will do you better if you plan to add any notching to it. Notching can be useful to offer an entire beam’s stability to hold a header, or whatever item you want to hold up.
Weather: 4×4 versus 6×6
Weather is another reason to choose 6×6 posts over 4×4. Simply put, if you live in an area with an excess of snow and ice, that puts a lot of pressure on your deck – literally! If you find yourself with a build-up of snow, then a 4×4 post might not be able to withstand the weight, which could have disastrous consequences for your porch.
Simply put 6×6 posts are more stable, especially in colder climates. The loads it can carry are much heavier, and it can make the heading for your roof much more secure. It is also
Cost: 4×4 versus 6×6
One of the biggest and most impactful factors which may make someone choose one kind over the other is the expense. In some surface-level research, 4×4 posts are around £30 ($40) whilst 6×6 start at £70 ($94). However, as many experienced contractors would recommend, the amount of benefit offered by using a 6×6 post would far outweigh the difference in price.
Again, if the pergola is higher than 3 feet tall, then 4×4 posts have a danger of bending and warping over time – or worse – in the weight of general use. In any other case aside from a 3-foot deck with 4 -6 feet between posts, 6×6 posts are by far superior.
Legislation: 4×4 versus 6×6
A small note to end this article; one of the many things which may push 6×6 over 4×4 is that depending on where you live, it may be nothing short of a legislative requirement to use 6×6 posts in your project. It is highly dependent on where you are in the world, so be sure to check your local laws.
Related Reading: What Size Beams Are Best For a Pergola?
In conclusion, for many reasons, 6×6 posts are a clear choice, with few benefits to a 4×4 post.
These advantages involve the fact that 6×6 posts can carry loads up to 78% more than 4×4 posts at 14 feet, the fact that 6×6 posts are much easier to apply notching to, that 6×6 posts are hardier against weather phenomena which add more to the load they are meant to carry such as snow and tornadoes, and may even be required by legislation depending on where you live.
Their only true disadvantage over 4×4 posts is the price, but with their stability over 4×4 posts in so many other circumstances, the only true exception to 6×6 posts being better than 4×4 is decking below 3 feet and pergolas with 4 to 6 feet between posts.