Fence Posts - What do I need?
Fence posts come in a wide variety of sizes and there are round ones and square ones too. Choosing the right post for the job depends on what you plan to build and I always gave my advice based on the circumstances of each job and what my experience told me would work best and why. The first question I would always ask is, will the fence posts be seen? If they are going to be hidden from view most of the time, then their aesthetic appearance probably isn't all that important, so plain square rough-sawn timber posts will be the best option. On the other hand, if the posts are in the front garden in prominent visible positions, then maybe consider a more pleasing look, with a dressed, planed and rounded edged post which will compliment the property. Some of the more interesting and attractive posts are in the decking sections of the DIY stores and timber yards so make sure to have a look in there as well as the builder's section where the general timber is stacked.
If the look isn't so important, and cost is a factor, then good quality rough-sawn timber posts are the general choice for most fencing. You want to check for timber certification and treatment to ensure the timber was sustainably sourced and that the treatment guarantees the longevity of the timber. Untreated timber won't last long outside in the ground so it's an important step to check on that before you buy.
Here are some posts to look at and you can scroll down to read more about choosing the correct post length and thickness for the job.
Post thickness and length are a choice but I always prefer to over engineer it slightly and just make absolutely sure nothing could possibly go wrong with any of my fence posts. I tend to choose a 4 inch or 100mm square post for most jobs, and if the fence was a more decorative job, then the 90mm x 90mm dressed or smooth timber posts with the nice rounded edges was what I would choose.
There are always a few exceptions to that, and if you find yourself changing out a damaged post, you will want to replace the post on a like for like basis with whatever post size is there at present, just for the sake of continuity and preserving the visual amenity of the fence.
50mm post or the 2x2 is generally used for pegging out a boundary or making a very low divider in a garden, and good for making planting boxes or bird tables and that sort of light duty work.
75mm post or 3x3 in inches is still a very popular choice of post size and does well for fences up to 3ft or 1m in height, and maybe 4ft or 1.2m in height too. These posts are widely available at longer lengths for higher fences but I always felt better using a thicker post if I was fencing above 4ft or 1.2m in height, just to cope better with the wind loading.
I'm not saying that a strong wind is going to snap a 3x3 fence post, don't get me wrong. My reason for concern is purely on the whole life of the post, and as the years go by, any post is going to be weakened over time by a variety of factors and my feeling is that a 4x4 would outlast a 3x3 by years, so for the extra cost, which isn't a huge amount, it always made more sense to me to choose the 4x4's and aim for a 30 year fence life.
Very occasionally we would be asked to erect a very high barrier fence to give privacy or security and then we would opt for 5x5 or even 6x6 posts and then space them at short intervals, but that was a rare day. You can have posts made to order for that sort of job and most yards will be happy to phone that through to the sawmill and have them to you off the next run of timber going through the mill.
If you are looking at our photos there and wondering what all that black stuff is on the foot of the posts, that's bitumen. I'll explain why we coat all of our post feet in bitumen.
When we first started building fences in 2003, we were snowed under with work from storms where fence posts had just snapped off at ground level and I got into conversation with the guys in the timber yard about that and what the main causes were, because it never used to happen as frequently as it seemed to at that time. Back then, they old guys put it down to the ban on creosote being used in domestic gardens and fence posts. The EU regulations were blamed too, saying the nasty chemicals we used to soak all of our outdoor timber in were far too dangerous, and so the formulas were watered down to make them less toxic. Fair enough I suppose, but now the timber is failing to do what it used to do, and that was to last 20-30 years in the ground. On top of that, the wood munching insects were feasting on the timber quite early on after the posts were set in the concrete and then there was the problem of water penetration and frost damage added to the mix to compound the problem.
Hence the bitumen. As you can see in the photo, we cover about a foot up from the concrete and seal off that small area where the posts would generally always fail. It always struck me how healthy the timber appeared both above and below that point too. It was like new inside the concrete and the section above there was always in very good condition too, so it seems the combination of insects, soil, moisture and of course that point being the fulcrum where the wind loading is leveraging all of that force, leaves that area vulnerable. That's why I add bitumen, to just protect and preserve that one spot and squeeze the longest life possible out of the posts, and it works.
It's messy lol, don't get it on you. Wear gloves, preferably long marigolds because that stuff is like super glue to bare skin, it's an absolute nightmare to get off. I buy cheap paint brushes from the pound stores and once the last post foot is painted up, just dump the brush in the hole. I never used to worry about ground pollution either.
The bitumen has high volatile organic compounds so it cures very quickly and sets to a stable, hard shell which can't be washed off or diluted by the rain, which is the whole point of using it. It's like painting anything else outside, to protect it and make it last longer, it's just that this particular piece of timber has a really tough job to do under some seriously harsh conditions, and bitumen is the answer, in my opinion.
It's relatively inexpensive too, used mainly for gluing roof felt to sheds or patching leaky flat roofs, so it's always generally available in the builder's yards or the big DIY stores.