Making a fire-rated joint correctly
In short: A fire-rated joint is never created by the sealant alone, but always by the complete tested assembly of building element, joint width, joint depth, backing material and sealant. In practice that means: take the joint dimensions, make the flanks sound and dust-free, install non-combustible backing cord to an even depth, inject fire protection sealant free of bubbles and smooth it within the skin formation time. The fire resistance duration that results – up to 240 minutes in the tested system according to EN 13501-2 – is not determined by the installer, but by the European Technical Assessment (ETA) of the system used. If you deviate from the joint width, joint depth or backing, you have an ordinary joint.
Which joints this guide applies to
This guide describes the linear fire-rated joint: the connection and movement joint in fire-resistant walls and ceilings made of masonry, concrete or drywall, for example at the connection of a partition wall to the structural ceiling or to a column. There are tested systems for this which you can carry out yourself as a skilled tradesperson.
You have to distinguish two situations. Cable and pipe penetrations are not a joint but a penetration seal: this requires a product whose approval expressly covers penetration seals – for the Sika Sikacryl-621 Fire Plus that is ETA 23/0964 alongside ETA 21/0889 for linear joints – and exclusively in the construction described there. And if the joint is part of proof to the building authority, a fire protection expert or a property insurer, the work belongs in the hands of a specialist company with a documentation obligation. Sika releases the Backer Rod Fire backing cord for commercial installers only anyway.
Shopping list
Basic set (always required)
Sika Backer Rod Fire backing cord A1
Mineral wool in a glass fibre mesh, approx. 250 kg/m³, reaction to fire A1 according to EN 13501-1. Seven diameters from 12 to 60 mm.
Intumescent fire protection sealant for indoor use, ETA 21/0889 and ETA 23/0964, up to 240 minutes according to EN 13501-2.
Birtel joint smoother set 4-piece
Four joint shapes for pressing in within the skin formation time. Without pressing in, the sealant does not fully wet the flanks.
View all high-temperature and fire protection sealants
Optional – depending on the installation situation
- Soudal Firecryl FR – if the joint is to be painted over later. Intumesces from approx. 120 °C, ETA 13/0335, up to 240 minutes according to EN 13501-2 in the tested assembly with PE backing according to EN 1366-4.
- Ottoseal S94 Fire Protection Silicone B1 – for connections to glass and fire-resistant glazing. Low flammability according to DIN 4102-B1 between solid mineral building materials, tested by the Holzforschung TU München.
- Ottoseal S54 Fire Protection Silicone – for difficult adhesion substrates such as asphalt, tar, stainless steel or aluminium, on which acrylic does not hold.
- Soudal Gun Foam B1 or Soudafoam B1 with combi adapter – only if a cavity, a wall opening or a frame gap behind the joint has to be filled.
- Birtel foam gun F8 – for metered foam application; thanks to its 2-in-1 valve, the Soudafoam B1 can also be applied with a spray tube.
- OTTO Cleaner T – for degreasing non-porous flanks such as glass, stainless steel and aluminium, flash-off time approx. 1 minute.
Tools
- Manual, compressed-air or cordless cartridge gun
- Utility knife and a blunt installation tool (tamping stick, joint scraper) for the backing cord
- Tape at least 40 mm wide for cutting the mineral wool cord to length
- Hand brush, vacuum cleaner, spray bottle with water
- Tape measure or joint gauge, safety goggles and gloves
Why a fire-rated joint only works as a tested system
The most common and most expensive mistake on site: „B1“ and „fire resistance“ are confused. These are two completely different things.
B1 according to DIN 4102-1 is a building material class and describes how the material itself behaves: low flammability. The Soudal Gun Foam B1 has been tested as B1 by MFPA Leipzig, the Ottoseal S94 according to DIN 4102-B1 between solid mineral building materials. But that says nothing about how long a building element withstands fire.
Fire resistance according to EN 13501-2 – that is EI 30, EI 90, EI 120 and so on – always applies to a tested construction, not to a product. Sika puts this unambiguously in its product information: the sealant alone does not turn a joint into a fire-rated joint. What counts are joint width, joint depth, backing and wall or ceiling type exactly as described in the declaration of performance and the ETA. The same applies to the backing cord: it achieves up to 240 minutes according to EN 1366-4 only in the system with the matching Sika sealant, documented via ETA 23/0088.
In practice this means: before you attach the first cartridge, you need the declaration of performance or the ETA of your system, and in it the line that matches your joint. If your joint width is not listed there, your work is not covered – regardless of how clean it looks.
The second adjustment factor is the diameter of the backing cord. It is deliberately installed oversized so that it sits under compression and limits the joint to the intended depth:
Rule of thumb from the data sheet: the diameter is around 15 % above the joint width. Specifically 12 mm for 7–10.2 mm joint width, 15 mm for 9–12.7 mm, 20 mm for 12–17 mm, 30 mm for 16–25.5 mm, 40 mm for 24–34 mm, 50 mm for 32–42.5 mm and 60 mm for 40–51 mm. How joint width and joint depth relate to each other in general is explained by our guide How wide can a silicone joint be?.
Step by step
Take the joint dimensions and determine the system
Measure joint width and joint depth at several points – structural joints rarely run parallel. Compare the largest and the smallest dimension with the declaration of performance of your sealant system and then choose the cord diameter (around 15 % above the joint width).
Why: The fire resistance duration depends directly on both of these dimensions. A spot that is too wide falls outside the approval, even if the rest fits.
Check: Every measured width lies within a line listed in the ETA. Note the measured values and the ETA number – that is your proof later.
Make the flanks sound and clean
The joint flanks must be sound, clean and dry, free of oil, grease, dust, cement laitance, paint residues and water repellents. Mechanically free porous substrates of loose particles, for example with a wire brush or grinding disc, then vacuum. Degrease non-porous flanks such as glass, stainless steel or aluminium with OTTO Cleaner T and a clean, lint-free cloth, flash-off time approx. 1 minute. Iron and steel need an anti-corrosion coating beforehand. With the Soudal Firecryl FR, pre-coat strongly absorbent substrates with diluted product (1 part product, 2 parts water).
Why: An intumescent sealant bead is useless if it detaches from a dusty flank in a fire. Adhesion is the weakest point of the system.
Check: Scratch over the flank with your thumbnail – nothing may come off as sand. An adhesive tape pressed on must stay clean when peeled off.
Foam cavities – only if present
If there is a wall opening, a frame gap or another cavity behind the joint, fill it with B1 foam. Shake the can vigorously approx. 30 times before use and screw it firmly onto the gun. Fill only about a third to half of the cavity, as the foam continues to expand. Application temperature +5 to +30 °C, optimum 15–25 °C. The Soudafoam B1 can be cut after approx. 65 minutes; then cut off any excess flush with a knife.
Why: Foam keeps the cavity smoke-tight, but does not replace the joint backing in the tested joint assembly. The cord is still required. More details in the guide Using PU foam correctly.
Check: Test cut with the utility knife – the foam must no longer pull at the blade, the cut surface is fine-pored and dry. If it pulls strings, keep waiting.
Install the backing cord
Unroll the roll slightly: the end marked green lies on the outside, the end marked red on the inside must not be pulled out. To cut it to length, wrap the cord with tape at least 40 mm wide and cut through the tape in the middle. Then press it evenly to the intended joint depth with a blunt tool – firmly seated, without twisting, without bulging, without gaps and without overstretching the cord lengthwise.
Why: The cord limits the joint depth and prevents adhesion to the joint base as well as the flanks. If the sealant also adheres to the joint base, it tears when the building element moves – and a torn joint is open in a fire. The tape when cutting prevents mineral wool from escaping from the glass fibre mesh.
Check: Run a finger along the entire length of the joint: the same depth everywhere, no section can be shifted sideways, no butt gap between two pieces of cord.
Check the climate and inject the sealant
For the Sikacryl-621 Fire Plus, air and substrate temperature must be between +5 and +30 °C, the substrate at least 3 °C above the dew point. The same +5 to +30 °C applies to the Soudal Firecryl FR; do not apply if frost threatens during curing. Inject the sealant free of bubbles with the cartridge gun so that both flanks are completely wetted. In vertical joints and overhead, work from bottom to top.
Why: An air bubble in the bead is a channel for hot gases in a fire. Below the dew point, condensate forms on the flank and the dispersion does not cure cleanly.
Check: The bead stands slightly above the joint plane and shows no gaps or sink marks. At a test spot, lift the fresh sealant with a spatula: it must adhere continuously to both flanks.
Smooth within the skin formation time
Press in with a smoothing tool and a little water. The time window is tight: the Sikacryl-621 Fire Plus forms a skin after approx. 15 minutes, the Soudal Firecryl FR after approx. 20 minutes. Therefore work in sections of two to three metres – first inject, then smooth, then carry on. Clean fresh residues and tools with water; cured material can only be removed mechanically.
Why: Smoothing is not a cosmetic step. Only the pressure of the tool presses the sealant against both flanks over the full surface. After skin formation you only move the surface and tear open the adhesion underneath.
Check: Let the residues of the bead run along on a piece of cardboard. If the sample still sticks slightly to your finger when tapped, the time window is open; if it pulls strings or feels dry, it is closed. Technique and choice of tool in detail: Smoothing a silicone joint perfectly.
Let it cure, check, document
Do not load the joint during curing and do not paint over it. After curing, the Firecryl FR can be painted over with most water-based paints – the paint must be elastic enough, a test coat is recommended. The Ottoseal S94 is paint-compatible according to DIN 52452, but expressly not paintable, and must be fully cured before it is exposed to temperatures above +50 °C.
Why: A fire protection element without proof is worthless in the event of damage. Photos, joint dimensions, product name and ETA number take five minutes and later decide the acceptance.
Check: Visual inspection over the full length – no cracks, no voids, no detachment at the flanks. Photos before and after closing, plus a sheet with date, joint dimension, product and ETA number.
Times and checkpoints at a glance
| Work step / product | Time value | How you can tell it fits |
|---|---|---|
| Soudal Soudafoam B1 – skin formation | approx. 8.5 minutes | Surface no longer sticks to the glove |
| Soudal Soudafoam B1 – cuttable | approx. 65 minutes | Test cut: foam pulls no strings, cut surface fine-pored |
| Sikacryl-621 Fire Plus – skin formation | approx. 15 minutes | Must be smoothed by then; residual sample still sticks slightly to the finger |
| Soudal Firecryl FR – skin formation | approx. 20 minutes | As above; afterwards do not move the surface any more |
| Application temperature acrylic sealants | +5 to +30 °C | Substrate at least 3 °C above the dew point (Sikacryl-621) |
| Application temperature B1 foam | +5 to +30 °C, optimum 15–25 °C | Can hand-warm, foam flows evenly from the gun |
| Application temperature Ottoseal S94 / S54 | +5 to +40 °C ambient, +5 to +35 °C bonding surface | Flank does not feel cold and damp |
| Ottoseal S94 – temperature load | only above +50 °C after complete curing | Joint cured through, elastic-firm even in depth |
Common mistakes – and how to avoid them
- „B1 foam alone is enough“. B1 is a building material class, not a fire resistance duration. An opening filled with B1 foam is not a tested penetration seal. Solution: foam the cavity, but additionally close the joint according to the ETA with backing and fire protection sealant.
- PE round cord instead of mineral wool. The cheap PE cord is combustible. It belongs in normal expansion joints – in fire-rated joints only if exactly this assembly is in the ETA, as with the Firecryl FR with PE backing according to EN 1366-4.
- Smoothed too late. After 15 to 20 minutes the window is closed. Anyone who injects 10 metres in one go and then smooths destroys the adhesion that has just formed at the start of the run. Solution: sections of two to three metres.
- Wrong sealant for the substrate. Firecryl FR adheres poorly to natural stone, bitumen, glass and metal; Sikacryl-621 Fire Plus is not intended for floor joints, sanitary joints, glass, natural stone or untreated steel. For glass and metal, a fire protection silicone such as the S94 or S54 is the right choice. If nothing holds despite choosing the right product: Which primer helps when silicone does not adhere.
- Acrylic outdoors. Both fire protection acrylics are expressly intended for indoor use and do not tolerate permanent moisture or weathering.
- Cord installed overstretched or twisted. Then the joint depth varies along the length, and in places the joint no longer corresponds to the tested assembly. Always press in evenly with a blunt tool, never pull lengthwise.
Frequently asked questions
Does the fire protection sealant alone make the joint fire-resistant?
No. The stated fire resistance duration of up to 240 minutes according to EN 13501-2 applies exclusively to the construction described in the declaration of performance or the ETA. Joint width, joint depth, backing and building element type must be listed there. On this, Sika states expressly: the sealant alone does not turn a joint into a fire-rated joint.
What is the difference between B1 and EI 90?
B1 according to DIN 4102-1 describes the reaction to fire of the building material itself – low flammability. EI 90 is a fire resistance class according to EN 13501-2 and describes how long a tested construction maintains integrity and thermal insulation. A B1 product can be part of an EI-classified joint, but taken on its own it is not one.
What diameter does the backing cord need?
Around 15 % more than the joint width. According to the Sika data sheet: 12 mm for 7–10.2 mm joint width, 15 mm for 9–12.7 mm, 20 mm for 12–17 mm, 30 mm for 16–25.5 mm, 40 mm for 24–34 mm, 50 mm for 32–42.5 mm and 60 mm for 40–51 mm.
Can I paint over a fire-rated joint?
That depends on the product. The Soudal Firecryl FR can be painted over after curing with most water-based paints and varnishes; the paint must be elastic enough, a test coat is recommended. The Ottoseal S94 is paint-compatible according to DIN 52452, but not paintable.
May I make a fire-rated joint outdoors?
Not with both fire protection acrylics: Sikacryl-621 Fire Plus and Firecryl FR are intended for joints inside buildings and do not tolerate permanent moisture or weathering. For weathered connections, silicones such as the Ottoseal S94 with very good weathering, ageing and UV resistance come into consideration – but here too the approval applies only to the tested assembly.
How much movement can the joint absorb?
For the system of Sika Backer Rod Fire and Sika sealant, the permissible total deformation is 7.5 %. The Soudal Firecryl FR absorbs around 12.5 % total deformation. Calculate the expected movement of the building element beforehand – if the joint width is not sufficient, the joint must be made wider, and that again changes the tested assembly.
Questions about the work?
If you are unsure which system suits your joint, give us a call: Baustoffheld, Lindenstrasse 56, 47623 Kevelaer, +49 (0) 2831 - 916 77 03. You will find an overview of all fire protection foams in the category Fire Protection Foam.
Important note on suitability for use
This guide describes the procedure in general terms. The fire resistance duration of a joint results exclusively from the respective tested system assembly of the manufacturer. What counts are its European Technical Assessment (ETA) or general building authority test certificate and the limit values set out there for building element, joint width, joint depth and backing. If the work deviates from this, there is no fire-rated joint – even if a fire protection sealant was used.
This page does not replace a proof of suitability or specialist design. Fire-rated joints and penetration seals in required building elements and in escape and rescue routes belong in the hands of a specialist company that documents the work. If in doubt, ask your site manager, the fire protection expert or us: +49 (0) 2831 - 916 77 03.
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