Installing decking is an excellent way to create an outdoor living space for relaxing and entertaining. Decking can either be freestanding or attached to your house.
This guide outlines the method for building raised decks with timber frame and horizontal deck board positioning. We’ll be using postcrete to set the posts, which is easier to use and sets quicker than concrete.
Before you grab your tools, watch our handy step-by-step video to see exactly how we tackled the project. Let's explore how to build a raised deck safely.
- Building a deck frame requires structural treated timber, secure postcrete foundations, and internal joists spaced no more than 400mm apart to ensure complete stability.
- You must maintain a 5-8mm expansion gap between every deck board to allow for natural wood movement and safe rainwater drainage.
- Before digging any post holes, you must use a Cable Avoidance Tool (CAT) to ensure your garden area is entirely free from hidden underground pipes or electrical cables.
Tools and Materials
Planning and Preparation
- Consider the size and location of your new decking carefully. Are there any existing features like trees or posts to work around? Is the ground level or does it have a noticeable slope? With good planning, building a deck on a slope is highly achievable. However, wet or boggy ground isn't suitable for timber structures.
- You might be wondering if you can build a deck over a concrete patio, and you can. It just requires a slightly different approach. You’ll need to secure the base frame directly to the solid concrete rather than digging deep post holes.
- You might want to build a pallet deck on uneven ground for a quick, budget-friendly option. But for a hard-wearing, long-lasting garden feature, we highly recommend using structurally treated timber and a fully secured frame.
- Always check with your local authority first as you may require planning permission for your raised deck.
- Always use treated timber and screws specifically designed for the job and treat all cut surfaces and drill holes with two coats of preservative. This will ensure a hard-wearing, long-lasting deck.
- Make sure you’re wearing appropriate protective clothing, such as gloves, a face mask, or safety goggles, when cutting or sawing timber, applying preservatives or other timber treatments, or using postcrete.
Fitting decking posts
Carefully measure and mark out your deck area using pegs and string. Measure diagonally from post to post and use a set square to ensure the area is exactly the right size.
Use a straight edge to cut the turf edges, then dig the soil out to a depth of 50mm. Keep some of the turf in case you need it later to fill in around your completed deck. Measure and mark out 100mm from each face of your frame corners.
Dig a post hole about 300 x 300mm in size and 700mm deep at each corner.
Top tip: It’s worth hiring a CAT (Cable Avoidance Tool) to ensure your site is free from hidden pipes and cables.
Split a concrete block in two using a brick bolster. Place a section of each block into each of the holes, making sure they are firmly in place.
Place a length of timber post into the hole. Make sure it is longer than you need so you can easily cut it down later.
Create temporary props for the posts by screwing in pieces of timber batten to hold them. Use your spirit level to check the posts are perfectly vertical.
Secure each post with postcrete. Use a trowel to create a slight slope in the postcrete to ensure water drains away. Allow the postcrete to set fully, then remove the props.
Wind a string line around the four corner posts to mark the outline of your deck. Find the centre point along the string between each corner and mark each of these spots with a timber batten. You also need to measure and dig a hole in the exact centre of your layout.
These new edge posts shouldn’t be more than 1500mm apart, so if you have long edges you may have to mark out two posts on each one, and each of these posts should have a corresponding post inside the layout.
As with the corner posts you need to dig out holes which are 300 x 300mm in size and 700mm deep. Then place a concrete block into each hole but do not install the posts at this stage.
Building the base of the deck
If you are working on a slope, always measure from the corner where the deck will be highest above ground level.
Mark on the post where the top of the frame will be.
Measure the distance to the opposite post and cut a section of joist. Line up one end with your mark and temporarily fix it with a 64mm deck screw.
To ensure rainwater runs off properly, factor in a fall of 2mm for every metre of decking.
Using the spirit level as a guide, position the first joist at its required level then secure it to the other corner post. You will probably need a helping hand to hold the joist in place. Repeat this process until this outer frame is fixed in place.
Mark on the frame where the midpoint of the corner post is, then use a set square to draw a straight line downwards. On this line mark guide holes at 40mm from the top and 40mm from the bottom of the frame.
Use a flat wood drill bit to make countersunk holes, then drill pilot holes.
Hang a washer on the end of a 100mm coach screw, insert it into the hole and hand tighten before securing firmly in place with a 13mm socket set. Repeat this process for each end of the outer joists.
Mark on the frame where the centre of the remaining support posts will be positioned, and repeat the process of marking and drilling the countersunk holes.
Place a post into the hole in line with your mark on the frame. Check that it’s vertical then clamp it into place. Secure the frame to the post as before, then repeat the process until all the perimeter posts have been fixed into place.
To add your central support joists, measure the distance between opposite points on the inside of the frame. Then cut your joist to this length. The central joists should run in the same direction as your deck boards.
Attach the first joist to the frame using countersunk screws and washers as before. Put the centre post into position, clamp into place and fix as you did with the other posts. Use a spirit level to make sure the post is exactly vertical.
Repeat this process so that the centre post is now sandwiched between the two central joists. Once the post and the two joists are fixed in place you can secure all your posts with postcrete.
Trim down the tops of all the posts so they sit perfectly flush with the joist frame.
Roll out your landscape fabric underneath the frame and cover it completely with gravel to prevent weeds.
Adding internal joists and noggins
Noggins give your frame extra stability. These are short pieces of timber that fit snugly between the internal joists. Make sure the distance between your noggins is never greater than 1200mm.
When figuring out how to build a deck frame safely, proper spacing is vital. Mark lines at 400mm intervals down the length of your outer frame.
This marks the centre of where each internal joist will sit.
Mark pilot holes 40mm from the top and 40mm from the bottom of the outer frame and drill using a 6mm drill bit.
Attach a joist hanger to each end of your cut internal joists using 30mm exterior screws.
Place the joist into position so the centre lines up with your 400mm mark.
Secure it to the main frame using 100mm external grade screws.
To secure the joist to an unexposed side of the frame, drill a skewed pilot hole on either side before securing with screws.
Once the joist is in place, fasten the final 30mm screws to the joist hangers, and repeat this until all the joists are in place.
Mark the centre of the noggin’s intended position on each joist and use a set square to draw a vertical line down the joist. Then measure and mark two pilot holes on each line, 40mm from the top and 40mm from the bottom.
Drill pilot holes before securing the noggin with 100mm external timber drive screws. Once all the noggins are in place, check that your frame is level, and all your fixings are secure.
Building the steps for a raised deck
Steps should be positioned on solid level ground. The step support housing (or “stringers”) shouldn’t be spaced more than 450mm apart. We will be following the sequence of adding the step stringers, then adding the deck boards, and then finishing the steps afterwards.
Hold each step stringer in position and mark its edges onto the frame. Measure the distance between the marks and cut sections of joist to this length.
On the cut sections of joist, measure and mark out three evenly spaced pairs of pilot holes. Then clamp the sections into place, drill the pilot holes and secure with two 100mm screws at each end.
Drill a final screw through the support frame and into the back of each step stringer, being careful to avoid the other screws that are already in place.
Take a section of deck board and mark where the fascia will go. Use a set square to help you mark a clear line.
Lining up with the mark, use the chalk line to extend this mark across the width of the steps. Then measure from the far end of the deck to the chalk line and cut a deck board to length.
Place the cut deck board into position in line with the chalk line. If you are attaching fascia boards to the decking as we are here, you will need to allow for an overhang.
Laying the deck boards
When you’re laying deck boards it’s best to pre-drill all holes with a 2mm bit to prevent damaging or splitting the timber.
Start from the outside edge. Screw through the grooves to fix the first board with 64mm decking screws.
Work your way across the deck. Use a suitable spacer to maintain a strict 5-8mm expansion gap between each decking board. This gap allows the wood to naturally expand and contract over its life.
Finish the steps
Once the decking boards are all laid, measure the gap on the steps and cut sections of joist to length. These will act as additional support for your steps and can be fixed into position using the same method as for the noggins.
Always start by securing all the risers first. Step risers are the vertical support pieces between one step and another. Measure the width of the step between its outer edges and cut a piece of deck board to length. Make sure you allow for enough overhang on each side so that the board will be flush with the fascia.
Pre-drill your holes in the board as before, using a 2mm drill bit. Then secure the riser in place using 64mm deck screws.
To attach the boards that will form the tread, i.e. the board you actually step on, drill one screw into the second groove down and into the riser below. Always use whole deck boards at the front of the tread where the most pressure will be applied.
If your tread is wider than a whole deck board, and you have a gap to fill, then measure and mark the required board size, remembering to allow for the 5-8mm expansion gap. Secure the board and cut with a jigsaw.
Once the sections are cut, fix them to the step stringer below using deck screws.
Adding fascia boards
Fascia boards give your deck a neat, professional finish.
Measure and cut the boards that you need to the right length. Mark each fascia board with a line at 50mm from each end. Then from the first 50mm line measure out and mark lines at 600mm intervals down the length of the board.
When fitting the fascia boards, make sure to maintain the expansion gap of 5-8mm between all boards. Drill pilot holes on the marked lines and then fasten the boards with 64mm decking screws.
If you need to add an angled board because your garden is sloping then start by attaching packers to the main joist frame. This will give you something to secure the fascia onto.
Take an offcut of deck board and slide it underneath the top fascia until it nearly makes contact with the ground. Then mark a line along the top fascia to show the position of the new board. Mark on the fascia where the far end of your angled board will be, then measure up to your line on the top fascia board.
Measure the distance between the end of the angled board and the ground. Then take back 5mm as you don’t want the board to be touching the ground at any point.
On the deck board you will be using as the fascia, mark the point where the full width will end and the angled section will begin. Then make a second mark where the angled section will finish. Connect these two marks with a chalk line and use a circular saw to make the cut.
Top tip: When sawing, use the 90-degree guide marker on your saw, or a set square, to make accurate cuts, and sand the cut ends to remove splinters.
Secure the board to the packers using 64mm deck screws, then repeat this process for any other angled boards.
Add gravel around the base for an attractive finish that aids drainage.
We hope this guide gives you the confidence to construct your own outdoor space. Building a deck takes time and effort, but the result is a beautiful area to relax and entertain. If you want to go a bit further and you’d like to add some railings to your deck, check out our guide on how to fit decking handrails.
FAQs
You might need planning permission depending on the height and size of your structure. Local planning laws can be complex and vary from place to place. We strongly advise checking with your local authority before you start digging any holes.
You must leave an expansion gap of 5-8mm between every board lengthways. This gap is vital because it allows rainwater to drain away easily and gives the wood room to naturally expand and contract throughout its life.
Your internal joists should be spaced no more than 400mm apart. This provides a strong, rigid base that safely supports the weight of the heavy decking boards and everyday foot traffic.
Yes, you can build a sturdy deck frame directly over an existing concrete patio, provided the surface is sound and has a slight slope to allow rainwater to drain away effectively.
