Building a Deck

I built a deck recently. Here’s how I did it. Learn from my mistakes.

Design Phase

The paved area at the back of the house has always been a bit awkward; weeds grow up through the pavers, but it’s too close to the south side of the house to bother pulling them up and planting more gardens. The house was also starting to feel a bit small once babby started crawling, so a bit of an “outdoor room” was a good cheap option.

The idea then was a low deck, extending almost along the length of the back of the house, straight out the back door, and covering most of the paved area, which takes it right to the edge of the vege garden beds. That door leads into the kitchen, so it will be a space to sit outside and drink cups of tea and look at the garden, with maybe an outdoor table in the sun as well. The little bit of height gives one a feeling of lording it over our backyard and gives a nice vantage over the strawberry patch.

Gratuitous Babby Photo

A deck consists of a couple of main features; the structure and the boards. It’s worth getting some of this language right if you’re thinking of building, as it makes it easier to talk to suppliers and order materials, and the reference materials available will make more sense as well. The picture below, from the internet, gives you an idea:

The structure includes everything below the boards; foundations, posts, bearers and joists. Posts sit on the foundations or mounting points, with bearers attached to the posts. The joists sit on top of the bearers, then the boards attach to the joists.

Contrary to what is intuitive, structures that humans use are not designed so that they don’t “break” but so that flex is managed. It is surprising how far something will flex before breaking, even something as simple as the standard 2″x4″ pine member. So a lot of these distances could be longer and members smaller, but it would flex appallingly. It’s a small thing, but aim to manage flex rather than ensuring the structure doesn’t fail catastrophically.

The distance between your joists is governed by the stiffness of your boards; the unsupported section between joists will flex if they’re too far apart. Industry standard is 450mm, which seems to be based on a standard 18mm pine board. I didn’t think too much about this as 450mm worked well with my 9m width.

The distance between your bearers is governed by the stiffness of the joists; again if the unsupported sections are too long the whole lot will flex and apart from being unnerving, flexing will loosen screws and make it squeak. As one edge of this deck was along the back of the house it made sense to use the existing brickwork to secure the bearer. Bolting to a brick wall is hard to beat for security. The other edge was 3.6m away, and without looking into it too much I decided that I would probably like to halve that distance, so the joists didn’t need to be too large. This meant 3 bearers, 1.8m apart. Then using this distance I could size the joists correctly.

In hindsight, this was a mistake. The timber I used (details of which I’ll get to) has detailed specifications available for span lengths and the appropriate beam section (it’s height and width). To ease construction I could have forgone the centre bearer altogether and sized the joists for a 3.6m span.

Alternately, as this is a low deck and the garden-facing edge will easily be seen, I could have moved the end posts in a little, to cantilever the front edge a bit. This would add a bit of drama, but also hide the bearer and posts from view.

The spacing of your posts is then governed by the size of your bearers. As mine is a pretty simple site, the number of posts could really be whatever I chose. I settled on a somewhat arbitrary 5 posts, 1.8m apart.

Materials

Building anything outside requires materials that are going to last; in wind, rain, blaring sun and changes in temperature, which can be extreme in Canberra. I considered steel for quite a while as there are some innovative flooring systems available now, like Lysaught’s Quika-Floor, but the main advantage with those systems is installation time. The materials are more expensive than timber, but the cost is recouped by reduced site time on construction projects. As my time at home isn’t really a project cost, there’s no advantage. I also think steel is a bit trickier to work with.

So timber it is. Timber is great, especially pine, as it’s very easy to work with, cutting easily with a hand saw and taking screws without pre-drilled holes up to 8 and even 10mm. This is a serious consideration when building a deck, as there are a lot of screws in 32 square metres of boards.

Previously I would have just used treated pine and been done with it, but I’ve got misgivings about it in a structural context and I think it’s pretty ugly. I’d been doing a bit of research into engineered pine products and stumbled onto Tru Pine

Generally speaking engineered timber products are not your standard “cut pieces of tree”, but are small pieces of timber, glued back together in interesting ways. Doing this improves the structural consistency of the timber, as defects can be removed and perfectly straight beams produced. This consistency means the strength can be very accurately determined, but also that construction is simple as everything is straight.

Tru Pine is an engineered plain beam, meaning the profile of each member is a regular rectangle. It is grown in FSC certified forests (which is a way of describing the sustainability of the product) cut and glued into a range of sizes, then treated with a Light Organic Solvent Preservative, to stop termites and rot. But the thing I really like about it is that it comes with span tables.

Span tables are what you use to determine what size beam is required for a certain distance, or span. The key distances when ordering your structural timber, as discussed above, are the distance between bearers (determining joist profile) and the distance between posts (determining bearer profile).

Using the link above, page 18 has spans for “Deck Joists”. I needed to span 1.8m, with a joist spacing of 450mm. The correct joist profile then is the one that allows a span of greater than 1800mm; in this case 138x42mm. As you’ll see in the tables, I could have gone all the way out to a 3.6m span by ordering a larger profile. The joists are then ordered for the full width of the deck, at 3.6m long.

Repeat for bearers. Add fasteners. Order. Our local supplier had all this material to me the next day, with free delivery.

The boards are recycled Blackbutt, mostly from houses being demolished in the local area. All hardwood (which was also used in the roof) was supplied by recycled timber specialists Thor’s Hammer. I’m a big fan of their work and extremely happy with all the wood used here.

I used gal-dipped bolts throughout the structural section; preferring bolts and nuts to attach the bearers to the posts, as this is significantly stronger than coach bolts which screw into the timber.

The post stirrups are a pretty standard hardware item, with a few different mounting configurations available.

To secure the boards to the structure I went with stainless steel screws. Stainless isn’t really a great material for screws, as it’s quite soft, meaning a lot of stripped heads while trying to drive them in. But, as this will be seen all the time, I did it for aesthetics, with a plan to use bright metal in the roof structure (which would also be visible) and match the whole lot together.

Tools

Beyond the standard things like drills and hammers a couple of specialist tools will make your life a lot easier.

Drop-saw/Mitre Saw – I wasn’t going to use one initially, but after about 6 boards I decided it was mandatory. As good as I am with a hand saw, and I use it for almost everything, it is impossible to cut two perfectly square ends on the boards so that they butt together neatly. Also, hardwood boards require a lot of work to cut, and for a deck this size there are as many as 200 cuts to be made. Spend the money or borrow one. It’s worth it.

A good screw-driving drill – I used a $50 supermarket drill that I’ve had for years, and it went “okay” but could have been a lot better. I used more than 2000 screws, and driving screws requires a lot of torque. This drill didn’t have any sort of clutch or ratchet mechanism, so each screw had to be judged by eye. That wasn’t a problem in itself, as a clutch wouldn’t have been that effective, due to the variance between boards and therefore the variance in resistance. What you want to avoid is injury, as the torque reaction gets taken up by the soft bits in your wrist and hand, and I was feeling it after a couple of days. If I was rich I’d buy one of the specialised screw-driving drills with mechanisms designed to stop specifically that problem. ~$400

The cheap drill caught on fire while driving the roof screws.

Counter-sunk drill bit – Use this for board holes. I was actually given the wrong size, which I persisted with because I was too stubborn to go back to the shops. The bit should be big enough to drill a clearance hole in the board (so the shaft of your screws don’t interfere with the boards) and just long enough to go through the board. There are a few of these available now, called Magic Bit or Smart Bit. It’s just a thin drill with a counter-sinking bit on the end. Simple, but very useful.

Long Clamps – Something like this: http://images.toolbank.com/images/full/Q-G546EL7.jpg

The boards will warp a little side to side; use these to pull them into position, then drive a screw in to hold it.

Method

Remembering that construction is all about reducing flex, any good deck depends heavily on its foundations. The job of your foundation is to engage your structure with the ground. This could happen in any number of complicated ways, but if you just remember that you want to distribute the load into the ground, and allow no movement now or over time, you’ll be fine. In this case I employed a couple of different methods, mostly due to what was already in place. Each post is attached via an engineered, zinc-dipped steel footing, to a 400 x 400mm concrete slab. Each of these slabs was then cemented into position.

Post on concrete footing, on concrete foundation.

On the end closest to the camera and a couple of spots along the front, I dug 600mm holes with a hand auger and filled them with concrete, mounting the slabs on top. In the middle where there are already pavers, I concreted the plates straight onto the pavers. The footing spreads the load, the concrete engages with the pavers. None of these things will move in the next 25 years.

I then mounted short posts in the post stirrups, lined up horizontally with a piece of string, then vertically with a spirit level. The footings I used have a degree of freedom which can be changed by loosening the bolt, so this was pretty easy.

Bearers next, and this is the key to the whole structure. Get these right and everything else just falls into place. I chose to bolt the bearers to the posts, so I had the freedom to move them up and down for a perfect level.

Select a datum point to measure everything against. For this project, it was the lip on the door into the kitchen. I wanted the boards to sit flush under this lip, so deduct the thickness of the boards (18mm), then the thickness of the joists (138mm) from this point, and that’s the height to use for the bearers. The bearer against the wall (some sources call this bearer a ledger) is probably the best place to start as it’s easy to install. Hold it in place, drill a clearance hole in the timber, then a 12mm hole in the bricks for your 12mm dynabolts. Secure with some fat washers under the nut and against the timber. Use a spirit level to make sure it’s flat.

Getting the first bearer against the wall flat is probably the hardest part. After that we used a 10m long clear vinyl hose with water in it, as a long distance spirit level. Get one water level next to the datum, then the other end of the house will be at the same level.

With the bearers in place, the joists just sit on top, their centres 450mm apart. I used some special ties to hold them in place, but it’s not that important, a few nails would do the job. Once the boards are on they won’t move ever again.

For a deck that’s close to the ground, I think this method is fine. But, if it’s raised and so that it’s possible to see underneath, I think I would put the joists inside the bearer (rather than on top) and use joist hangers. It would be more work, as each joist will need to be cut to size, then 6 screws for each hanger, but the result will be neater, like a flat box hanging in space. Something to consider.

I then put a coat of paint onto the pine for appearance and some extra weather protection. I got mixed messages about whether or not it needed paint, and erred on the side of caution. Up to this point took about a weekend, with a mate helping.

Putting the boards on was epic. About six, 8-hour days with variously 1, 3 and 2 people working on it. The sequence for each board, goes like this:

Cut end square (if it’s not square already)

Measure other end so it’s in the middle of a joist; cut

Set the gap between boards by tapping a nail into the joist

Drill holes

Drive screws

Repeat.

We broke a lot of screws early on; the small holes in the board meant they were experiencing a lot torque which stripped the heads. Failure rate was about 1/10. In hindsight I should have gone and bought a larger drill bit, but instead I smeared glycerine on the shaft of each screw and they went in a lot easier. Screwing the boards on was terrible work, but worth taking the time to do properly. Sore back and hands are to be expected.

Roof

Putting the roof on was pretty simple, so I’m not going to go through it with the same detail. It’s the same sort of process as the deck structure; bearers, with ‘joists’ on top (rafters), but with the addition of ‘purlons’ so the sheets have something to attach to. Again, use materials appropriate to the environment and designed to minimise flex.

We used Laserlite 3000 as the sheet, which purports to reflect heat but allow light transmission. We haven’t had a summer yet to test the efficacy though.

Timber is all recycled hardwood, which looks glorious, but is incredibly hard to work with. Drilling holes in those posts was an ordeal. Worth it though, and again they should be there in 25 years.

Along the back edge I used these complicated brackets called Roof Extendas or something; in any case something similar will be available from a decent hardware store. This puts a bracket directly onto the frame of the house roof. The problem here is this necessitates cutting holes in roof tiles. I drilled 4 holes 50mm apart and joined the holes with an angle grinder. Have some spare roof tiles.

Finished Product

I’m quite happy with the result, but as always with these things it’s the little things that are wrong that I remember. How to do better next time. Like these stairs. They’re terrible.

EB