How to prepare the base for Decking

You’ve decided that you want a deck and you know what type of deck you are going to build. Our next blog tells you how to decide on the structure to lay the base. Our instructions are based on best practice, giving you a solid and durable base.

Tools you will need:

    • Spade
    • Pick Axe (if the ground is solid where you are building)
    • Post Hole Digger (not essential but strongly recommended)
    • Tape Measure
    • Screwdriver (or screwdriver attachment for drill – recommended)
    • Hammer
    • Cement Mixer (handy if you are planning on mixing your own concrete)

Materials you will need:

    • Concrete (ready mixed will be easier)
    • Gravel
    • Posts to support the joists (100mm/4 inch recommended)
    • Joists (50mm/2 inches wide and 150mm/6 inches deep – note, you can use deeper wood for greater support)
    • Heavy Duty Polythene Sheet
    • Galvanised Point Staples
    • Easydrive or Self Taping/Drilling Screws
    • Anchor Bolts (if fixing the joists to a wall)
    • Webbing (to prevent weeds)

Before starting the project it is important to work out what quantities you will need to buy for each material. In the instructions below you should be able to work out what you will need based on the measurements you have. We will in a future blog give some guidance on quantities, but this does vary from project to project.

The first step to building your decking is to prepare the ground. This involved ensuring that it is as level as possible and is free from any weeds. The ground level should be at least an inch below the bottom of the decking joist when fitted. Assuming that you use joists that are 50mm x 150mm you should aim for your ground level to be around 200mm below the top of the deck.

Once you have prepared the decking the next step is to place posts in the ground to support the joists. The joists should be no more than 400mm apart. They can be nearer if you want a sturdier base. Where there are special features such as stairs or where you may put extra weigh on the decking (a hot tub for example) then 300mm would be more appropriate.

Once you have worked out the best distance between the decking you should put your vertical posts in place. Here the best approach is to use a post digging tool which allows you to dig a small but deep hole to put the post into. We recommend that this should be around 500mm deep ideally but this will depend on how hard the ground it.

When you have the hole dug it’s time to prepare the post. If you take some of the plastic sheet you should cut this and wrap it around the base of the post. This can then be secured using the galvanised point staples. This will ensure that the post lasts longer and doesn’t rot quickly once in the ground.

At this point it is sensible to put down some form of covering on the ground. This is designed to stop weeks growing through the decking from ground below. Plastic sheeting is very good but there you can also buy some webbing from garden centres and DIY shops specifically for this task.

Here you have two options you can leave the post longer then required and cut later or cut it to the right size now. The decking will sit directly on top of the post and the joist so you need to cut these to be around 25mm (standard decking depth) below the finished height.

We would suggest you start in one corner and work in a line to get on edge. Once you have done this then work in a line at 90 degrees to get the second edge. If your decking is not square or rectangular you should cover the main area first using this technique and then extend out for here as required.

For each post you should place a small amount of gravel into the bottom of each hole. Once you have done this you can pour in the concrete on top. If you use post concrete this comes in small bags and can be emptied in dry and then water poured on top to make it wet. This is by far the best thing to use as it also sets quickly. Ensure that the posts are covered in the plastic to above ground level. Also make sure that they are held in a vertical position whilst drying. This can be done by putting a couple of blocks or heavy items around them.

Once you have laid out all of the posts and the concrete has hardened you can start to affix the joists to these. The joists normally come in different lengths, so it is good to plan ahead and purchase lengths that require the minimum number of cuts and keep wastage to a minimum.

Affix the joists to the posts. If you have already cut the posts to the correct length, then you will be ready to fix the decking in place once this step is complete. If you have left the posts longer you will need to cut these down to size using a hand saw or a multitool. Ensure when you complete this stage you allow for a slight decline to allow water to run off the deck. This will minimise the risk of it freezing and becoming slippery in winter. The water ideally should run away from any walls or the house and drain onto grass or gravel unless you intend to install some drainage gutters as the end you are getting the water to run away to.

The best way of fixing these joists to the posts is using Easydrive or self-tapping screws. Around 120-150mm are ideal and using a drill with a Philips screwdriver piece the easiest way to screw these in.

Ensure you check heights and decline on a regular basis whilst fixing the joists.

When you have laid the joists in one direction (we suggest at ninety degrees to the direction of the boards) you can then lay joists in the other direction to provide strength to the structure. This should result in a frame made up of squares between each post.

You are finally ready to fix the boards to the frame! Well done!