Silicone joint keeps cracking – causes and solution
If a silicone joint keeps cracking open, the backing cord is almost always missing: without it the silicone adheres on three flanks instead of two and cannot absorb the movement elastically. Other causes are a joint that is too narrow, a dirty substrate or the wrong silicone. Remedy: remove the joint completely and redo it with backing cord, a clean substrate and a suitable sanitary silicone.
Contents
What you need
Project template “Renew the silicone joint in the bathroom”If a silicone joint keeps cracking open, a missing backer rod is almost always behind it: without one the silicone adheres to three instead of two flanks and can no longer take up the movement of the joint elastically. Further causes are a joint too narrow for the movement to be expected, a soiled substrate or the wrong silicone. The solution: remove the joint completely and run it again properly, with a backer rod, a clean substrate and a suitable sanitary silicone.
Why does the silicone joint keep cracking?
A silicone joint has to take up the movement of the adjoining components elastically and permanently. If it cracks open again shortly after sealing, that is almost always down to one of these four mistakes:
Missing backer rod (three-sided adhesion)
Without a round cord the silicone additionally adheres to the back of the joint instead of only to both side flanks. This three-sided adhesion stops the joint from being able to stretch – it usually cracks open in the middle.
Joint too narrow for the movement
Connection and expansion joints have to be dimensioned wide enough to take up the movement of the components. A joint planned too narrow is permanently under stress and cracks.
Substrate not clean and free of grease
Grease, dust or remains of the old silicone prevent lasting adhesion to the flanks. The joint comes away at the contact face and cracks there first.
Wrong silicone used
Construction or window silicone has elasticity values different from sanitary silicone and is not designed for heavily loaded movement joints in wet areas.
Solution: renew the joint properly
A cracked joint cannot be repaired – it has to be removed completely and run again, this time with a backer rod and the right preparation.
| Step | What to do | Check |
|---|---|---|
| 1. Remove the old silicone | Remove the cracked joint mechanically and completely with a cutter or joint scraper, including the remains on both flanks. | No silicone residue visible or detectable by touch |
| 2. Check the joint depth and insert the backer rod | As a rule of thumb a ratio of joint width to depth of about 2:1 applies. Press a PE round cord backer rod of the right diameter in evenly with a blunt object, without perforating the surface. It is closed-cell to DIN 18540 and prevents three-sided adhesion. | Round cord sits evenly and undamaged in the joint |
| 3. Clean the substrate | Make both flanks free of grease, clean and dry with Otto Cleaner F surface cleaner. | Substrate feels free of grease and has grip |
| 4. Seal again | Seal again with OTTOSEAL S100 – an acetoxy-curing 1K silicone sealant to DIN 18545 part 2 group E, with fungicidal and bacteriostatic additives. Application temperature +5 °C to +35 °C. | Even, bubble-free bead of sealant with only two contact faces |
| 5. Tool the joint | Tool it in one pass with smoothing agent and the OTTO joint smoother spatula before a skin forms. | Smooth, closed surface with no air pockets |
| 6. Let it cure | Let the silicone cure right through before the joint gets wet or is put under load. Work out the material needed in advance with the Ottoseal silicone usage table. | Scratch test: the surface feels firm and can no longer be pressed in |
You will find detailed instructions for sealing a shower completely in our How to: seal a shower with liquid membrane and sealing tape.
How to stop the joint cracking again
Always use a backer rod in deep joints
With joint depths over about 6 mm, always fit a matching round cord in order to avoid three-sided adhesion.
Dimension the joint width correctly
Plan movement and connection joints wide enough for the component movement to be expected – in case of doubt rather a little wider than too narrow.
Only sanitary silicone in wet areas
In damp areas use sanitary silicone with suitable elasticity exclusively, no construction or window silicone.
Let the joint cure completely
Let the fresh joint cure right through before it gets wet or is put under load – otherwise it cracks before it reaches its full load capacity.
Common mistakes – and how to avoid them
Applying new silicone onto old
New silicone does not adhere lastingly to old, cured silicone. The old joint always has to be removed completely.
Forgetting the backer rod in deep joints
In deep joints in particular this is the most frequent reason for recurring cracks – the round cord always belongs in these joints.
Joint planned too narrow
If the movement of the components is underestimated, the joint is permanently under stress and cracks open again sooner or later.
Loaded or wetted too early
If the joint is used before it has cured completely, the material can deform and then cracks at that spot.
Frequently asked questions
Why is my new silicone joint cracking again after only a short time?
Usually the backer rod is missing, which makes the silicone adhere to three instead of two flanks (three-sided adhesion) so that it can no longer stretch when there is movement. A joint width too narrow for the movement to be expected, or a soiled substrate, also leads to cracks.
What is three-sided adhesion and why does it harm the joint?
If the silicone additionally adheres to the back of the joint instead of only to both side flanks, it can no longer take up the movement of the joint elastically. The stresses that arise usually crack the joint open in the middle.
How deep and how wide does a silicone joint have to be?
As a rule of thumb a ratio of joint width to joint depth of about 2:1 applies. The exact dimensioning depends on the movement to be expected of the adjoining components.
Do I need a backer rod in every joint?
In deep joints and movement joints a backer rod such as the PE round cord is compulsory in order to prevent three-sided adhesion. With very narrow, shallow connection joints it can be dispensed with in individual cases.
How long does the new joint have to dry before it may be put under load?
The scratch test serves as the check: only when the surface feels firm and dry and can no longer be pressed in may the joint get wet or be put under load. The drying time depends on the layer thickness.
Can I simply apply new silicone over the cracked joint?
No. New silicone does not adhere lastingly to old silicone. The cracked joint has to be removed completely, the substrate cleaned, and the joint renewed properly with a backer rod and new silicone.
More on choosing the right colour and product for wet areas in the Guide: Which silicone for the shower? and in the Guide: OTTOSEAL S100 vs. S105 in detail. You will also find suitable products directly in our category sanitary silicone.
More guides on this topic
All guides on “Problems with silicone joints”
Instructions
Guide: cleaning silicone joints – what helps and what harms
Clean silicone joints properly: scratch test, household remedy check and deep clean
Problem solving
Guide: preventing and removing mould in silicone joints
Remove mould from silicone joints, when the joint has to be renewed and how to prevent it
Problem solving
Guide: silicone joint is pulling away from the wall – causes and fix
Why silicone joints come away from tile, bath or frame and how the new joint holds