Decking, avoiding the pitfalls


DIY softwood decking is a great project for those handy with wood and screws, with a few power tools and several weekends to work at it. You can do so much with decking. You can really be creative by running boards at different angles and directions to make shapes and patterns. Mitring ends to make frames around the decking really makes a statement. It really is extremely flexible. But there are many pitfalls surrounding decking the average DIY’r wouldn’t consider.

Preparation

As with any job, planing and good preparation is the key to success, and decking is no exception. Start off by getting out there and measuring. The design your deck. Try to work to full deck board lengths where possible. If you can’t then work to lengths where you can stagger joints like a brick wall.
An important thing to consider is the height from the ground to your damp course. The top of the deck should be 150mm lower. Many people prefer to have the deck at the same floor height as indoors. This makes it feel like an extension to their home.

Using 10mm gaps around the edge you bring the deck almost level with the damp course, but you should never be above it.
The gap between ground and damp course will determine if you need to dig out further or if there is a concrete slab in place already, you may have to reduce the height of some of the joists, and give them more support.

Starting out

Firstly clear your site of plant life, if it is to go over turf then dig that up. Some don’t bother, they just weedkiller it, but why risk it? It’s more work taking a short cut now if it starts to grow again in the future!

Ideally you now have an area which is flat, with a slight fall away from your house, usually 10mm for every metre. So a 3m projection of deck will be 30mm lower at the furthest end from the house.

Spray the entire area which will be covered by the new deck with a strong weedkiller, then cover with a heavy duty landscaping fabric. Overlap joints by 100mm and peg it down. Now cover the fabric with gravel, leaving areas where the deck’s supports will go clear. The gravel prevents the membrane from lifting during winds. Missing this step will result in weeds coming through under your deck and shooting up between the boards.

 

The frame or sub-base.

How many times have you walked on a deck which flexes underfoot, or shudders when someone moves. It’s a signal that the sub-base is not designed correctly. Often you will find joists are too far apart so that the deck boards flex, or they are too thin to cope with the span between ground supports and the joists bend with any weight.

There are various discussions about the optimal gap between joists. They quote differences on board thickness versus spacing. I have seen allsorts of measurements advertised as acceptable gaps, some up to one metre! Even with the thickest deck boards, a 1m spacing is far too wide in my opinion.

Our decks are made up of timber studs 50mm x 150mm with joists placed at 400mm centres, These studs are supported from the ground at 1.8m intervals in each direction.  If in doubt how strong it is and if you have enough support bounce up and down on a joist timber. If theres even a hint of movement put another ground support in, the cost is negligable!

For decks where the studs are close to ground level use paving slabs to support it. These should be laid on a firm base, ideally a mortared, or well compacted granular sub base bed.

There may be occasions where you cannot use a paving slab to support your deck. This is nearly always due to an awkward slope, or the deck spans multiple levels of existing concrete or patio. If this is the case then you will have no choice but to put your deck on legs to compensate.

Deck on legs

Legs should be fixed in the same positions as if the deck was resting slab supports. Use 100mm square timbers cut flush with the top of the joists, extending to the ground. These legs should be well fixed to the joist, with a couple of M12 bolts, not screws.

The bottom of the support legs should never rest on bare ground. Over time foot traffic will cause the leg to create a depression in the soil. This will happen even if well compacted, and create bounce in the deck. The post should rest in the centre of a slab, which is prepared on a well compacted granular sub base or mortar bed as with the slab ground supports or even better on a purpose cast concrete pad.

Fixed supports

The supports on the front edge (furthest edge from the building) should always be fixed to the ground, not just resting on slabs or concrete. Often these corner posts are concreted into place. While this is acceptable, the post will inevitably rot at the bottom, just like fence posts do. This is where the moisture is in constant contact with the timber. I prefer to use a square steel mounting bracket, which is bolted onto a concrete pad. This ensures no movement of the deck, and the posts can be easily replaced when required.

Treat your frame

Once completed the subframe of the deck and the support posts will be hidden completely. This means you have one shot at treating the timber. Do not use decking oil on the subframe it needs reapplying every year or two. Instead give it two coats of a quality wood preservative that will protect against water, fungal and insect attack. You won’t ever stop the rot completely, but this is your chance to slow it down. On the end grains of the frame, especially the legs it’s better to dip them into the preservative. Doing this ensures it penetrates deeper into them in the crucial areas. Angling the upper face of legs rather than a flat cut helps the water run off more easier.

Fixing the subframe to the building

This may seem pointless as a finished deck weighs a lot, so how can it move? Wind is a powerful element and can flip decks if not properly secured.

Many advise you to fix a timber directly to the wall and then use joist hangers to attach the joists to it. I don’t like doing this for two reasons.

  • Firstly using joist hangers is not as strong as a properly screwed frame.
  • Secondly you are putting a moisture trap right up against your house!

I prefer to create the subframe as one solid unit with the structural joists fixed properly and only the intermediary joists hung with joist hangers. Then offer it up to the wall drill holes and fix using M12 rawl bolts every 1.5m. Before tightening the bolts use spacers either side of the bolt, like the ones window fitters use. This allows an air gap of 10mm between the wall and timber of the majority of its length. It also allows you some adjustment, rarely will you find a house wall that it perfectly square. This way your woodwork can be square, adjusting it without pulling it out of shape to follow a wall.

Before adding the boards

By now you will have a frame fixed to the wall, supported off the ground by legs or resting on slabs, under it will be a flat weed membrane or geotextile covered with a layer of gravel.  Before you get excited about laying your boards there are a few things to consider. Do you want, or need ballustrades around your deck? Or do you want lighting on the deck? If the answer is yes, then you need to fit these now.

Fitting the newel posts

Newel posts are the square profile posts the rise from the decking to support your rails, usually 100mm square. The first job is to rebate the bottom of the post nearest the wall. The post will fit onto the inside of the subframe joist which is attached to the wall, butted right into the corner. Rebate the post so that the post remains 10mm away from the building wall. When rebating, the bottom of the post should align with the bottom of the joist to give maximum area for fixing to the frame.
Fix the bottom of the post inside the corner of the frame with 3 decent screws. Then drill through the top of the post and drill and plug the wall. You can now screw straight through the post into the wall giving it strength at the top. Between the post and the wall add stainless steel washers to ensure the post can be screwed up tightly, while keeping the 10mm gap.

The position of the next post is dependant on what will be between the posts. For wooden rails with spindles, or rope swags you can create posts that are equidistant along the deck. Simply measure the deck length and determine how many posts you need to fit the chosen handrails. You will probably have to trim the handrails to length. For steel spindles or glass panels the positions will be determined by their length. Cutting every panel isn’t a viable option, so work out the number of full spans you can do. You will be left with a short span which will need trimming to length, this looks better closest to the house.

Fixing the second post is more simple. Align the bottom of the post with the bottom of the joist and temporarily clamp in place. Check for plumb in both directions, remember joist timbers can be sometimes twisted slightly. Drill 2 holes through the post and the joist and bolt them together with M12 bolts and penny washers. If you need to adjust the vertical because of a twisted timber. Carefully plane the bottom 150mm of the post until the vertical is plumb.

Then repeat fitting newel posts at the correct intervals until you reach the other end.

The corner post should be fitted right into the inside corner of the frame. Bolt this through both sides. Ideally 2 bolts each way if the joist is of sufficient height.

Lighting

If you are to put lighting in, then it’s easier to get the lighting transformers and cables in now. Doing it while you sill have access to the full subframe  means you run them under the deck tidying the cables.

I’m not going to run through this here, I’m not an electrician, but is really simple to install, especially if you have an outdoor socket already.

You could do this yourself, but make sure you stay safe, have an RCD and get a qualified electrician to check your work!

Laying the boards

Now for the fun part, you can start laying your boards.

If you have designed your deck and made your frame accurately you should be working with whole boards. I always start at the furthest edge from the house and work back. Working this way means that if you have to cut any boards along their length, they will be next to the house and less noticeable.

Fascia boards on the outside hide the subframe and prevent pets from going under the deck. If you are having them then roughly fix them in place. Ensure the top edge is flush with the top of the joist, do this to the sides as well, dont worry about lengths being too long or too short, you will cut and fit these properly later.

Now the first board. Accurately measure and notch the board using a jigsaw to fit around the newel posts. The furthest edge from the house should be flush with the outside edge of the facing board, with a 5mm gap around the posts. The board should reach approximately half way across the newel posts.

Now repeat the process, but on the inside of the newel posts this time, leaving an expansion gap between the boards. Its not absolutely critical what this gap measures, just that its there, and it matches what you specified in your plans. I use the width of a decking screw as my spacer. This ensures uniform gaps over the whole deck. It’s worth noting that cheap softwood decking can often be warped along it’s length. These will require more patience to get the gaps correct.

Now just work away fixing with two screws on every joist, keeping your gaps uniform, on wider boards it may even be possible to put three screws in. Ensure the screw heads are countersunk well. Don’t underestimate the amount of screws you will need.

When cutting the boards to length remember to cut them flush with the outside edge of the fascia boards.

If your deck is too long for one board then try to stagger the joints like a brick wall if possible.

Fascia boards

It’s now time to add your fascia boards properly.

Starting with one side of the deck measure your length and mark your board, now cut a 45° mitre across the width so the outside edge is a board width longer. and fix it to the frame (you will need to add a timber to each leg to ensure the subframe and leg are flush). Space the next board down the same as the top of the deck, stopping 50mm short of the bottom to allow a gap for airflow.

Cut the opposite mitre on the board to go on the front of the deck and work along it. When you reach the other end repeat the corner again and finish the other side.

Weak points

Inevitably around the cutouts for the newel posts you will get weak points where the deck boards are cut. You may even find that there is hardly anything to screw the board into. If this is the case you must strengthen that area. Cut noggins to fit and toenail the screws in if you have no access to go through into them, or it maybe easier to add another joist timber onto the newel post when you bolt it up to the subframe. There’s no right or wrong way as long as it is strong.

Note about Decking Screws

There are many screws available, some specifically designed with a tooth pitch angled to minimise pull out. I use Timbadeck screws which are self countersinking, self drilling to reduce splitting and coated in various colours to match your deck colour and maximise their resistance to the weather. Using cheap screws will result in them going rusty quickly, pulling out and often splitting the wood.

Treating the finished deck

Preparation again is crucial.

Sand, plane or router any rough edges now. Don’t wait until after and then try and touch it up, the colour will be different with any coatings that have stain in them and it will look naff. Once sanded sweep any wood shavings, sawdust or general grit and dirt from the deck before you start.

What to use is up to you, personally i like to use an oil based deck oil (you can get water based oils).  Some ‘decking’ paints wear very quickly, and result in you repainting more often than you like, so i try to avoid them.  Where possible try and spray your treatment on, the finish is better and it helps you get in all of the crevices. If you do spray it, do it on a still day and mask off the areas you dont want covered with tape and paper or plastic. Nice white rendered walls don’t look great splattered with natural cedar or autumn gold!

If you end up painting it with a brush, follow along each board completely in one go, don’t let areas dry and then try and work up to the dry edges, you will get a line and it wont go.

Building Regs and Planning

Believe it or not these DO apply to decking if it meets certain criteria.

Planning permission does not need to be granted if:

  • The decking isn’t over 30cm from ground level
  • It doesnt cover more than 50% of the garden including any outbuildings.

There are exceptions to this where planning permission is required. Such as any building which has had a change of use, barn conversions for instance, flats and maisonettes, or other buildings like warehouses, pubs etc. Or where there is a planning condition, article 4 or other restriction.

Building regs are applicable to any decking structures that require planning permission.

I hope that this helps you avoid the pitfalls of decking to get the deck you want. If it all sounds too much, then please get in touch and we will be pleased to have a chat about your ideas and quote you.

Decking is also available in hardwood, composite and fully recycled plastic materials.