Learning how to build a solid base for a summer house or log cabin will ensure your garden building can remain structurally sound and level for years to come. The base is just as important as the garden room itself.
The first key element to building your garden room is creating a high-quality and durable base that will stand the test of time. All of the other work you carry out after will be riding on the base being level and stable. Even the lightest summer house is going to weigh hundreds of Kilograms and the last thing you need is the base failing while you are hosting drinks with friends inside.
Various examples are available throughout the internet of those who have taken delivery of a garden building, only to realise once they have started building, the base is unsuitable and going to require a re-work. We cannot stress enough that a garden building requires a base which is 100% level and solid. The old patio at the bottom of your garden may look level, but be sure to measure it properly to ensure all sides and the centre are level.
A level base is even more important for a log cabin due to the way it will be assembled. The building is built up piece by piece in interlocking layers until the roof is reached. Precision cutting is used to ensure each piece fits in perfectly and creates a tight seal, with little in the way of tolerance. If part of the base is not level and the pieces do not line up correctly, this will affect the overall structural integrity of the garden building and may leave gaps in the walls. Over time, as the wood becomes strained, the cabin could become warped and windows or doors may be difficult to open.
Table of Contents
Location Decisions
When purchasing a garden building, the dimensions given can vary from some manufacturers. While some measure the walls, others include the overall distance of the roof as well. This is important as the roof can overhang, sometimes by up to a metre on the front. The total external dimensions should be used when planning where to position the base for a garden building. The area where the base is placed should allow for sufficient space in each direction upwards above the height of the building. Meaning no trees or garden fences can be in the way.
As well as during construction, on-going yearly maintenance such as painting and checking for damage can be required with a garden room. This should be taken into account when choosing a location and ample space should be left available for access. If you are planning on a large garden room close to a boundary, planning permission may be required.
Positioning your garden room in relation to the sun can make a big difference to how warm it feels inside, particularly in colder months. For use as a garden office, positioning so the windows are facing the sun can provide lots of natural light and create a feeling of warmth. Keep in mind that any screens may be difficult to see if pointing directly towards the window. If positioned near a tree, debris could potentially fall off and cause damage over time.
Footprint
The size of the base for a summer house or log cabin should be just over the size of the buildings wall dimensions. Any larger will encourage water to puddle. This is a nightmare scenario in the winter when standing water can be sat for days, damaging the wood at the bottom.
A base which is too small will also not provide the support required on all sides. Creating a base the exact size of the wall dimensions can create a nice flush effect on the edges and also allow for any water to run instantly down the sides. But this leaves no room for tolerances in the measurements or slight movement during assembly.
Concrete Base
A concrete slab is recommended where possible to ensure a solid and level base. A simple concrete base is fairly easy to put together and if done yourself, can be relatively inexpensive. Concrete and paved bases cannot be placed on top of the soft (muddy) ground and require a sub-base first. The base should be dug out to allow for 8cm of sub-base and then a further 8cm of concrete. With the sub-base made up of type 1 hardcore which is compacted into the ground and then covered with a layer of ballast to fill any gaps.
To start the concrete base, a wooden shuttering should be used to create the outline of where the concrete is going to be filled. A spirit level should be used to ensure all of the sides are level and the spacing is accurate. The dimensions inside should match or be slightly over the wall dimensions of the garden building.
The concrete is poured inside the shuttering and filled until it is level at the top. A tampering board is run along the top to help level out and compact the concrete. Building a concrete shed base is usually a two-person job.
You may choose to use a damp proof membrane with a concrete base. This works by stopping damp rising up from any water drainage in the surrounding mud or soil. The damp proof membrane is usually placed several centimetres above the surrounding ground level. Plenty of time should be left for the base to completely dry out before installing a garden building. This can not be done the day before and left to set overnight.
For larger constructions, steel may be required to reinforce the concrete base. We recommend consulting a professional builder for advice. If you are not building the base yourself, a local builder or landscaper should be able to help.
Paving Slabs
The construction of a base using paving slabs is similar to a concrete base. Paving slabs are pretty easy to install and can also be lower cost when compared with a concrete base. Using an existing patio may be suitable. But many are designed to allow rainwater run-off and may not be 100% level, which is no good for a garden room.
For paving slabs, a sub-base is also required. Again, this should be constructed using hardcore which is compacted into the ground. Mortar is created using a mix of sharp sand and cement. The formulation should have enough moisture to stick together, but not run off or lose consistency.
Flat and smooth slabs should be used throughout the base. Each slab will be installed one at a time and a mallet used to tap them into position. Getting every slab level and flat is critical to ensuring your garden building can be built properly.
Ground Screw & Timber Base
Ground screws are effectively very large screws which go into the soil to create a firm base for a garden building. They make a good solution where a concrete base is not possible or cost prohibited. A benefit to using ground screws is that air can flow freely underneath the base of the building to ensure the frame remains dry and out of the way of damp.
While ground screws can be placed right down to the ground level, they can also be left slightly raised. If there is a risk of flooding near your garden room, a raised building can help to prevent this. Where we would recommend the use of ground screws is for sloping bases. If your garden contains a slope, ground screws can save the need to dig out the ground to create a level base.
With a ground screw system, the base can be installed at the same time as the garden room and immediately weighted. There’s no need to wait for anything to dry as with a concrete base. The timber frame is then screwed into the ground screws using brackets to remain firmly in place. Be careful to check your garden building comes with a timber base which is suitable for use with a ground screw system. Many log cabins and summer houses will have flat flooring which is not capable of attaching to the brackets.
Decking
Suitable for smaller garden rooms, timber decking which you already have in place can be used as a base. We find decking creates a great look all the way around a garden room and can also be extended as a veranda to take advantage of the space outside.
Unlike a concrete base, decking does require maintenance, generally once a year. This involves wood treatment to prevent damp and keep the decking looking new. As decking is raised, air can circulate underneath and prevent damp from rising and causing damage to the bottom of the garden room walls. If you are installing new decking to use as a base, we recommend using pressure treated wood for longevity.
Adjustable Risers
Riser base pads are designed to be used in conjunction with timber frame and decking bases. For example, if you are using a timber frame on an old and slightly uneven (but solid) patio, adjustable risers can be used to level the base. The adjustable pads make it easy to achieve a level result without the need to dig up any existing foundation.
The risers come in multiple designs made by lots of different garden building manufacturers. Most are made from ABS plastic which creates a surprisingly sturdy system, and some are formed using steel. Several sizes are available to cope with different inclines and slopes. Each riser can be adjusted individually using the central ring for adjustment. We recommend these when your base is nearly there but just needs minor adjustments to achieve a level surface.
Summary
Creating a good base is a key step to installing a garden room. Everything stands on the base, so if that fails, everything will. A garden room is heavy and needs ample support, especially with multiple people inside at once. A base which is perfectly level is essential as summer houses and log cabins are manufactured with precision and small tolerances. Finding out your base is not level on the day of installation is going to cause delays and added cost.
While we usually recommend a concrete base, the best solution is going to depend on the space where your garden room is being installed. Ground screws make a very good solution where it is not possible to lay concrete and can be installed at the same time as the building.
Checklist
- Level and smooth in all directions
- Solid construction capable of holding significant weight
- Just over the size of garden room
- Damp-proof membrane if required
- enough access space for building maintenance
Which base will you be using? Let us know below.
I have an existing hardstanding that is quite level (I think, I will check this shortly), however this was designed many years ago for a brick garage in mind, which we never built. Also, the hardstanding is very thick and reinforced, so it’s not coming up.
The garden room we are looking at is approximately 4m x 4m, but this is significantly smaller than the hardstanding, so the timber joists cannot be laid directly onto the base. I was thinking of using either concrete breeze block pads or the adjustable riser pedestals (so simplify any issues in level of the hardstanding), however, I can seem to find much information about the loading these plastic pedestals can take. All information I can see from the manufacturer relates to using them for lightweight deck and paving areas.
Is there anything suitable for the weight of garden room?