Backing Out Joints
Backing out joints is a preparatory step in the process of mastic sealant joint sealing prior to application. As it’s not always an essential choice, joint backing is ambiguous to many people and is a somewhat misunderstood process. Let’s break it down and explain some important points to consider when deciding wether a joint backer is to be used.
What is a joint backer
Exactly as it sounds, a joint backer backs out joints, gaps and cracks prior to the application of a wet or paste-like filler or sealant of some kind, providing a controlled depth, sound base and backstop for the material to cure against. Usually produced from flexible, polyethylene foam and known the world over by many different names, such as backing foam, backer foam, backing rod, backer rod, caulking foam, caulking strip, cork, chalk, filler foam, backing cord, backer cord, polycord etc. etc.) it’s technically the same thing.
The need for backing out joints
Why do we need a backer and when should it be used versus not used? The answer to this question is often subjective and decided on by the end user or ‘applicator’ at the time of preparation. Some professionals would choose to use a joint backer for certain applications but not use in other applications depending on the circumstances or their style and/or method of work. In other cases, a specification for the work or project by an architect, designer or project manager may state that a joint configuration requires a joint backer to be used and may then also elaborate as to the specific material(s) suggested or recommended, depending on wether the ‘spec.’ is open or closed. One factor in the decision to use a backer is to break the adhesion at the bottom of the joint or gap, thus preventing a 3-sided bond, which would tear or shear upon movement. The elasticity of a joint is greatly improved Typically, facade joints, mechanical joints, structural joints and expansion joints would all require a backing, whereas decorative and/or capping joints are shallower and a sealant or filler would be used uniquely.
Further explanation (from wikipedia)
Backer rod, also called backer material or back-up rod, is a flexible foam product used behind caulking to increase elasticity, reduce consumption, force the caulking into contact with the sides of the joint creating a better bond, determine the thickness of the caulking, and define the cross-section hour-glass shape of the caulk. The backer rod also acts as a bond breaker to keep the caulking from sticking to the bottom of the opening—called a three-sided bond—with the caulk only adhering to the sides of the opening in an hour-glass shape it can flex more easily and is less likely to tear. Backer rods can also be used to reduce consumption of the caulking by filling part of the joints.
Backer rod is often round and rope-like in shape but many special types and shaped such as square, rectangular and D-shapes are available.
Porous so it will let gasses through which could otherwise cause blistering of the sealant. Also, open-cell backer rod allows air to get to the back side of the sealant which accelerates curing when used with air-cured sealants such as silicone. Open-cell rod is more compressible than closed-cell foam and should be 25% larger diameter than the joint.
Does not absorb water and is impermeable. Closed-cell rods are less compressible and should not be compressed more than 25%. Closed-cell rod will also lose firmness and out-gas if damaged during installation or overcompressed or at sharp bends. The gasses cannot pass through this backer rod and can deform, weaken, and even cause holes (leaks) in the sealant as it escapes. Out-gassing is the reason open-cell backer rod was developed.
Advantages of backing out joints
As well as the obvious advantage of backing out joints using some sort of gap filler prior to applying a wet sealant, of drastically reducing the actual depth and overall volume of the joint, (meaning less space to fill with the more expensive joint sealant material) there’s also some very good reasons to using this additional method of gap filling, take a look below:
- bond breaks the bottom of joint
- prevents 3-sided adhesion
- reduce mastic sealant caulk consumption
- lightweight & compact
- unaffected by bacteria, mould & mildew
- doesn’t degrade or rot
- provides backstop for sealant
- controls depth by expanding to fit
- improves performance
- provides insulation
- provides air seal
- prevents 3-sided adhesion
- provides acoustic properties
- won’t conflict with the material
- vibration dampening
Joint configuration is generally based on width and depth calculation, with the length then measured to determine the total volume. As the width of the joint and indeed the total length of the run is non-negotiable, the depth is a dimension that is variable and can be manipulated with the addition of a compressible filler. We have a dedicated GUIDE to mastic sealant estimating on a separate blog here.
Backing out different joint configurations
As mentioned above, a basic joint configuration can be calculated by the width and depth of the joint. SQUARE JOINTS (vertical or horizontal
Where a square (or rectangular) shaped joint is required, square section or round section backer caulk can be used.
Some examples of applications where square mastic sealant joints are situated are as follows:
- connecting floor joints
- saw cut floor joints
- wall connection joints
- between wooden panels (or wood to other substrate)
- between concrete panels (or concrete to other substrate)
- between tiles (or tile to other substrate)
- brickwork or blockwork expansion joints
- between timber sheets (or timber sheet to other substrate)
- connections in cladding
As a rough rule of thumb, the dimensions can be floor (horizontal 1:1 ratio – vertical 2:
Manufacturing backer foam
If we’re looking at which type of foam to use when backing out joints, it’s important to understand the differences. Firstly, there’s the actual material used in the manufacture of the products. We’ll stick to the most common material and indeed what all of the backing foams we stock are made from, Polyethylene (or PE) foam. This versatile, lightweight man-made material is produced as a result of the polymerization of ethylene. (From Oxford Dictionary website www.lexico.com: ‘Ethylene is a flammable hydrocarbon gas of the alkene series, occurring in natural gas, coal gas, and crude oil and given off by ripening fruit. It is used in chemical synthesis, especially in the manufacture of polyethylene.Alternative name: ethene; chemical formula: C₂H₄’) Many commonly used everyday products are constructed from this flexible and lightweight material, such as swimming pool learning floats, packaging filler materials, gym mats and many sports goods. Other industrial applications of polyethylene foam that are useful include appliance dampeners, various nautical flotation items and of course, backing fillers.
Types of backer foam
Take a look at the various shapes and sizes that backer foams are packaged for sale in.
(note the bold measurements are the packs that the various sizes are sold in)
Square/Rectangular Caulking Strip (Texell 3 medium density, course cell, non-crosslinked, closed cell PE foam)
12mm x 12mm x 200m / 15mm x 12mm x 200m / 20mm x 12mm x 200m / 25mm x 12mm x 200m / 30mm x 15mm x 200m / 40mm x 20mm x 100m
Round Section Circular Rod Caulking (medium density, course cell, non-crosslinked, closed cell PE foam)
6mm 2500m / 10mm 1150m / 13mm 750m / 15mm 550m / 20mm 350m / 25mm 200m / 30mm 160m / 40mm 2m / 50mm 2m
Black Square Section Foam 2m Strips (medium density, Plastazote® LD45, cross-linked, closed cell PE foam)
(Please note; called black foam but more dark grey/anthracite coloured)
12mm x 10mm x 2m / 20mm x 10mm x 2m / 25mm x 10mm x 2m
Flat Caulking Foam In 15m Coil Rolls (fine cell, cross-linked, closed cell PE foam)
(to be discontinued mid 2020 – limited stock available as of February 2020)
6mm x 50mm x 15m Coil / 6mm x 75mm x 15m Coil / 6mm x 100mm x 15m Coil / 12mm x 25mm x 15m Coil / 12mm x 50mm x 15m Coil / 12mm x 100mm x 15m Coil
Technical advice & assistance
As the science is often over-looked in mastic sealant application, with preparation more than often rushed or skipped completely, mistakes are frequent with rectification proving costly and time-consuming. It costs nothing and is always an excellent starting point to do some research yourself on the internet and/or contact your suppliers’ technical department (or even a manufacturers’ technical representative) to ask questions and get some advice or feedback on your proposed work regarding the backing in a joint configuration (providing you have a specification or your own idea of which mastic sealant you plan to use). Technical data sheets and Health & Safety sheets (MSDS or Coshh) are usually available for reading and/or downloading (we have sheets on each, individual product online on the E-Shop).
What the professionals say
(Karen is president of Building Diagnostics® Associates, a Hollywood. Fla., firm that specializes in the analysis of roofing and waterproofing problems. This quote is from her informative article on Building Diagnostics® which is no longer available click here to see Karen’s Linkedin) Quote From Karen Warseck
A summary of backing out joints
Wether prepping for the sealing of brickwork, blockwork, flooring, internal or external cladding, frames, sanitary ware or any other substrates, there’s no doubt that backing out joints prior to application can be an important step with many benefits. As conditions and variables dictate the many types of joint configuration, each and every job needs to be reviewed, calculated with a solid a plan of action before any work is undertaken. Choosing the correct type and dimensions of backing foam caulk is essential to achieve a successful finished task. Hopefully, this blog has gone some way to helping you decide which type of product you require.