DIY Decking | Part 2 – Structural Design

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This guide covers all the steps to confidently take you from the concept design you did in step 1, to having a structural decking solution that will stand the test of time & ensure your deck will feel solid underfoot. Plus, I’ll teach you a little bit about structural principles as we work through a couple of examples, hopefully so that your deck will be designed correctly, even if it isn’t the same shape or size as mine. If you have any suggestions or edits, let me know

Jump straight to…

I’d be very surprised if your decking was exactly the same as mine. Therefore, I’ll try to keep things as generic as I can and make sure I explain how to do things that apply to all decking, not just mine.

Designing Your Deck – The Structural Frame

I strongly advise producing a plan showing the structural frame, for a number of reasons:

  • It helps with ordering the correct amount of wood. At £5-7 per metre length of pressure treated structural timber, you’ll want to make sure you don’t over order.
  • This also applies to the fixings, such as joist hangers and bolts.
  • The frame is the most important part of the deck, get this wrong and you may not be supporting the decking correctly, it could be unsafe or just plain wobbly.
  • The frame dictates where the posts will go, so it’s crucial to know this accurately before the build commences.
  • It ensures you don’t install more posts than you need, reducing soil waste that you will have to dispose of and time in digging an unnecessary amount of post holes.

Now that we’ve got the benefits ironed out, it’s time for a bit of structural design. Carrying on from Part 1 – Planning & Design, we have the following decking layout:

final plan on decking showing the boards, proposed plants, lighting and a bench
This is the layout of the decking in my garden.

We’ll also look at a simple rectangular deck, seeing as this shape will be the most common for you guys:

simple top down view on rectangular decking
Simple rectangular deck

I’ll start with a helpful diagram to show the components of a deck and its frame.


This decking diagram may look a bit daunting at first, but take the time to study it. If you do, you’ll start to see patterns, such as:

  • The joists run perpendicular to the direction of the deck boards.
  • The distance between each post is no more than 2m.
  • The joists are spaced out evenly – remember that the screw heads will be visible where the deck boards are screwed to the joists. Spacing the joists equally apart keeps the screw heads looking neat.

These are all things that are crucial to the design.

a couple of decking boards with a stringline going over the top, which is lined up with the  joist below. There are four screw heads showing (two per board) that are in-line with the screw heads
Remember that the decking screws require joists beneath them, and you’ll be able to see the screw heads. So, space the joists out in regular, parallel intervals to ensure the screw heads look neat, it’ll make a huge difference to the look of the finished deck

The frame can be split into 3 parts:

  • The posts.
  • The secondary beams – beams that take the loads from other beams, these usually form the outer frame.
  • The primary beams – the “filler” beams between the main beams.

Start with the secondary beams, which is the skeleton of the frame. Take the decking layout that you produced (this is the top down view on the deck boards) and draw a line around the edge. The frame will be contained within this line, you don’t want it poking out from under the deck boards.

To be able to design a suitable decking frame, I need to teach you a little basic structural mechanics. Don’t be alarmed, it’s really easy stuff and will help ensure your decking frame will last as long as possible.

Start off with the top down view on the decking that you drew up previously. To make things as clear for you as possible, let’s do a simple example, say a rectangular shaped deck, alongside my deck, which is fairly complicated (as far as decks go). So, Here’s mine again, plus a rectangular deck:

simple top down view on rectangular decking showing a 4.062 by 2.095 metre deck with the deck boards spanning along the greater length
Simple rectangular decking top down view
final plan on decking showing the boards, proposed plants, lighting and a bench
My decking top down view

The frame design starts off with the edge pieces. So I’ll add those in now, we’ll call them perimeter joists.


top down view on the simple rectangular decking showing the outer, perimeter frame only
Simple rectangular decking showing the perimeter joists only. The outer limits of the deck boards are shown as the dashed line

Now, there are a few things to consider when adding the perimeter joists around the proposed decking, I’ll cover them now

  1. Ideally, The top boards need to overhang the edges to some degree. The amount of overhang depends on aesthetics, it’s about how you want it to look. It’s not really to do with keeping running water off the edges as there are gaps between each deck board anyway. I’ve gone for a 10mm overhang, no particular reason, just thought it would look best. Therefore, I’ve drawn a line 10mm in from all of the perimeter:


  1. Work out where you want cladding, basically on all the sides that are visible


Draw on the cladding, making sure its drawn at the right thickness. You’ll notice that I’m using my deck boards as cladding also, makes sense and keeps things simple. My decking is 21mm thick, so I’ve drawn this thickness on the plan.


  1. Along the fence edges I will not fix cladding boards to the perimeter joists. This is because no one will be able to see these edges. Therefore, the perimeter joists will go up to the 10mm overhang.


  1. Support needs to be added to the cladding. You can fix it to the flat face of perimeter joists, like so:


On plan, it looks like this:


Notice how I’ve got curved and straight edges where my cladding goes. Straight edges are pretty straightforward, curved edges are a different beast altogether. I’ll cover how to curve wooden boards later on (insert link) , for now we’ll just stick to the design.

Also, the curved cladding is supported by joists that have been cut to a curve, the joists themselves haven’t been bent or warped. These form segments along the curve that will be securely attached to the main frame.

Next up, we can add in some posts. Decking posts spaced every 2m is a good rule of thumb. I suggest using 100mm square pressure treated softwood posts for this as a minimum. Thicker posts and those made from hardwood, such as Oak, will add longevity.

The first posts go in the corners, tucked inside the perimeter joists. Don’t worry about how to go about fixing the joists to the posts, that’ll come later, we’re just getting the layout sorted for now. On my frame layout below, I’ve added in the corner posts.


The reason you need to put posts in all the corners is so that the frame is supported fully at all edges. The closer you can get the posts to the perimeter joists the better.

Once you’ve got the corner posts sorted, it’s time to add in some more along the straight edges. For me, this was along the fences. I therefore used my fence posts to support the deck, rather than install new posts. I was happy to do this as my fence posts are formed from 125mm square Oak. In other words, they’re not going anywhere for at least 20 years. If my fence posts were 100mm square softwood, I’d have built the deck frame independently of the fence. Also, the fence posts were spaced about every 1.8m, perfectly within limits for the decking.


You’ll notice I have also added in some posts around the curved edges of the deck too.

If the width of the deck is more than 2m, consider adding in some posts within the centre of the deck. I only needed one for mine, it’s quite a small deck after all.

At this stage the post positions are not precise – we’ve still got to add all the inner joists, which may slightly alter the position of the posts anyway. It’s a little bit of trial and error with the design until it’s just right.

Next up, we can look at positioning the inner joists.

The inner joists are arranged perpendicular to the deck boards. They form the bulk of the support to the deck. As the deck is ultimately screw fixed to these joists from above, their position needs to be carefully positioned and orientated.

Picture of screws neatly aligned

With the picture above, notice how the screw heads are neatly aligned in rows, with equal spacing between rows. These screws go through the deck boards and attach to the inner joists below. If the inner joists weren’t neatly aligned, or didn’t have equal spacing between each member, then the screw head wouldn’t look aligned either.

One final consideration for the inner joists is how far apart to space each one and how far each joist should span between supports. This is a structural question and depends on a number of factors, such as the size of joists that are used and the type of wood they are made from. It also depends on the type of decking used.

My decking was finished with Yellow Balau Hardwood, which is a very stable and dense type of wood. To give you an idea, European pine is around 350-500kg’s per cubic metre, English Oak is 740kg’s and Balau is around 850kg’s. If I were to run through the structural calculations for this, I could prove that the joists could be spaced further apart for Balau deck, compared to pine deck. Don’t worry though, I’m not going to load this page with calculations, nor do I expect you to carry them out yourself. The reason for this is because I don’t think it’s necessary and I’ll explain why.

With reference to a very good manual for deck building – “Timber Decking, the Professionals’ Manual” 2nd Edition by Trada, they give span tables for all sorts of joist and deck board types. It says that for my type of deck boards, I could space out my joist as much as 400mm apart and they could span as much as 3m between supports.

Insert simple sketch of joists showing the spacing above.

Structurally, that would hold. However, it would feel “bouncy” underfoot. The joists would bend so much you could just about feel it, plus the space between the joists mean the deck boards would also feel bouncy. I placed my joists 270mm apart, and they spanned no more than 2m between supports. The decking feels reassuringly solid underfoot. Granted, it cost more in materials and took longer, but if you haven’t guessed yet, I like to do things properly and would much prefer to put in extra work and materials to ensure a quality build.

At most, I wouldn’t go any more than 300mm apart. This is based on my own experience of building decks (I’ve done several in my Landscaping job)

If you’re using a manufactured type of deck board, such as Millboard, Ecodeck, Timbertek etc (there are loads), then you’ll be able to get the structural information from the manufacturers. Millboard for example, states that joists should be no more than 400mm apart for residential and light commercial applications. I’ve built a few Millboard decks and this seems structurally fine, albeit a little bouncy.

Right then, back to the design.  As mentioned earlier, you’ll need to space out the inner joists the same distance apart from each other, so that the screw heads reflect the same uniformity as the joists below. Take the longest distance that’s perpendicular to the deck boards, for me this was XXX.


Divide this distance by 300mm, or 0.3m. You’ll likely end up with a number to quite a few decimal places. For me this was XXX. So I rounded this to XXX. Let us see how this fits on my decking plan.


Remember how I mentioned that some of the post positions (insert link) may need to be tweaked slightly? Well, this is the time to do so. If you’re happy with your inner joist positions, you’ll need to tweak the post positions to work with the joists, so we’ll do that now.


Once tweaked, you have now designed the main layout of the decking frame. If you’re still unsure at this point, ping me an email with your plan so far, a bit of a description and an explanation of why you’re stuck, and I’ll take a look for you. Email me on Just to be clear, if you haven’t made a reasonable attempt at designing the deck yourself, I’m afraid I’m not interested in doing it for you.  I’m all about self learning and pushing myself out of my comfort zone, and this website is for like-minded people. If you want to offload all the design to someone else, then employ a structural engineer to do so and pay them accordingly.

Thanks for taking the time to read through this page, I appreciate it. If you have a comment or would like to get in touch, fill out the email form below and hit subscribe, I look forward to hearing from you.

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