Many homeowners start planning their terrace with glazing in mind — which system, which tint, which locking mechanism. But in reality, a proper terrace project begins with the foundation. And not just any foundation, but one that accounts for the specific requirements of the glazing system.
In this article, we explain why the foundation is critically important when it comes to glazing, and what you need to know before starting construction.
Why a standard terrace foundation may not suit glazing
A terrace without glazing needs to withstand the weight of people, furniture, and the elements. Tolerances are generous — a movement of a few millimetres makes no difference.
But with glazing systems, everything changes. Malmerk Klaasium frameless folding terrace glass panels allow a structural movement of only ±2 mm. This means that if your terrace structure moves more than 2 mm, the glass panels will jam and no longer slide smoothly. For framed sliding systems the tolerance is slightly larger — ±7 mm — but that is still very little.
These figures mean that the terrace foundation and structure must be far more stable than those of a standard open terrace.
Frost heave — the greatest threat
In Estonia’s climate, the greatest threat to a foundation is frost heave. In winter, the ground freezes and moisture in the soil expands. This pushes the foundation upward. In spring, when the ground thaws, the foundation settles back down. This cyclical movement repeats every year.
If the foundation has not been built to a sufficient depth (below the frost line) or if frost-resistant solutions have not been used, the entire structure moves — and along with it, the glass panels.
For an ordinary terrace, this is not a problem. For a glazed terrace, it is a catastrophe.
What to do correctly
Foundation depth. In Pärnumaa, the frost line is approximately 1.2–1.4 metres. The foundation must extend below this depth to avoid movement caused by frost heave.
Soil type. Sandy soil is more stable than clay. In clay soil, frost heave is a greater problem and requires additional measures — a drainage layer, insulation around the foundation.
Structural rigidity. Even with a correct foundation, the upper structure must be sufficiently rigid. With timber beams, bending and drying must be taken into account. A steel frame is more stable. Glued laminated timber (glulam) is the best compromise between strength and aesthetics.
Measurement and planning. Before construction, it must be established which glazing system will be used — this determines the load, the location of fixing points, and the tolerance requirements.
Why the builder and the glazing company should be the same team
Typically, a homeowner commissions the terrace from a builder and the glazing from a glazing company separately. The builder constructs it the way they always do — without knowing that the glazing system requires a ±2 mm tolerance. The glazing company arrives, installs the glass, and leaves.
Six months later, the problem appears: the structure has moved and the glazing no longer works. The glazing company says the structure has shifted — not their responsibility. The builder says no one told them about the special structural requirement.
When one team handles both the construction and the glazing, this problem simply does not arise. Dimensioning is done correctly from the outset.
Summary
When planning terrace glazing, think about the foundation first — not afterwards. The tolerances of glazing systems are tight, and a stable base is a prerequisite for them to function correctly.
If you are planning terrace glazing and are not sure whether your existing structure is suitable — or if you are planning to build a new terrace — contact us. We carry out a structural audit and give honest answers.
ATKA Klaaskoda is the official Malmerk Klaasium dealer in Pärnumaa, Saaremaa, and Hiiumaa.
