Make no mistake, you have total control over your project. A professional flat roofing specialist would never let you make decisions without all the essential information, though.
One of the things that your average roofing contractor may not talk about is the danger of adding an OSB layer over your PIR insulation.
Another layer of protection seems like a simple way to create a sturdy roof surface, but there are some factors to consider first.
We’re here to give you the full picture so you can make an informed decision for a better outcome (whether you’re doing the work yourself or hiring a contractor!).
Here are the 7 things you need to know about OSB over PIR insulation, so you can make the best choice for your project.
Weight Matters…
A roof is a careful balance of strength and weight. Adding anything extra can tip that balance and lead to serious issues.
With this in mind, it might seem like a good idea to add a layer of something rigid that would potentially add structural strength. But the stronger the addition, the heavier it will likely weigh.
With this in mind, an 18mm OSB board can add significant weight to your roof (around 11-14 kg per square metre!) and tip those precarious scales. The extra weight can strain your roof structure, potentially leading to sagging, deformation or even long-term damage.
By trying to make your roof sturdier with OSB over your PIR insulation, you can actually do more harm than good.
Fixing Frustrations: Aesthetics and Functionality
Imagine that you decided to cover your PIR insulation with OSB. To keep the sheets secure within the structure of our roof, you’d need to use the proper fixings. This would be to prevent movement, slipping and protect your roof in high winds and bad weather conditions.
Fixing OSB to PIR requires specialised fixings that can be tricky to install since your PIR insulation needs to maintain its insulating quality, and any fixings will create tiny punctures in the foam. It wouldn’t be wise to simply use construction adhesive to bond the two together, as this may not stand up to the pressures your roof will be under, even in normal conditions.
This means just the attachment of your OSB could be reducing the effectiveness of your PIR and potentially causing future issues.
The worst part? The fixings can stick out, affecting the roof’s appearance, perhaps not giving you the clean, modern appearance you were hoping to get from your flat roof in the first place.
The truth that your average contractor may not want to tell you because they’d have to explain all of the above is that fixing directly into PIR (without OSB) often gives a cleaner, more streamlined finish.
Moisture Mayhem: Condensation Risks
Condensation. The word that all roofers dread.
Most roofing designs are made to avoid condensation build-up, which can lead to damp, leaks, mould and, eventually, structural damage.
The problem is that OSB can trap moisture, increasing the risk of condensation under the OSB layer itself. This trapped moisture can damage the PIR insulation, reduce its effectiveness and even cause the OSB to rot.
Proper ventilation or a vapour control layer are essential to prevent these issues – not OSB.
Fire Risk: Playing with Fire
PIR insulation is already combustible – adding OSB increases this fire risk to, well, risky levels. You want your roof to be something you can trust in and pretty much forget about, even though it’s above your head all day. Adding another fire risk element to the mix can steal your peace of mind.
Make sure your roof design complies with clear fire safety regulations to keep your property safe.
PIR is Tougher Than You Think: No Need for OSB Backup
It’s understandable that you want your roof to be as sturdy as possible, but OSB isn’t needed.
Modern PIR insulation is designed to handle foot traffic and maintenance activities without needing an extra layer of OSB.
It’s strong enough to distribute weight evenly and maintain its performance, so if a contractor is using accessibility as a reason to add in OSB over PIR they might not be taking into account the built-in strength of your PIR insulation board.
Industry Guidelines: Don’t Void Your Warranty
While a general contractor may agree to add OSB over your PIR insulation, they might not be aware of one big problem…
Leading manufacturers and industry standards don’t recommend adding OSB directly over PIR insulation. Doing so would go against a widely acknowledged standard that only seeks to make your roof safe, strong, effective and long-lasting.
Adding OSB might even void your warranty on certain materials, leaving you with no protection if problems arise after the contractors are long gone.
Costly and Complex: Is it Really Worth It?
If you have a strict budget for your roof build, then OSB sheeting might not be your friend.
Adding OSB increases material and labour costs without offering significant benefits. In fact, it could cost you more in the long run, as it is likely to create issues during installation as well as in the future.
It can also make future repairs more complicated and expensive, and we would always recommend factoring in the long-term upkeep costs of your chosen roof system.
What’s The Recommended Approach?
At Proactive Flat Roofing, we follow industry best practices when constructing, replacing and repairing your flat roof. We’ll always give you transparent guidance about your roof structure, so you can make the right choice for your requirements.
So how would we manage PIR insulation in a roof installation?
We use high-quality single-ply membranes or other approved waterproofing to protect your PIR insulation. This ensures compliance with regulations and enhances the durability and efficiency of the PIR, giving you the best of both worlds.
With specialist roofing contractors who have experience with different flat roof systems, you can rest assured that your PIR and your roof overall, are in safe hands.
What Now?
You might be tempted to add OSB over your PIR insulation, and that’s because it makes sense on the surface of it. But it’s important to weigh the pros and cons carefully for your specific roof build.
Want to work with flat roof specialists who will always empower you to make the best decisions for your project? Get in touch with our experts today to get your free online quote.