Johnathan Brown, Fixfast
This article was originally posted Here
Constructing or refitting a flat roof might seem like a simple job – especially if you’ve been doing it for years.
But as we all know, nothing is ever as straightforward as we might like!
So much depends on the design of the roof in the first place, whether the flat roof is part of an extension on a building, a living roof, or contains multiple skylights. Single-ply flat roof design and repair needs careful consideration – if best practice is not followed, this could significantly reduce the lifespan of the roof or lead to leaks appearing.
Simple things such as how the roof is fixed can have a massive impact on the overall result.
So, without further ado, here are 10 tips to help you up-level your flat roofing knowledge…
Tip #1: Don’t make your flat roof completely flat.
It might sound counterintuitive, but you really shouldn’t make your flat roof completely flat.
Because a roof that is completely horizontal at a 0-degree angle provides nothing but a nice way to collect rainwater. Having rainwater sitting around on a roof for long periods could eventually damage the roof, so it’s better to construct a flat roof with a slight angle so that rainwater drains off the roof.
Experts say that the main concern with designing a roof to be completely flat is you will likely end up with back falls. You don’t want this – trust us – as it creates standing water (which is heavy). All that extra weight will likely lead to deformation of the deck. Worst case scenario: the complete collapse of the deck. This can be made worse if the standing water starts growing algae – which a very scientific Google tells me is highly toxic to both humans and animals, and can make the damage from a leak significantly worse (just picture a ceiling caved in and damp with a tonne of algae growth – yuck).
On top of the risk of everything caving in, ponding water can become a slip hazard for maintenance folk – especially in freezing conditions.
Tip #2: Add an air gap for cold roof build-ups
Without an air gap, there is a chance that the timbers could rot and the roof could collapse.
There are three main ways to build up the layers of substrate on the roof, and of the ways, the cold roof build-up is the most problematic for this reason.
If not constructed properly, a cold roof could collect condensation, which could lead to rotting timbers. The way to avoid this is by ensuring that there is an air gap between the underside of the deck and the top of the insulation. The edges of the building will need ventilation, too.
Tip #3: Avoid cold roof build-ups where possible
Due to the amount of trouble they can cause, a cold roof build-up is not usually recommended as they can be something of a nightmare to manage.
Industry guidance even states the above (though in more technical terms), and has done for a good few years.
So what’s the solution?
Instead, use a warm roof build-up.
Tip #4: Stick to a warm roof build-up
This is for your own sanity.
Well, and for the longevity of the roof.
A warm roof buildup consists of (going from bottom to top layer): the structural deck, the vapour control layer, the thermal insulation, and finally the waterproof membrane.
Why use a warm roof buildup?
It’s down to simplicity. Warm roofs are by far the simplest way of constructing a roof and achieving good U-values. It’s also industry best practice.
Tip #5: If you’re building a green roof, or a roof garden, stick to an inverted warm roof build-up.
Notice the word ‘inverted’. This is because the layers are added in the reverse order to a standard warm roof.
Most inverted warm roofs have a concrete deck as the build-up can be very heavy, and typically paving slabs or the living roof growing material will hold the membrane and insulation in place.
Tip #6: Do get a pull-out site test
A pull-out site test looks at the fixing and the deck and provides information about what fixings would be right for the deck.
This is particularly recommended for all new-build concrete decks and all refurbishment projects.
Without one, you could run into trouble.
Because a pull-out test ensures that the mechanical fixings specified are suitable for the structural deck in place. And if you don’t carry out a pull-out test, you run the risk of losing your flat roof if the mechanical fasteners are not compatible with the structural deck.
Tip #7: Profile metal decks must be a minimum of 0.7mm thick
Profile metal is often used because it is so lightweight. However, using anything less than 0.7mm in thickness will mean that threaded fasteners simply won’t work, and you’ll struggle to fix it to the structure.
So check carefully!
Tip #8: When using a fastener to secure a flat roof build-up to a metal deck, ensure that the fastener penetrates the underside of the deck by 15mm.
Because most fasteners have a drillpoint and lead-in threads. The threads are the part of the fastener that actually hold it in place – so if the fastener is not drilled in far enough, the threads won’t be gripping the deck properly.
Conversely, overdrive the fastener, and you’ll probably run the risk of stripping the thread, which could result in losing your roof!
Using the 15mm rule ensures that the fastener has good pull out resistance. Which you’ll want, because no one wants an insecure roof.
Tip #9: Only fix to the crowns of a metal deck
You might well think it’s a great idea to get a fastener that is long enough to fix into the crown or the trough of the metal deck.
But although it might seem like the easy option, it’s not actually good practice.
Put simply – a fastener has a head, a smooth section called a ‘thread free’ section, and then the screw part with the threads on it that hold it in place.
Getting one length of fastener that is long enough for the troughs as well as the crowns ultimately means that the fastener won’t be secured properly. This is because you will need to drive the fastener further through the deck more than is necessary, resulting in a decreased pull-out and back-out resistance.
Ultimately, if you hit the trough, it will be unsupported by the insulation. This makes it easier to overdrive the fastener, resulting in stripping and deformation of the deck.
In simple terms, your roof will be wobbly and insecure.
So just get one type of fastener that is suitable for fixing into the crowns and you can avoid the wobbly roof problem.
Tip #10: If your structural deck is plywood, it must be a minimum of 18mm thick.
Industry best practice strikes again!
If your structural deck is plywood or OSB then it needs to be thicker than the metal deck – and 18mm is the recommended thickness.
Remember that chipboard is not an approved material as it isn’t strong enough for roofing.
What’s your best tip for repairing or building a single ply roof?