Sealing the Deal: Building Sealants. A Useful Guide (Part 1)

Background of Building Sealants

We commonly find significant problems with building sealants, problems range from:

  • Unsealed critical junction details
  • Poorly applied sealant
  • Poorly re-applied sealants
  • Incorrect choice of sealant
  • Premature sealant failure
A vast range of sealants are available but how do you choose the right product?
A vast range of sealants are available but how do you choose the right product?

Sealants are possibly one of the most under-appreciated products in the construction industry and yet they often provide the critical technical detailing that will determine the long-term durability of you project. They can protect a multi-million pound external wall insulation project from failing prematurely, provide essential fire protection detailing or just be used as simple caulking to aid in providing a high quality internal decorative finish.

However, there are such a wide range of products available how do you choose the right product for your project? What are the dangers in choosing the wrong product, what supplementary factors also need consideration when choosing sealants and how should sealants be applied?

Sealant technology has progressed significantly over the last 30 years and we now have intumescent sealants available for sealing service penetrations in fire walls, lead sealants used as an alternative to mortaring lead flashings and even bacteriostatic cold room sealants to be used in areas where contact with food is possible.

Historically, ‘caulking’ was used to make heavy wooden ships watertight by soaking hemp in pine tar (Oakum) and driving it into the joints of the ship. The term caulking, is still used loosely to describe the sealing of building joints but more often it refers to internal decorative caulking, whereas we generally refer to ‘sealant’ as being required for external building applications. To my mind there have been some worrying developments in sealant marketing, rather than technology, in particular the marketing of lead sealants as a direct replacement for mortar and we are already encountering numerous cases of premature flashing failure due to premature sealant failure; particularly since the lead pegging is often omitted when sealant is chosen over mortar.  To  my mind lead flashing sealants should never be used for anything other than a temporary fix or at best you should accept that flashing durability will be significantly reduced when compared to mortar.

Causes of Failure

There are two types of failure found in sealants, adhesive and cohesive failure.

Screen Shot 2016-06-12 at 08.13.37
Adhesive failure on left and cohesive failure on right.











Adhesive Failure

More commonly seen adhesive failure of lead mastic sealant on new construction
More commonly seen adhesive failure of lead mastic sealant on new construction

Adhesive failure is recognised as a failure occurring at the joint interface with the building substrate and usually occurs for one of the following reasons:

  • Poor preparation of the joint, perhaps primer should have been applied.
  • Dusty, dirty or contaminated joint interfaces.
  • Sealant applied to damp surfaces
  • Lack of joint backing
  • Lack of tooling during application
  • Incorrectly specified sealant (Expansion & contraction characteristics not suited to application. Failure to choose low modulus sealants for PVCu windows & doors is a common problem)
  • Thermal movement occurring in the building substrate before the sealant has set
  • Old sealant not removed prior to reapplication.
  • Incompatible coatings applied over sealant. Some paints may chemically react.
  • Inadequate depth of sealant
  • Overly wide joints

Cohesive Failure

Less common cohesive failure
Less common cohesive failure

Cohesive failure is relatively uncommon when compared to adhesive failure but is recognised as the splitting of the sealant material rather than failure of the bond with the substrate; it usually occurs for the following reasons:

  • The thermal movement in the building joint exceeds the sealants elastic limit.
  • Lack of joint backing causes inability for movement to be spread across the whole sealant joint.
  • Inconsistent depth of sealant.
  • Thermal movement occurring in the building substrate before the sealant has set
  • Movement joint too small due to poor design and inadequate allowance for thermal movement in the building envelope.
  • Cheap, poor quality sealants.
  • Poor mixing of two part sealants.
  • Air or moisture entrapment results in expansion of air or vapour that can cause the sealant to bubble or fracture.
  • Age related loss of properties

Sealant Omission

It is incredible to think that constructors would complete a project worth hundreds of thousands or even millions of pounds then put that project at risk by omitting to use sealant at critical junction details but we commonly see precisely this problem, particularly on external wall insulation projects. Then there is the common problem we see of timber windows being sealed with hard inflexible mortar fillets, which quickly crack and fail after application and more recently the trend of fixing in windows and doors using nothing more than expanding polystyrene foam as both a fixative and a sealant, when in fact in can not adequately serve either role.

Newly applied OPC mortar fillets to window frames cracked within days of being applied. Note complete absence of sealant.
Newly applied OPC mortar fillets to window frames cracked within days of being applied. Note complete absence of sealant.
Window fixed and sealed using expanding polystyrene foam.
Window fixed and sealed using expanding polystyrene foam.












Tools & Workmanship

Poor application of sealant is a common problem we find, even on new build sites were you would almost definitely expect a high quality finish and of course this is often more than an aesthetic problem. Untidy or un-tooled sealant often lacks the durability of a well finished joint. Contractors tend to approach the application of sealant in one of three ways:

  1. They employ a site finisher who applies the sealant and finishing touches to a project.
  2. They may outsource sealant to a professional sealant company
  3. They give it little thought due to programme pressures and it gets forgotten or applied, often grudgingly, by any tradesman in the area who doesn’t really see it as his job.

The latter approach tends to result in sealant that looks like this…

Poorly applied and unsightly silicone sealant
Poorly applied and unsightly silicone sealant


Various cheap plastic tools exist to provide a high quality surface finish to sealants so why not use them?
Various cheap plastic tools exist to provide a high quality surface finish to sealants so why not use them?

It costs a couple of pounds to buy a plastic tool that will give a high quality finish to the surface of sealant, facilitate better edge adhesion and properly tool the material into the full depth of the joint but sealant is comply untooled or simply finished by running a thumb or finger along the joint. Given the cost and obvious benefit in using these tools it’s hard to see why they are not more commonly used.

Width & Depth of Joints

There is a minimum depth of sealant that should be applied and generally speaking this is around 5mm but can be up to 15mm for some sealants and you should check individual manufacturers instructions. A more common problem we see is where sealants have been used in gaps that exceed their maximum permissible joint width. We’ll include a joint width guide in the table published in part 2 but applying sealant to an overly large gap can result in almost immediate adhesive failure of the sealant on curing, as in this case.

Large gap between window frame and masonry is too wide for the choice of sealant here.
Large gap between window frame and masonry is too wide for the choice of sealant here.












Which sealant should you choose?

We’ve put together a handy guide which will be published in part 2 to help you choose the right sealant  for the particular job under consideration.

Removing Old Sealant

When re-applying sealant it is critical that old or existing sealant is removed back to bare substrate. Applying fresh sealant over the top will simply lead to premature failure. It can be challenging to cut away or peel off old sealant but there are various tools and products on the market that will make this job easier. There are special tools to help cut away sealant and gels that can be applied to to break down acrylic and silicone sealants.

Sealant removal gel
Sealant removal gel
Special sealant cutting and profiling tool
Special sealant cutting and profiling tool

12 responses to “Sealing the Deal: Building Sealants. A Useful Guide (Part 1)”

  1. Nigel Morgan FRICS avatar
    Nigel Morgan FRICS

    Really good stuff – although my understanding to date had been that a flexible lead sealant was likely to perform much better than (particularly cement) mortar which experience suggests all too often fails within days and commonly simply drops out within a few years, especially where retro-fitting of lead is involved. Surely a flexible seal between a relatively rigid and a highly mobile material must (if properly executed) be beneficial?

    1. Joe Malone avatar
      Joe Malone

      Thank you Nigel and I think you make a really important and interesting point. Durable leadwork was originally pegged at regular intervals with lead wedges and then pointed into the bed joint with a lime mortar, which is far more flexible than OPC mortar and has the ability to self heal. Lime mortar was of course overtaken by the use of OPC mortar and subsequently we started to see far more stress failures and cracking in the mortar. Lead mastics are as you say far more flexible, but their use does not dispense with the need to peg the leadwork at regular intervals. This pegging will always inhibit movement at the bed joint and of course we still have maximum permissible limits for lengths of leadwork to keep the coefficient of expansion within acceptable limits. You will be lucky to find any sealant with a guarantee beyond 10 years, most lead mastic sealants fall well short of this but another problem I am seeing is the the use of lead mastic is also causing installers to dispense with the pegs, despite the fact that even NHBC guidance states that lead should be pegged at minimum intervals of 450mm or less. I am consistently finding premature failure of leadwork due to a combination of adhesive failure of sealant, failure to turn lead into the bed joint by the minimum required 25mm and the omission of lead pegs. To some degree I turn back the clock when specifying leadwork and specify lime mortar, which when used in combination with adequately pegged and secure leadwork significantly outlasts the modern lead mastics. Another big issue with lead mastic is that it must be applied to a sound and clean surface. If the mortar is raked out when still green ready for the leadwork then it leaves a cleaner joint, with far better chance of getting good adhesion with the lead mastic. However, this is often forgotten and bed joints are cut into when dry using an angle grinder, which leaves a dirty joint that is often not cleaned ahead of applying the mastic. It is a common reason for premature adhesive failure of lead sealant failure and we do see a high degree of user error when inspecting lead sealants.

      1. Nigel Morgan FRICS avatar
        Nigel Morgan FRICS


        I suspect the problems with lead sealant may well lie more in the workmanship and detailing than in the sealant. But I’ve used some recently at home so will monitor!

        1. Joe Malone avatar
          Joe Malone

          I wouldn’t disagree Nigel. I wouldn’t have such an issue with sealant being used if workmanship and detailing was generally completed to a higher standard but I am still of the opinion that it will be less durable in the long term than lime mortar.

          1. Peter Clarke avatar
            Peter Clarke

            I am a building surveyor now but did spend 15 years in the roofing industry on the tools doing exactly stuff like this.
            The main problem with lead flashings is trying to get them to stay in the brick joint while you fix it. You form the 90 degree turn-in on a timber baulk then offer the piece into the chase/joint. Sometimes whilst working the lead it takes a slight curve, sometimes not. Sometimes the brick course was not laid perfectly flat. So, in most cases you are relying on some mechanical force to hold the lead into the joint, hence lead wedging. Forget the textbook cross sections, they show the finished product. Silicon comes out the tube wet – by definition of a fluid, is unable to accommodate a shear force so it cannot hold it in or force the lead to lay flat on the bricks beds or resist a slight bend in the flashing – you need a mechanical fixing- any that are mentioned in the Lead Development Association guides. It won’t stick to dust either. I’ve even sealed a dusty cut chase to improve adhesion. Also lead will not follow the minor dips in the bed in between the wedges. If you just squeeze sealant in it does nothing to push the lead down whereas a semi dry mortar properly pressed in will. But there are applications where sealant is desirable – maybe if the guy before you didn’t cut a wide enough chase, or where overlaps reduce the available space for a decent thickness of mortar.

  2. Suzanne Crighton avatar
    Suzanne Crighton

    An interesting article Joe,
    As you know we are currently in dispute with our contractor on a very poor EWI job to our private residence. Quite apart from not fitting the correct proprietary sealant strips required for the system (for want of a better description) below the render finish, the contractor was one of those reluctant contractors you describe in the article who had to be called back to seal the system with sealant. This was a job they did badly and with what was clearly an inappropriate and/or cheap product such that it failed almost as soon as it was applied. It is also worthy of note that the BBA certification on our property calls for the sealant used to be the proprietary product of the manufacturer of the system – it wasn’t and therefore renders the system non BBA compliant (though sealant is just one minor area of non-compliance on this job).
    Had everything else been done correctly, they would have ruined it at the eleventh hour…not the case on our job where they did that in the first hour and continued in that vein to the end!

    1. Linda Griffiths & Linda O'Connell avatar
      Linda Griffiths & Linda O’Connell

      Hi Suzanne,

      We also had a poor EWI job, our contractor was not only incompetent regarding the actual installation but also assured us that the EWI system would weatherproof our leaking and damp home! After 3 years we are still struggling to get a resolution and our house now leaks worse than ever. The latest (cheap) sealant debonded in less than two months though the BBA certificate doesn’t specify a particular sealant but does say that the system we have shouldn’t be used on wet or damp walls. We are the Carmarthen clients mentioned in Joe’s December 2015 blog. We’ve been told that we are the only failure, It would be interesting to know what manufacturer etc. you used and where you are. We would be happy to swap stories it might help us both.

      1. Suzanne avatar

        Hi Both,
        I am really sorry to hear you too have been subject to the poor standards maintained by some in this industry.
        Unfortunately for us, litigation seems to be the only answer now, so I am unable to discuss anything – I am sure you understand and hope that you are able to resolve your issues without following this route – I doubt it however after 3 years fighting and like us, you may have to bite the proverbial bullet and engage a solicitor.
        Before you do that, I would suggest you get an independent technical report as without this, there isn’t much you can do – I thoroughly recommend Joe who has done ours and the report is excellent & very comprehensive.
        Good luck & maybe we can swap stories when this is all over!!

  3. Gareth avatar

    Thanks for another great article Joe.

    Looking forward to the guide in part 2 and any views on/experience with Geocel 201 Polymer Mastic Sealant, advertised as a polysulphide alternative. It seems to promise a whole lot, but I wonder if it’s still unproven technology or trying to solve an issue which doesn’t really exist. This is an area I know nothing about.

  4. Linda O'Connell & Linda Griffiths avatar
    Linda O’Connell & Linda Griffiths

    Hi again Suzanne,
    I’ve been wondering how you got on with your litigation. It’s 5 years now since we made the biggest mistake of our lives and believed the EWI industry literature and our contractors totally wrong assurances that EWI would weatherproof our building. We can’t afford litigation, we engaged a solicitor to try to get the contractor to mediation but they stopped replying to our solicitor. We have started writing letters, WE told Ofgem that there are no carbon savings here, please remove us from any carbon figures, the house is colder and wetter than before we started. EDF funded our ECO part of the cost and they spoke to the manufacturer who effectively told EDF that this was our fault and the problems are ‘unrelated construction issues’!! We replied to EDF saying we can’t tackle the construction now because: 1) we would have to cut into and remove the EWI, and 2) we’ve spent our money on the EWI and don’t have enough left for more major works. The BBA report found there was a ‘significant installation failure’ with regard to the water ingress we had but the fact that the BBA have effectively confirmed EWI shouldn’t have been used at all on a wet building is being ignored.
    Sorry for the rant, we simply can’t believe we were sucked into believing that this would work and now it’s proven that it shouldn’t have been applied here we are still being brushed off, the industry needs a good shake-up. We’ve put some videos of the water ingress etc. on you tube, channel called An EWI Failure and we’ve set up a facebook group page to try to get the word out that EWI isn’t weatherproof or the answer to everything. Small problem is we’re not used to social media so we’ve got a lot to learn! Someone is doing a small website for us too, not expensive so we thought we’d give it a go so we can put our full story on there and at least try to help others,

  5. Mo avatar

    Great article, many thanks.

    I don’t know if these comments are monitored, but if so, please could you let me know if a Part 2 was ever made? I’ve tried looking for it but with no joy.

    Also, when it comes to using lime mortar instead of sealant, do you/anyone have suggestions as to the type of lime (hot lime vs NHL, etc), and more specific suggestions as to mix ratios for whatever lime products are most suitable?

    Many thanks

    1. Joe Malone avatar
      Joe Malone

      Hi Mo,

      Unfortunately not; I’ve just never had time to write that follow up.
      With regards to lime, those decisions would need to take into account environment and exposure. For instance, for roofing/high level work, you’d be looking at using something like NHL 5.


      Joe Malone

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