HOW TO BUILD A DECK
HOW TO BUILD A DECK
Building a deck from timber is one of the easier DIY projects that you can undertake that will have a lasting positive impact on both your lifestyle and your home’s value. This is the first of a three-part DIY series on how to build yourself a deck.
Once you’ve decided to undertake the project, the most critical part is the planning and calculation of the timber you require for your joists and bearers in order to properly support your decking structure.
The process involved in planning for your deck involves several steps, firstly, you need to calculate the floor load width of your bearers, then select your bearer size and post spacings based on this information, and lastly select the correct size of your joist based on their required span. The use of span tables and floor load width calculations will be explained in detail further in this article, and hopefully you will finish reading with a good idea of what’s required to build a great deck for your home.
Please keep in mind as you read this guide that if you would prefer to have all of the calculations and planning work done for you, the teams in our stores at Softwoods are more than happy to do it all for you at a moments notice. If you’d like a free quote on your decking project please get in touch at any time.
In order to create your own deck, familiarity with the parts is required. Bearers are the timbers that are attached to the stumps or posts that support the deck and Joists are the timbers that are attached across the bearers to which the decking boards are in turn attached.
In addition, the concept of single and continuous span must be explained. A bearer that is considered to be single span, is fixed to stumps at each end of it’s length with nothing in between to support it. A continuous span bearer is one that is supported at more than two points along it’s length. For example, a deck that is 1 metre long and 1 metre wide is likely to have only two bearers, one either side (running the length of the deck) and will be considered single span as each bearer will be supported only at each end. Alternatively, a deck that is 1m wide and 10m long will still have two bearers each running the length of the structure, but they will need support along the 10m length from several stumps, and as such will be considered to be continuous span.
The load that you put onto your deck can at times be very large indeed, so correct calculations of the stiffness and strength of the internal timbers of your deck are essential. The flexibility of timbers and their strength is taken into account in the Australian timber framing code that is used to create span tables for both Bearers and Joists. Selecting the correct timbers and the layout of your deck is relatively simple once you are able to interpret these tables which are based on engineering calculations that are far too complicated for most non engineering types.
To correctly calculate the minimum strength and placement of the timber that you should use on your deck, you need to first establish the floor load width of your bearers, and then the spacing of your posts. The image and text below is a simple guide to the concept of floor load width.
As you can see, there are three different possible variables in a floor load width calculation.
First is bearer A, which absorbs half of the load of distance Y, as well as the overhanging distance X
Bearer B is different in that it is internal and has a bearer on either side of it. Due to this, the floor load width of bearer B is half of both distances Y and Z
Bearer C is another different calculation in that it is only bearing a load in one direction. As such, the floor load width of bearer C is half of the distance Z
In order to calculate your own floor load width you will need an idea of the size of deck that you want to build.
As a practical exercise we will choose a deck that is 10 metres long and 6 metres wide. The deck will have four bearers of continuous span running the 10 metre length of the deck. Two are on the outside of the deck, and two internally, creating three gaps of approximately 2 metres
**(the best number of bearers, posts and joists is a highly variable proposition(both in terms of value, labour and structural integrity) and can be best determined by people with lots of experience in building and designing decks)
In order to establish the required size of your bearers and the post spacings for your deck you need to calculate the floor load width of your structure. We will use the diagram below for reference.
Both Bearers A and D are only bearing a load on one side. This load is equal to 1 metre each as they are bearing half of the distance between them and the nearest bearer.
Bearers B and C are however bearing a load on either side of themselves. In both instances the floor load width is equal to the sum of half of the total span on either side of the bearer (total span = 4m floor load width = 2m) for a visual representation of the floor load width, see the sketch below
Once you have determined the floor load width of your bearers you can proceed to selecting the size of your bearers and your post spacings using a span table. At Softwoods we use only F7 Kiln dried treated pine in our decking substructure. It is possible to use timber of a lower stress grade, but we feel that in order to guarantee the long life and performance of your deck it is worth using quality materials.
Span tables are made up of variables based on the engineering within the timber framing code of Australia. There are many differing span tables available, it’s recommended that you enquire about those that relate to your area.
Please be aware that there are also many variables with regards to the engineering of decks that are raised above ground. The use of double bearers, as required in AS 1684.2 is important to note and we recommended that if your deck is to be over the height of a metre that you should contact us for a quote and to help you with your plan to ensure your safety and compliance with your local council or shire.
For the purpose of our exercise we will use a span table for Decking bearers with a continuous span. In order to select the correct bearers we need to have established our floor load width, which in our case is 2 metres (always use the largest floor load width from within your structure)
In order to use the table, select the floor load width from among the top line of the table (in this case 1200mm, 1800mm etc) our example is 2m (2000mm) and use the corresponding column (2400mm) below to select the appropriate size of bearer from the list on the left, and the post spacing measurement to match from within the column.
For example, our floor load width of 2m means that we need to use the 2400mm column to determine our bearer size and post spacing. We can select 190×45 bearers, which will require posts every 2.1 metres along our 10m long deck (resulting in a total of 6 posts per bearer) or we can use 190×70 bearers which will need a post every 2.6 metres (5 posts per bearer) As you can see it’s a case of determining what is the best and most economical, yet structurally effective combination. (**note** these calculations are based on the inclusion of a post at each end of the bearers, which is not always necessary) Please also note that the post spacing specified is a maximum of 2.1m. Although it’s not necessary to have your posts evenly spaced, you must not exceed 2.1m between any two posts. This allows you some flexibility in case there are any obstructions beneath your deck that you need to work around (the same concept of maximum spacing also applies to your joists)
Once we have our bearers and post spacings determined, we have to use another span table to decide on the correct joist size for our structure. The Joist span table below is for joists with a continuous span on a deck below 1metre in height.
To determine the correct joist required, we need to establish the spacing of our joists. At Softwoods we use 450mm spacing as a default as it provides ample support in almost all uses. We do use 400mm spacings with clear treated pine as it is slightly more flexible than our other decking materials. Please see an example of 450mm joist spacing below
With your joist spacing determined you can use the 450mm column to determine what size timber you will need for your joists. The numbers in the 450mm column refer to the distance that the joists are required to span between bearers. In our example we have a 2m span between bearers, so we need to scroll down until we find a number over 2000 and select the corresponding joist size from the list in the left hand column. As you can see on the table above, this will be a 140×35 joist. You can then determine the number of joists you will need by dividing 10 metres (10000mm) by 48.5 cm (485mm) which gives you 20.62 or 21 bearers. (**Note** 45mm thick Joists are usually preferred as it is much easier to affix decking boards that join over a joist when you have a little more space to work with)
The last thing that you need to do to calculate what you will need for your deck is to calculate the number of decking boards you will need. For our exercise we will use 90mm decking boards with a 5mm gap to allow for any movement. As our deck is 6m wide, we will need 6m (6000mm) divided by 95mm (each board and gap) which gives us 64 boards. Our deck also has to run the length of 10 metres, so we multiply 64 boards by 10 metres to give us 640 lineal metres of decking. When using random lengths of decking (as will usually be the case) an allowance of 15 to 20% of wastage is also recommended. You can also at this point calculate the number of nails you will need for your project by dividing the total lineal metres by .225. In this case we have 640 lineal metres, which divided by .225 gives us a total of 2844 nails.
Once you have calculated the layout of your deck and have purchased the appropriate timber, you can start to build the substructure of your deck. Although it may not seem like it, the substructure of your deck is the most important part of the deck and if installed correctly it will ensure that the finished decking will be stable, structurally sound and most importantly, level.
You will need to start by marking your post locations. With any luck you won’t need to excavate the site, but if you do this is your first step. Before digging anything it’s also wise to check with relevant services and authorities to make sure that you’re not about to rupture a sewer or an electrical line. If you are excavating, it makes sense to establish how far down you will have to go before you start to dig. It also makes sense to have a plan regarding the drainage of the site once you’ve built the deck, to ensure that you don’t have issues in future.
To calculate how far you will need to dig, you need to establish your joist to bearer attachment method (covered in more detail later) I’d recommend butting your joist into your bearers to remove the need to excavate the additional height of each joist. In getting your calculation exactly correct you need to leave allowances for your post shoes (we recommend that all posts be housed in shoes as this will ensure that your deck lasts for the longest possible time without being affected by pests or rotting of any sort) You will then need to add the height of your bearers and decking boards to establish the distance you need to dig (for decking that is at ground level)
If your deck is not at ground level, you can stake your site and run a string line at the level of the top of the decking boards. When you have your site ready and you’ve marked your post locations according to the plans (using a string line and measuring tape) you can dig holes for each of the footings. The hole size required for your particular footings and area will vary, so please check with your council/shire as to what’s required. Generally speaking for stirrup footings you need at least 200mm underground (and your hole needs to be another 100mm deeper than that)
tornado footings need to be 300mm underground, as well as another 100mm deep.
Please don’t take these measurements as gospel though as it’s important to ensure that your footings are adequate. When digging holes for your posts, you should taper the hole so that it looks like the bottom of a champagne cork, this will add stability to the finished footing. Once you have your footings in place, attach your posts to your post shoes. Prepare and pour concrete into holes, and sink post shoes into the wet concrete to the appropriate level. Clamp or prop each post at the correct angle so that it won’t move until the concrete dries completely. Allow the footing 24 hours to set before doing anything else.
Once the concrete has dried and your posts are set in position you will need to cut the tops of your posts to the correct height. Getting this correct is essential to a level deck. Use the string lines that you’ve set up at the desired level of the deck as a guide and cut the tops off of each post at the correct height. The post tops can be a little lower than the tops of bearers so the cut doesn’t need to be perfect. Use either a circular saw or a hand saw to do so. You will need to saw at 90 degrees to the ground, so you’ll need to be extremely careful and work safely at this point. Depending on your saw setup you’ll need to establish the safest way to complete this task without putting yourself or anyone else at risk.
If you are using a handsaw, be sure to brace the post so that you don’t cause any unwanted sideways movement of the footing. With your post tops all cut and level, you need to address how your bearers will attach to your posts. AS1684.2 1999 stipulates that decking construction requires the provision of double bearers. This means that each post requires two bearers to join it and the next post. There are a variety of acceptable options to achieve this. The first is to house a bearer on either side of the post (diagram below)
This requires that you cut a 20mm deep notch in the posts on either side to house the bearers, and bolt them both through the post. If you use this method to secure your bearers also use blocking pieces of timber at 900mm centres from each post, fixed with nails so that there is no movement of your bearers. The second method you could use is to notch the post 45mm and attach two bearers together, bolting through the bearers and posts to attach (diagram below)
If you use this method, nail the bearers together at 300mm centres to prevent any movement. The third method that you can connect with is to notch the post 20mm on each side at different levels so that one bearer is flush with the top of the post and the second has enough clearance over the post to butt into the joist and sit flush with the top of the joist. This method allows you to have a neat edge on the outside of the deck if it is visible. (diagram below)
If you are limited in clearance and excavating is difficult you can use just a post shoe to house your bearers and eliminate the posts.
Once you’ve selected your attachment method, mark your post tops and cut the appropriate notches into each post. Once complete, you can lift your bearers into position and clamp them into place, then drill and bolt them together using Cup head bolts. Once you’ve done this you can cut the ends of the bearers to size. You can reinforce the framework for your deck by using L brackets on the inside corners of the framework (or where the bearers meet the ledger (if this is the case))
The final step in building your decking substructure is to attach the joists to your bearers. Again there are a few variables at play here, which we will go through. The first step in attaching your joists should be to mark out where they will run as per your plans so that you have a clear idea of what you need to do. If you are taking your deck all the way to the wall of your home, you will need to determine the manner in which you’d like to connect the deck to the wall and how this will affect the placement of your joists.
You will need to install what is called a ledger (a piece of timber that is bolted into your wall) to which your joists will connect. Where the ledger is installed is determined by how your joists connect to your ledger. There are two generally accepted methods, firstly you can place the joist on top of the ledger and up against the wall, connecting the joist to the ledger with nails and a trip l grip bracket (image below)
(Trip L Grip Bracket)
Secondly, you can choose to butt the joist straight into the ledger and use a joist hanger bracket to fasten the joist in place with nails. (image below)
(Joist Hanger Bracket)
In connecting your joists to the bearers you can either run the joists directly over the top of the bearers or butt the joists into a bearer (if your bearers are laid out in the same manner as example 3 above) Depending on which approach you take you will either have to use trip l grip fasteners or joist hangers respectively. (images of each option displayed below)
Once you’ve connected your joists, trim the ends to size, and you are ready to install your decking boards. Please remember that if you have any unanswered questions about DIY decking we’d love to hear from you. We’d also love to be given the opportunity to quote on your decking project, regardless of where you are in Australia, we can ship a decking kit to you at great prices, designed to your exact specifications. We are also able to offer you a huge range of decking boards, Merbau, Kapur, a full range of Australian hardwoods and Clear Treated Pine.
If you missed it, or you want to review the basics of building a deck, check out the previous post on floor load width and components. Or you can move on to the third and final post in this series and learn how to install the decking boards.
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