Spanbilt Shed FAQ

We have recently received a number of questions from our DIY friends for advice on laying a slab for their garden shed, outdoor storage unit, carport or garage.  So the team here at spanbilt thought we’d post a how to as a bit of an extra resource.

A Bit of Background

Many people use different methods as a base for their garden shed, depending on the position and what the use of the storage shed is for most do the job pretty well.  As manufacturers we generally recommend pouring a concrete slab … why you might ask … well while sheds can be built on paving slabs, flooring kits, treated timber or gravel base, concrete is by far the toughest and strongest.

If it’s a garage of carport its concrete piers or a slab, your engineering will determine the appropriate method and specifications.

Pouring a slab is not as difficult as it may seem, but it is a lot of work and may cost a bit more than some of the other options, also make sure you allow a couple of days for construction and curing. The job is always easier with an extra pair of hands so we recommend you get a couple of coldies in the fridge and find a mate that owes you a favour.   A quick tip don’t start your slab without reading the instructions and confirming the slab plan, slab plans may differ from the dimensions of the shed so we recommend you check and double check well before concrete hits the ground.

First steps and critical to a good finish is the preparation … it all starts with the ground work  and levelling the site, then you build the formwork, followed by a moisture barrier and steel reinforcing mesh. Next up, you should be ready to pour the concrete … again a quick tip it’s better to pour the concrete early in the day, and then finish as the day progresses. Don’t be tempted to pour concrete late in the afternoon, as several hours of drying time will be needed before you can finish it off, it often pays to plan to do the slab over a couple of days to be safe … preparation and formwork day 1 then the concrete day 2

Some further concrete curing tips …

Generally concrete reaches about 50% -60% of its strength after about a week, and about 98% after around a month, but only if it is cured properly. It is essential the cement hydrates completely with excess water after it sets.

Proper hydration will prevent the surface becoming powdery and reduce the chance of shrinkage cracking. The easiest way to cure it is to gently hose the slab morning and night for at least a week, and more often in hot, windy weather. Other methods include covering the concrete in plastic, so evaporating moisture condenses and returns to the slab. You can also cover with sand or hessian, keeping them wet. Leaving the formwork in place for as long as possible will also help.

The following is a step guide to constructing a slab for your garden shed or outdoor storage unit.

Recommended Tool list

Spirit level; lump hammer; wooden stakes; mattock; shovel; cordless screwdriver; 75mm screws; moisture barrier (builders film); duct tape; bolt cutters or angle grinder; 40mm bar chairs; steel reinforcing mesh; gloves; wooden float; gumboots; concrete edging tool; wheelbarrow.

Reminder – Check the instructions 1^st for the slab detail, recommendations and rebates before you begin.

Step 1 – Mark out rough area of slab using landscaping paint or wooden stakes. Measure a little larger at this stage, as it is only for levelling ground (even an area that looks flat can have a small fall, so check). Mark lowest point and starting from here begin to excavate to level the ground for your slab. Remove any vegetation, especially roots, which will rot over time and leave voids. Also remove any soft spots in soil and protruding rocks. Keep checking levels as you dig, as you don’t want to go too deep. An easy way to do this is to use 2 bricks on edge to allow you to check levels at 2 points, fill any remaining depressions with road base, compacting it well. You should aim to end up with a flat area that is level in every direction and about 100mm wider all around than final size of slab you intend to pour. Spread a 20-30mm layer of clean sharp sand over your prepared surface, then screed level. This allows for drainage and protects the waterproof barrier, which will be placed under slab to prevent moisture rising into concrete.

Step 2 – Begin to construct the formwork which holds the wet concrete in place while it sets, the formwork must be strong enough to withstand the pressure of the wet mix. Using 90 x 45mm pine is recommended, however, you could use any pieces of old timber you may have lying around, making sure all lengths are not bowed. Screw the timber lengths together to create your formwork. All of your fixings should be on the outside, as later on you will have to strip the formwork and remove. Level and position the front of formwork carefully, because once your slab is poured it cannot be moved. Drive wooden stakes on outside of formwork at each end of front length until top of each stake is just below top edge of frame. Place your spirit level on the formwork and adjust if necessary. Screw high end of the stake to formwork from outside, ensuring frame is at desired height. Check front length of formwork with level, adjust if necessary and screw to stake at other end to secure. Now, measure both diagonals to ensure they are equal, which means formwork is square. Place your level along 1 side of formwork and, being careful not to knock out of square, drive in the stake at back corner and screw to side. Repeat for opposite end. Check again the back length of the formwork and adjust levels if necessary. Drive in and screw remainder of the stakes to the formwork. While driving in the stakes, sight along timbers to ensure they are straight. If your formwork is solid, level, and the top edge finishes at the height you want your slab to be, you can rely on your formwork to give correct levels of concrete without having to constantly refer to a spirit level while in the middle of a pour.

Step 3 – Now it’s time to line the formwork with a waterproof membrane to prevent damp rising into slab and add the steel reinforcing. Unroll the moisture barrier, overlapping formwork by about 200mm. Allow plenty of slack so you can push it into corners to minimise air pockets underneath slab. Overlap the membrane by about 200mm, then seal join with tape. Cut steel reinforcing mesh to size using bolt cutters or an angle grinder. Reinforcing mesh gives concrete tensile strength and helps to control shrinkage cracking. Steel reinforcing must be surrounded by 40mm of concrete on all sides to prevent premature corrosion, which in turn can lead to ‘concrete cancer’, so cut 40mm short of all sides. Large sheets of reinforcing mesh can be simply cut to size, but if you buy precut sheets, you will need to overlap them by 200mm. Position reinforcing mesh on top of bar chairs every metre or so to hold steel up from bottom of slab. Where required tie overlapping mesh together with wire, ensuring wire tails will point into concrete and will not be exposed.

Step 4 – The pour. Turn on mixer and add water, then add 4 or 5 bags of concrete mix with more water. Continue adding water and mix until it is just wet enough to pour. Once well mixed, pour into formwork and have a helper shovel or spread it into corners as much as possible (the steel will limit it somewhat). Keep mixing and spreading more mix until concrete is about 10mm above formwork. Your concrete is now ready for screeding. This is the action of compressing and levelling concrete with a screed board. Use a purpose-made screed or a long and straight length of timber that is a good 500mm wider than area being concreted. To screed, work from 1 end and pull back towards yourself so you do not have to stand on freshly finished concrete. With 2 people and screed on edge, use a backward and forward sawing and chopping action to work concrete down to level of top of formwork. Remove excess concrete from in front of screed as you work, or use it to fill hollow areas and re-screed. Once you have made your first pass with screed, tap formwork with a hammer to release any trapped air bubbles, then fill any voids that may have appeared.  While still wet, run concrete edging tool around perimeter so you can dislodge and remove any aggregate preventing a rounded edge. With most of the hard work done now might be the ideal time to crack that 1^st beer.  Next wait for the surface water to evaporate (this is why it’s best to pour in the morning). If you try to finish concrete now, the surface will be weak. Check on slab every half an hour or so. Dried surface can be left with a rough screeded finish, but most people prefer a sandy wooden float finish. When there is no water remaining on surface, work float in broad sweeps across surface to smooth. Once complete, redo perimeter with edging tool to remove sharp and brittle edge. Cure concrete for at least a few days to a week so slab can gain strength, then remove formwork. You should now be ready to erect your shed.