I am lifting limestone flags, insulating, installing underfloor heating, then relaying the original flags. The flags are uneven depths (between approx 80 & 120mm). How should I ‘trim’ the flags to make them an equal depth and give an even floor surface?

Many thanks & any other tips gratefully received!

  • Patrice Belle

    The flagstones could be sent away to be cut to a uniform thickness – but this will be expensive, and success may be relative (damaged flags, etc.).

    Normally, one would calculate the floor finish depth to be equal to the thickest flagstone on its mortar bed (e.g. 120mm + 10mm = 130mm). The remaining flagstones are then laid on the depth of mortar that brings them up to this same level. Thus, the thinnest flags (80mm) would be laid on a 50mm mortar bed. The thickest flagstone is often laid first to establish the finished floor level on site.

    This is a substantial floor finish above the UFH (Under Floor Heating): Check that the screed is sufficient to take the weight, and that the UFH is designed for this scenario (slower thermal reaction time, but increased thermal storage). Check also that the mortar for bedding the flagstones is appropriate for the task, with UFH. Also, remember movement joints in your floor finish (especially at doorways) to avoid unsightly cracks where you do not want them: These should be coordinated with movement/pouring joints in the floor slab, and the UFH should take account of these movement joints.

  • J F

    Many thanks Patrice; that all makes sense. I’ll go back to the ufh suppliers for more advice.

    Regarding the movement joints, the whole floor comprises a 5m x 3.5m rectangle with an adjoining area of 1.4m x 1.4m. I was intending to use a single zone ufh system, aiming to achieve even heat throughout. Is the area large enough to be concerned about cracking?

    Does anyone else have any experience of using ufh under a similar depth of floor finish? This is to be the sole source of heat on the ground floor of my (tiny) cottage, so I need to be sure it’ll be effective!

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