The short answer: In the sanitary area the minimum width is 5 mm, for movement joints in building construction 10 mm. At the top end the limit is less sharp: up to about 25 mm a joint can be run cleanly; above that it becomes hard to smooth and tends to sag – then a structural solution belongs there. What matters is not the width alone but the ratio of width to depth: up to 10 mm width about as deep as wide, above that half as deep as wide. And the most important rule of all: the sealant may adhere only to two flanks, never to the back of the joint. A PE round cord ensures that.
Why the joint width matters at all
A silicone joint is not filler material but a component with a task: it is to take up the movement between two components without cracking and without coming away from the flanks.
How much movement it tolerates is stated by the manufacturer as the permissible total deformation – for sanitary silicones usually 20 to 25 percent. That is a percentage value, and that is precisely the point: a joint 5 mm wide with 25 percent deformability takes up just 1.25 mm of movement. A joint 10 mm wide of the same material manages 2.5 mm.
Joints built too narrow are therefore the most frequent reason why a joint cracks or comes away on one side – not poor material and not sloppy smoothing.
The minimum dimensions
| Type of joint | Minimum width | Note |
|---|---|---|
| Sanitary joint, connection joint in the bathroom | 5 mm | Bathtub, shower, washbasin, worktop |
| Movement joint in building construction | 10 mm | Expansion joints, connections between components |
| Floor joint, bay-dividing joint | 10 mm | Width depends on the bay size |
| Glass sealing | excepted | the specifications of glazing construction apply here |
These values are lower limits, not target values. Where the components move more – long joint sections, wood, large temperature differences, young screed – the joint has to be made wider.
Calculating the width
For movement joints in building construction the required width can be estimated:
Joint width = expected movement ÷ permissible total deformation × 100
Example: 2 mm expected movement with a sealant with 25 % permissible total deformation gives 2 ÷ 25 × 100 = 8 mm. Rounded up to the minimum dimension of 10 mm for movement joints.
In the domestic bathroom nobody works that out, and it is not necessary either. There the practical rule applies: rather 8 to 10 mm than barely 5 mm. The difference costs a little more material and doubles the life of the joint.
The ratio of width to depth
A silicone joint is meant to be able to stretch. The thicker the material cross-section in the middle, the more force is needed for that – and the sooner the joint tears off at the flanks. That is why the following applies:
| Joint width | Joint depth | Ratio |
|---|---|---|
| up to 10 mm | about equal to the width | 1 : 1 |
| 10 to 20 mm | about half the width, at least 8 mm | 2 : 1 |
| 20 to 25 mm | 10 to 12 mm | 2 : 1 |
| over 25 mm | check a structural solution, such as a profile or cover strip | |
The depth is not determined by the components but set by the backing. Simply letting a deep joint fill up not only wastes material – it makes the joint stiff and short-lived.
Joint-back adhesion – the mistake you cannot see
This is the most important term in this subject. A joint works only if the sealant adheres to exactly two flanks and can move freely in between. If it additionally adheres to the back of the joint – the third flank –, it is held from three sides when stretching and tears.
This becomes visible only after months: the joint develops fine cracks in the middle or comes away at one flank, although the material and application were right. The cause then lies in the geometry, not in the product.
Joint-back adhesion is prevented by a closed-cell PE round cord, which is pressed into the joint before sealing. It sets the joint depth and at the same time acts as a separating layer from the back of the joint. If the joint is too shallow for that, a PE separating tape takes on the same task.
Inserting the round cord correctly
Closed-cell, not open-cell. Open-cell material soaks up water in damp rooms – the sponge effect promotes precisely the mould that the joint is meant to prevent. For bathroom and outdoors only closed-cell PE round cord comes into question.
Choose a diameter around 25 percent larger than the joint width, so that the cord wedges and does not wander when sealing.
Do not pierce it. Press the cord in with a blunt tool or the back of your finger, never with a screwdriver or knife. A damaged closed-cell cord takes up water and gives off gas as it cures – bubbles form in the fresh joint.
Even depth. The cord determines the joint depth. If it runs wavy, the cross-section fluctuates and the joint tears at the thinnest spot first.
The triangular fillet joint in the sanitary area
Between floor and wall or at the edge of a bathtub a rectangular joint often cannot be made – two components meet there at a right angle. The usual solution is the triangular fillet joint.
It too needs a separating layer in the angle so that no joint-back adhesion arises. For that a thin round cord is laid in the corner or a PE separating tape is stuck in. The leg length should correspond at least to the minimum dimension of 5 mm, 8 to 12 mm is usual.
By design, a triangular fillet joint takes up less movement than a rectangular joint of the same width. Where a lot of movement is to be expected – for example at a bathtub that sinks when filled –, it should be made rather generous.
Running wide joints – in practice
From about 15 mm width, sealing becomes more demanding. Three things help:
Cut the nozzle to suit. The opening should correspond roughly to the joint width and be cut at a slant. Openings that are too small force repeated restarting and produce air pockets.
Work in one pass. Run long joints in sections of at most one to two metres and smooth each section immediately – silicone forms a skin within minutes, and over an edge that has set it can no longer be smoothed cleanly.
Sparing with smoothing agent. One spray onto the smoother is enough. Too much smoothing agent runs into the joint and disturbs adhesion at the flanks. With wide joints better to draw twice with light pressure than once with a lot of agent.
With very wide connections over 25 mm, silicone is no longer the right answer. A structural solution belongs there – a cover strip, a profile or levelling of the substrate before sealing.
Joints outdoors and on the facade
Outdoors different standards apply, because the movements are bigger. A component that reaches 50 °C in summer and is below zero in winter expands noticeably over a longer joint section.
As a guide: for connection joints at windows and doors 10 to 15 mm are usual, for bay-dividing joints in facades the width depends on the bay size – the longer the continuous section, the wider the joint. With precast concrete elements and long runs of joints 20 mm and more occur.
Two points are added outdoors. First, the sealant must be UV- and weather-resistant; a sanitary silicone does not belong in a facade joint. Second, the joint absolutely needs a closed-cell backing, so that no water stands behind the sealant and tears it open in frost.
Which sealant for which joint?
| Joint | Suitable sealant | Why |
|---|---|---|
| Sanitary joint, shower, bathtub | Sanitary silicone, neutral | with fungicidal additives, permanently water-repellent |
| Natural stone, marble | Natural stone silicone | no migrating plasticisers, no edge discolouration |
| Connection joint that is painted | Acrylic or paintable hybrid | Silicone cannot be painted over |
| Movement joint in building construction | Construction silicone or hybrid | high permissible total deformation |
| Floor joint, walkable | Hybrid or PU, higher Shore hardness | has to withstand foot traffic |
| Metal, window construction | neutral-curing silicone | Acetic acid attacks metals |
| Permanently wet area, swimming pool | Special silicone with primer | constant immersion in water |
The value that matters when choosing is in the technical data sheet: the permissible total deformation. It states what percentage of the joint width the sealant may take up permanently – and goes directly into the calculation of the required width.
Tools and procedure
For a clean joint you need little, but the right things:
- Cartridge gun with anti-drip mechanism – without it material runs on and the joint becomes uneven
- Utility knife for cutting cartridge and nozzle
- Closed-cell PE round cord in the right diameter
- Joint smoother or smoothing tool, for triangular fillet joints with the right radius
- Spray bottle with smoothing agent
- Masking tape for masking visible edges
- Lint-free cloths and isopropanol for degreasing
The procedure in brief: clean and degrease, prime if necessary, press the round cord in to depth, mask if appropriate, apply the sealant bubble-free in one pass, smooth immediately and pull off the adhesive tape immediately – not only after it has dried on, otherwise the edge tears out.
Substrate and primer
The best joint geometry is of no use if the sealant does not adhere. The flanks must be dry, free of dust and grease and free of loose parts, paint residue and rust.
Some substrates additionally need a primer. It does not replace the cleaning but comes after it:
| Substrate | Primer needed? |
|---|---|
| Glazed tile, glass, enamel | as a rule no |
| Sandstone and other absorbent natural stones | yes, special natural stone primer |
| Concrete, plaster, aerated concrete, fibre cement | yes, primer for absorbent substrates |
| Metal, coated metals, sanitary acrylic | adhesion promoter recommended |
| Permanently wet area, swimming pool | yes, special permanent-wet primer |
With natural stone, besides the primer the choice of sealant is also decisive – normal silicone produces dark edge discolouration. More on this under Which silicone for natural stone.
The most frequent mistakes
Below 5 mm even a good sealant can hardly take up movement. The joint cracks or comes away, often already in the first year.
Joint-back adhesion, stiff cross-section, wasted material – and a joint that tears exactly where it should be working.
It sucks up water and holds it permanently behind the joint – the best precondition for mould.
Gas escapes from the damaged cell into the fresh mass. The result is bubbles and holes in the finished joint.
It gets between sealant and flank and prevents adhesion exactly where it is needed.
Frequently asked questions
How wide may a silicone joint be at most?
Silicone can be applied cleanly up to about 25 mm width. Above that the mass sags before curing and the joint can hardly be smoothed evenly any more. For larger connections a structural solution belongs there – a profile, a cover strip or levelling of the substrate.
What is the minimum width of a silicone joint?
In the sanitary area 5 mm, for movement joints in building construction 10 mm. Excepted are glass sealings, for which separate rules apply.
How deep must a silicone joint be?
Up to 10 mm width about as deep as wide. For wider joints about half as deep as wide, but at least 8 mm. The depth is set via the round cord, not via the components.
What is joint-back adhesion?
The sealant adheres not only to both joint flanks but additionally to the back of the joint. It then cannot stretch freely and tears. This is prevented by a closed-cell PE round cord or a separating tape.
Does every silicone joint need a round cord?
Every joint that is meant to take up movement, yes. With very shallow connections, where no cord fits in, a PE separating tape takes on the same task. Only with pure sealings without movement – such as a glass sealing – can it be dispensed with.
How do I run a wide silicone joint cleanly?
Cut the nozzle to the joint width, work in sections of one to two metres and smooth each section immediately, before a skin forms. Spray smoothing agent sparingly onto the smoother, not into the joint.
How much silicone do I need for a joint?
Usage in millilitres results from width times depth times length. A joint 8 mm wide and 8 mm deep uses around 64 ml per metre – a 310 ml cartridge is therefore enough for about 4.8 linear metres. With triangular fillet joints about half. Allow a 10 to 15 percent surcharge; exact values per joint dimension are in our usage table.
What happens if the joint is too deep?
A material cross-section that is too thick makes the joint stiff. It can then take up movement only under high tension and therefore tears off at the flanks sooner. In addition you use material unnecessarily. The depth is set via the round cord, not by filling it full.
Does the joint have to be masked?
It is not necessary, helpful with visible edges and on absorbent substrates that are hard to clean. What matters is the moment of pulling off: immediately after smoothing, while the mass is soft. Anyone who waits until a skin has formed tears the joint edge off with it when pulling off.
Why does my silicone joint keep cracking?
Usually the geometry is not right: joint too narrow, joint-back adhesion through missing backing, or a substrate that moves more than the sealant tolerates. Less often it is down to the product. The causes are dealt with in detail in our guide Silicone joint keeps cracking.
Can a joint that is too narrow be widened afterwards?
Yes, but only with effort: the old silicone has to come out completely, and the flanks have to be reworked mechanically – with tiles that is hardly possible without damage. In practice a joint that is too narrow is therefore usually left as it is and renewed more often.
