ᐅ Natural stone slabs for terrace, fixed installation with open joints yes or no?
Created on: 22 Oct 2025 08:24
A
Andreas_1_2_3_A
Andreas_1_2_3_22 Oct 2025 08:24Hello!
I’m new to the forum on recommendation and would like to discuss the following topic with you:
We have just installed our terrace using 2cm (0.8 inch) natural stone slabs, laid in a one-third bond pattern.
Construction layers:
Concrete foundation with a slope towards the garden – 3%
Coarse single-sized concrete from the ready-mix plant – 3%
Drain concrete with 2/4 mm (0.08/0.16 inch) gravel – 2%
Natural stone slabs glued with a 2% slope as well
My question now:
Is it absolutely necessary or at least strongly recommended to fill the joints, or can they remain open if the slabs are bonded? The location is in Tyrol/Austria, so snow and similar weather conditions are to be expected.
I am aware that open joints may allow weeds to grow. My main concern, however, is whether the slabs could suffer frost damage more quickly if the joints are open, since more water would generally pass through the joints.
If joint filling is advisable, which material would you recommend? I’ve heard good things about Mortec Soft joint filler but have no real experience reports from other homeowners. Perhaps someone can share their experience.
Looking forward to your feedback!
Many thanks!
Kind regards
Andreas_1_2_3_
I’m new to the forum on recommendation and would like to discuss the following topic with you:
We have just installed our terrace using 2cm (0.8 inch) natural stone slabs, laid in a one-third bond pattern.
Construction layers:
Concrete foundation with a slope towards the garden – 3%
Coarse single-sized concrete from the ready-mix plant – 3%
Drain concrete with 2/4 mm (0.08/0.16 inch) gravel – 2%
Natural stone slabs glued with a 2% slope as well
My question now:
Is it absolutely necessary or at least strongly recommended to fill the joints, or can they remain open if the slabs are bonded? The location is in Tyrol/Austria, so snow and similar weather conditions are to be expected.
I am aware that open joints may allow weeds to grow. My main concern, however, is whether the slabs could suffer frost damage more quickly if the joints are open, since more water would generally pass through the joints.
If joint filling is advisable, which material would you recommend? I’ve heard good things about Mortec Soft joint filler but have no real experience reports from other homeowners. Perhaps someone can share their experience.
Looking forward to your feedback!
Many thanks!
Kind regards
Andreas_1_2_3_
Hello Andreas,
From what I understand, a drainage screed or drain concrete was installed with a sufficiently steep slope, onto which the porcelain stoneware tiles were then laid.
I would definitely recommend sealing the joints, even if it is only to protect the edges of the tile surface.
A mineral-based grout is never waterproof, so any surface water that passes through can be easily drained away by the single-size aggregate concrete.
Frost damage is highly unlikely if the porcelain stoneware tiles are generally rated for use in exposed outdoor areas.
And frost damage to drainage concrete or drainage screeds is impossible...
Regards, KlaRa
From what I understand, a drainage screed or drain concrete was installed with a sufficiently steep slope, onto which the porcelain stoneware tiles were then laid.
I would definitely recommend sealing the joints, even if it is only to protect the edges of the tile surface.
A mineral-based grout is never waterproof, so any surface water that passes through can be easily drained away by the single-size aggregate concrete.
Frost damage is highly unlikely if the porcelain stoneware tiles are generally rated for use in exposed outdoor areas.
And frost damage to drainage concrete or drainage screeds is impossible...
Regards, KlaRa
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