Do orangeries and garden rooms need foundations?

Just like any other building or structure, orangeries and garden rooms need foundations to keep them supported and distribute their weight evenly. There are a few types of foundations that can be considered, but it’s not easy to specify the most appropriate approach until the topsoil has been removed and the soil conditions can be assessed…

The simple answer to this question is yes, orangeries and garden rooms absolutely need foundations for stability. Foundations are needed to support a structure by transferring their weight evenly across the ground and helping them to stay strong and sturdy. The kind of foundation you need will be dependent on your soil type, which can’t be specified until the ground conditions have been assessed. 

The first step is to remove a layer of topsoil until we find good, undisturbed ground; meaning ground that has not already been built on. Sometimes, soil that is close to the house could have been previously backfilled, perhaps from where drains have been laid or foreign objects have been added to the soil to level out a site. Building the foundations can’t be started until completely undisturbed ground is first found. 

Strip foundations

Best for: most subsoils that have an appropriate bearing capacity

Also known as strip footings, strip foundations provide a continuous, level strip of support to a linear structure, such as an orangery or garden room wall. Strip foundations are typically 450mm wide and 1000mm deep but can be up to 600mm wide and 1800mm deep where required. 

If any subterranean utilities are exposed, these should be protected with compressive fibreglass sheets. If the soil may experience some movement, then steel reinforcement can be used to mitigate the effects of soil expansion and contraction, therefore protecting the foundations from damage. 

Once the foundations have been dug and properly prepared, it is filled with a pre-mixed concrete and left to cure. Curing times will vary depending on the weather conditions, humidity, and ground type but typically, the foundations are ready to build on after 24 hours. 

Cavity brickwork is then built up to the height of the damp proof course and a layer of MOT Type 1 is infilled across the entire footprint of the building, which acts as a stable substrate. This is then ‘blinded’, meaning it is compacted and covered in a finer material to create a level surface to work from. Finish things off with a layer of damp-proof membrane and a 100mm thick slab of concrete which is reinforced with steel. Once cured, this concrete ‘slab’ will then be the basis of the orangery or garden room’s floor. 

Raft foundations

Best for: ground with underlying issues that cannot be supported by a strip foundation alone

A slab or ‘raft’ of concrete is poured into the dug-out ground at a uniform thickness, typically 150mm thick, which covers the entire footprint of the extension. This spreads the load from the walls over the whole area of the foundation and as a result ‘floats’ on the ground. The concrete rafts include steel reinforcement in the form of a mesh, which helps to prevent cracking.  Areas of the raft may be deeper to support specific areas; such as the perimeter where walls will be built or where internal columns will support beams. 

Raft foundations are usually faster to construct than strip foundations, however, they are not suitable where structural loads will be focused on a few concentrated areas such as columns or a single load-bearing wall. Only implement raft foundations for standalone buildings where significant excavation would be required to form strip foundations, or where excessive settlement is likely.

Pile foundations

Best for: where foundations deeper than 1.8m are required due to unstable subsoil

Pile foundations are driven deep into the ground where they can reach more solid and compacted subsoils and strata. These types of foundations are especially useful when the ground is prone to excessive settlement, or when tree roots are likely to contribute to ground heave.

There are a few different methods of installing piles, but the ‘caps and beams’ method works well. Piles are driven into the ground in a line, with the pile head or ‘cap’ at the surface. This cap is bound to at least two other piles with reinforced concrete to create a continuous concrete beam.  These beams are then used to distribute the weight of load-bearing walls.

Beam and block system

Best for: ground with voids which are too deep to backfill

Sometimes blockwork, the DPC above pile foundations and occasionally strip foundations can create a void, which is too deep to backfill. When this happens, it’s best to build a new surface suspended above ground level by a series of precast blocks supported by precast beams. This is known as ‘Beam and Block’, ‘Rib and Block’, or ‘Lintel and Block’. 

The spacing and laying pattern used will be determined by the overall span between the foundations and/or the existing walls that the floor will be adjoining. Large spans may require a supporting wall (also known as spine wall) between the foundations on the outside perimeter and the inside wall.  This would require additional piles to be installed before construction.

Once the foundations are in place and the beams are in position, the process of placing the infill blocks can be completed within a day.

Screed floors

Best for: laying your finishing floor

Screed floors give a flat, uniform surface for us to lay your finishing floor. Whether you have a concrete slab, raft, or beam and block floor (depending on your foundations), add a layer of insulation followed by the underfloor heating if it is included in the project’s specification. Next, a self-levelling layer of anhydrite and sand mix is pumped in, and after the first seven days, the surface is ‘scabbed’, which involves removing the thin top layer of the screed to let it cure faster. Typically, the curing process can take up to five weeks however running the underfloor heating at a low setting can reduce this to four weeks.