How To Build A Deck

Using a can of florescent paint, mark on the ground approximately where your deck will sit.

Set a height somewhere, called the datum point. This is normally near where the door leads out onto your deck. There are different laws relating to the step down required; in our example for a non-cantilevered slatted deck the finished deck height will be the same as the finished floor height on the building (see E2/AS1 7.1.1b). Take your time setting the height correctly. This will be the crucial height referred to continuously during your build.

From your datum point, set a height. Some choose to set the height at the top of the piles (this helps when you cut the piles), some choose to set it at the top of their joists (which helps when laying joists). We will set it at the finished deck height because you can always measure down during the build; if you set the height too low it can become redundant later in the build.

Place your profiles approximately a metre outside where you deck will be. If in doubt set them further away rather than closer to the building area. When it comes to building your deck you need to be able to move freely inside your profiles; there is nothing worse than tripping over them. Drive 50×50 (or larger) pegs into the ground as shown. You want your profiles to be pretty firm and steady because they are crucial to the accuracy of everything later on.
Transfer the height you have selected off the house onto the pegs and attach the top of your profiles to those marks on the pegs.

Mark out your pile locations starting with placing one pile at each corner of the deck. For rows A & B we will place two holes in between at 1.3 metre centres. The reason for this relates to the floor load requirements for a wet in service subfloor framing requirement and the dimensions of timber commonly available (see NZS 3604:2011 section 6.4(b)). SG8 is the most commonly available stress graded timber and according to the 2kPa load requirements this can be achieved by using 140×70 (hard to get) at 1.3 metre centres or 140×90 (two pieces of 140×45 is what we will use) for our bearers.

We will place each row of piles 2.35 metres away from each other (excluding brace piles which will be explained further on). The reason for this is we require the 2kPa floor loading requirements for SG8 timber wet in service and we are planning to use joist spacings of 450mm (decking requirement). Your choice of decking dimensions determines the maximum joist spacings plus the dimensions of the joists you choose affects the span. In our example we are using commonly available VSG8 140×45 and at 450mm spacings we can span 2.35 metres maximum (see NZS 3604:2011 section 7.1(b) for other options).

Next, place the piles in the holes. We will use H5 treated 125×125 rough sawn piles (for round options see NZS3605). Our piles will be 500mm above cleared ground level. Your piles may not normally finish less than 300mm from cleared ground level; special rules allow for lower heights but you need to check with the NZS3604:2011 for additional protection from dampness. Ordinary piles can have a maximum height of 3 metres but anchor piles, cantilevered piles and braced piles have lower limits (see NZS3604:2011 section 6).

Most of our piles will be ordinary piles and require a minimum hole that is 225mm square and 225mm deep. Of that 225mm deep we need 25mm of compacted granular bedding on good ground and then 100mm thick concrete base (see NZS3604:2011 section 6 table 6.1). However we are going to need some brace piles which will be 350mm square by 450mm deep. Again you will need 100mm concrete base. We will place our brace piles at opposite corners of the deck.

You can place your piles either at the correct depth or cut them off after the concrete has set. In most cases it is difficult to get them at exactly the correct height so it is easier to cut them afterwards. After your holes have been dug and the 100mm concrete base has set, use the string lines from your profiles to place your piles in the correct line and then set the piles. In our case the exception will be our brace piles which will be off centre 82.5mm to allow for bearer connection (see bearer section for more).

After your piles have set, measure down from your selected datum line and mark your piles. In our case we measure 301mm down from our datum point i.e. 21mm down from our finished height (decking) plus 2x 140mm down (joists and bearers, except brace piles as they are 170mm down). It is often more accurate to mark one pile at each end and then using a chalk line, mark the intermediary piles. Our brace piles need to be 140mm taller than the rest of our piles to allow for bearer connection. If you have a laser level then it pays to check your heights on each row of piles to ensure accuracy. All cut piles must be treated with creosote, zinc naphthenate, TBTO(bis-(tri-n-butyltin) or TBTN (bis-(tri-n-butyltin)naphthenate) (see NZS3604:2011 section 6.4.3.3).

After your piles have been cut to the correct height, you will need to knotch your brace piles in preparation for connection to bearers. Don’t forget to treat the cut piles.

We will be using two pieces of 140×45 to make up our bearers. These need to be nailed together at 180mm centres in two rows, 45mm from each edge from alternate sides (see NZS3604:2011 2.4.4.7).

Lay your bearers in the centre of your piles, double skew nail each edge and use two wire dogs to attach to the piles. Our brace piles will require 12kN connection, so use a M12 bolt with 50x50x3mm washers (see NZS3604:2011 6.8.5 Figure 6.3).

Because we are using 100x25mm decking, fix joists to bearers at 450mm centres (see NZS3604:2011 7.4.3).

Fix each joist to the bearer using two skewed 100×3.75 mm nails (see NZS3604:2011 Fig.6.8) except joists that will be used in the bracing element (see NZS3604:2011 6.8.6.1 and 6.8.6.2).

Directly above where a brace will be attached, joists must be attached with either a 6kN or 12kN fixing. We will attach one end of a brace to the pile and require two joists in the area directly above our pile to have a minimum of 6kN fixing. The joist where the other end of the brace will be connected requires a 12kN fixing.

Due to the height of our deck we will do away with the 6kN fixing and just fix our joists with 12kN fixings. We will use 4 stainless ceiling ties to meet the 12 kN requirements. You need to check with each product manufacturer what the ratings of their products are. There is no specification in the NZS3604:2011 for the 6kN and 12kN fixings, only that the connection must meet this requirement.

You can either attach your joists cut to length or you can leave them uncut and cut them later, however it is recommended that you leave the joists longer than necessary and cut them later (we will explain why further on).

This is the point where you need to check the height of your joists very carefully. If you have your profiles still set up you can measure down from a line as you lay the joists and check that they are all the perfect height. Alternatively you could fix one joist at each end of the deck and run a string line between them to keep checking the heights of the joists. When you come to lay the decking it can be difficult to fix a sag or bulge so it’s important that you ensure all joists are at the correct height at this stage. If something is too high you can plane off a little from the bottom of the joist or if its too low pack it up with some damp-proof course.

Decks that project more than 2 metres from a building must have subfloor bracing (see NZS3604:2011 7.4.2.2). In reality you have to work this out at the beginning of your project in order to design your piles correctly. It can look a bit intimidating but work through it.

HOW TO WORK IT OUT
The rule for decks is half the bracing demand required by table 5.8 (in NZS3604:2011) for light/light/light cladding at 0 degrees roof slope and subfloor structure.
So this is 0.5 x 15 (value from table for light/light/light 0 degrees) = 7.5 (this is what bracing units you require per square metre).

In addition to this you need the multiplication factor for the soil type and earthquake zone your deck is built on. In this example we will use the worst soil class and worst earthquake zone in order to cover all situations. At the bottom of table 5.8 (in NZS3604:2011) soil class deep and very soft and earthquake zone 4 give us a multiplication factor of 1.5.

Therefore the equation now looks like this: 7.5 x 1.5.

The last part of the calculation for brace requirements is the area of your deck. In our example the deck is nearly 4 metres long and 4.8 metres deep. In order to work out the area just multiply the length by the depth i.e. 4 x 4.8 = 19.2 metres square.

Now the equation looks like this 7.5 x 1.5 x 19.2 = 216, meaning that this deck requires a minimum of 216 bracing units.

In order to work out your requirements, the rules are stated in section 5.5 of NZS3604:2011. Try to think about it like this: you will need to brace the deck in each direction to ensure that it is stable and will not wobble. The rules very simplistically are half of the bracing units required divided by how many braces you have in that one direction or 15 times the length of the external wall.

In this example, for the lines running parallel to the house we plan to have two braces in that direction (one in each corner of the deck) so that is 0.5 x 216 (half requirement) divided by 2 (how many braces we have in that line) 0.5 x 216 / 2 = 54. So we need 54 BU’s but the calculation of the length of the wall is 15 times 4 (length of deck) which equals 60. At first glance this might appear to be all we need, but in accordance with the NZS3604:2011 5.5(e), we actually must use “bracing greater than 100 BU’s” so we need at least 100 BU’s along string A and string C.

For string B and string D, which are running perpendicular to the house, we do the same 0.5 x 216 / 2 = 54. 15 x 4.8 = 72. Again we must have at least 100 BU’s so between B and D combined they need 100 BU’s.

Be careful with how many braces you intend to use and what the requirement is. For example, in our second calculation if we had planned to only have 1 brace then the equation would have been 0.5 x 216 / 1 (number of braces) then our bracing requirement would have been 108 and that would have been greater than the minimum required.

You can use 100×75 or 100×100 for bracing. 100×75 has a maximum length of 3 metres and 100×100 has a maximum length of 5 metres. Each of these provide 120 bracing units, so technically our deck could have had only 1 of these in each direction and it would have met all of bracing requirements (even in the single brace calculation) but we want to make sure our deck is never going to move so will put in a couple of pairs.

We will attach two of our 100×75 braces from brace pile to brace pile. Holes for bolts must be at least 90mm from the end of your brace and bolted with a M12 bolt and a 50x50x3 washer(see NZS3604:2011, 6.8.4). The bottom of the brace will be connected to the bottom of one brace pile between 200 and 300mm. Timber may not be within 150mm of cleared ground level without protection (see NZS3604:2011 6.8.3.5). The top of our brace will be connected to the top of a brace pile between 90mm and 150mm from the top of the pile (see NZS3604:20011, 6.8.4.3 (a)).

Our second pair of braces will be connected to the bottom of a brace pile and then onto a joist (the one that we used the 12kN fixing on). The bottom will be connected as above where we use M12 bolts and connect between 200mm and 300mm from the ground. The top will be connected to the joist not less than 50mm from its lower edge and not more than 200mm from the centre of the nearest pile. This can be slightly off set but no more than 150mm from the bottom of the brace (see NZS3604:2011, 6.8.4.3 (c)).

When you first start to lay your deck, consider where it is going to finish before you start. If you start laying your deck from the outer edge and move towards your house you might end up with a very small slither of timber being required to finish the deck. If you start from the house and work out again you might end up with a partial board being required at the outer edge. As we are leaving our joists uncut, when we get to the edge of the deck we might be able to trim the joists to match a full board width.

Start from the house and lay the first board 12mm off the wall of the house (see E2/AS1 7.1.1b).

There is no requirement specifically for fixing decking slats except for durability. If your decking is within 600mm of cleared ground level then it requires 304 stainless steel fixings (angular grooved nails minimum). Most decks would be classified as exposed areas so a minimum of galvanised steel nails would be required to meet durability standards. (see NZS3604:2011, Table 4.3 for nails and screws requirements). In our case we will use stainless steel screws. The reason being, regardless of galvanised or stainless nails, there is a tendency for deck fixings to pop up. Decking will inevitably swell up and shrink when exposed to rain and sun, nail heads provide a lot of resistance and nails will tend to pull out of joists rather than crush into the decking timber. Screws will provide a better long term outcome.

We will also lay our decking smooth side up. Except for the main access to a building which requires resistance of 0.4 in the wet (see NZS3604:2011, 7.4.4) you can use the decking smooth side up rather than grip tread up. We will use the smooth side up for two reasons: it feels better to walk on without shoes, and it is much easier to keep clean.

Prior to laying the decking it’s important to air season the timber. This is a crucial step to building a good deck. Most pressure treated timber in New Zealand is described as treated wet which means nothing happens to the wood after pressure treatment. It normally comes from a treatment facility and is stored outside at timber yards. As a result the ends tend to dry out a little but the timber in the middle can be completely saturated. When you are laying your decking all of your joins and gaps can be affected by the moisture content of the wood you are using. There is nothing worse than coming back to your new deck a couple of days after its been laid to find that gaps have opened up between boards and between end joins. Timber has to be quite dry before it can be machined into its final profile. Normally timber is very accurately machined and only alters in size due to moisture content. Once wood has been left to dry naturally for a while it will return to quite an even accurate size.

Leave a gap between boards. It is quite common that builders will lay wet decking hard up against each other, the objective is to have a gap between each board once the timber has dried. However if the decking had not been air dried you never know what you are going to end up with. If you have air dried your decking then hopefully the timber will never get as saturated as it was during treatment and in summer it shouldn’t dry out too much more. Leave a 3mm to 6mm gap between boards to allow for swelling and shrinking. Leaving a larger gap will not affect the way the decking feels underfoot but it will make it much easier to keep clean.

Fix the decking with two screws at each joist. If you are joining a run then ensure the joins land in the centre of the joist and fix each board with two screws, 15mm from the end. Leave a 2mm gap between butt joins to allow for longitudinal movement. It would be worthwhile pre-drilling the holes at the end of the boards; if the timber is going to split from the pressure of the screws (nails) then it is most likely to cause damage at the ends.

Cut your joins square. Some builders like to cut a scarf on their butt joins. When using pine it is best to avoid all mitre joins of any kind. The reason being is the long points of any cut will almost inevitably warp and distort more than the rest of the cut and you could end up with sharp points sticking up, which will eventually leave ugly gaps.

So lay your first board 12mm from the house, fix it at every joist with two fixings, leave a gap between board of up to 6mm and if you need to butt join boards leave a 2mm gap and pre-drill the holes at the end.

When laying your decking start attaching the board from one end and work your way down. Timber is not normally dead straight so when you start to lay it place the board with the bow in – that means the arch is facing towards the previous board and the free end is sticking out away from the deck. The reason is as you lay the decking it is very easy to push the decking in to get it straight rather push in the centre and pull out the end.