I get a lot of enquiries through my contact form where homeowners make a mention that the roof leaks only during heavy rain.
In my experience, roof leaks can be separated into 3 categories:
- The roof leaks every time it rains;
- The roof only leaks when it rains heavy;
- Only prolonged rain makes the roof leak.
On this post, I will examine the second category.
Why does my roof only leak during heavy rain?
The most common reasons why roofs only leak during heavy rain are:
- No sarking to catch leaks driven in by heavy rain
- Leaking low pitched tile roofs with sarking
- Heavy rain (and blockage) causing the valleys to overflow
- Flat profile roof tiles not coping under heavy water load
- Downpipe spreaders dumping too much water over leaking roof tiles
- Dirty water courses overflowing during heavy rain
- Flat corrugated metal roof leaking at laps
- Top end of roof sheets leaking due to ‘turn – up’ problems
- Box gutters not draining quickly enough in heavy storms
- Eaves gutters overflowing into wall cavity
- Debris blockage (together with heavy rain) causing leakage at skylight
Leaks on tile roofs only when it rains heavy
No sarking under the roof tiles.
If you go into your ceiling cavity and you can see the underside of the roof tiles – then you have no sarking. Stay there during heavy rain and you will feel a fine mist coming through the gaps between the tiles. Usually, this is not a big problem because it is not enough moisture to wet the ceiling. And there are no ceiling stains.
It is not the lack of sarking that causes the leak in heavy rain. It is because there is nothing under the roof tiles to catch the leak and drain it away.
If you do get stains on your ceiling, it makes our lives (as roof leak busters) a lot easier. The leak is right above the ceiling stain!
Low pitched roof tiles.
Roof tiles start to do poorly when the roof pitch gets less than 20 degrees. Manufacturers of roof tiles have limits on roof pitches because of this. Yet, many roofers ignore these for the sake of ‘looks’. To compensate, tile manufacturers relax the pitch requirement – provided sarking is installed. But this does not stop the leaks. The sarking will hold water until it gives way and pours water into the house.
It does need heavy rain to overflow the tops and edges of the roof tiles. If you don’t have sarking, then the ceiling gets wet quite immediately. If you have sarking, then it is quite possible that the leak is contained and you don’t know that your roof is leaking.
Overflowing roof valleys.
Leaves and valleys don’t work well together.
During long dry spells, leaves (and perhaps other debris) gather in the valleys. The roof tiles along the sides of the valley also help in trapping the debris. Then you get the storm with the heavy rain. Water rushes down the valley and hit the clump of debris.
Whilst some of the water can percolate through this rubbish, the heavy rush of water packs the rubbish tighter together and the water floods over the edges of the valley – and into your house.
Flat profiled roof tiles.
Roof tile manufacturers pander to consumers need for ‘modern, fashionable’ roofs.
The ‘slate’ look is still quite fashionable and this brings about the increased use of flat roof tiles. Why go with the expense and limitations of slate – when you can use a flat tile and achieve a similar look?
I feel sorry for the designers of these flat roof tiles. You just cannot make a roof tile flat, and also have it being durable and highly serviceable. (I will be doing a special post on flat roof tiles soon).
Flat roof tiles have no defined troughs for water to flow. In heavy rains, water runs uniformly across the face of the tile instead of along the trough/valleys. This means more water load on the water courses. And since these water courses trap more debris and are prone to breakage, water leaks along the edges of the tiles are quite common.
Another problem with flat profiles roof tiles occur when the side of a roof meet a wall. Water often run across the tile, under the wall flashing and into the roof cavity.
On two storey roofs, the top gutters usually discharge onto the lower roofs via downpipe spreaders.
Roof tiles are designed to carry rainwater up to a certain rainfall intensity – assuming that this rain falls directly on the roof tile and the roof is of a certain length. Even though the length of downstairs roofs are usually quite short, a downpipe spreader can dump over 10 x the design carrying capacity of the roof tile.
The water courses will leak, and on concrete roof tiles, water will also leak over the upstream edge of the tile.
Dirty or broken watercourses.
If the roof is in a tree zone, leaves are washed into the watercourses and over time, this leads to clogging and eventually, blockage during heavy rain. The resultant overflow will cause leaks.
Certain roof tiles have very fragile watercourses, and these will break. Since the watercourses are under the overlap of the adjacent roof tile, it is not easy to spot. Whilst the leaks from clogged and broken watercourses can be minor during light rain. It can get substantial during downpours.
Leaks on metal roofs during heavy rain
Corrugated low pitched roofs.
The standard corrugated roof is designed to be laid on standard pitched roofs. The roof sheeting manufacturers state that the minimum slope is 5 degrees. Build a corrugated roof flatter than 5 degrees (It is quite common), and it will overflow during heavy rain.
The laps on the roof sheets get flooded when excessive water fill the corrugations and overflow under the laps. The screw holes at the laps will also start leaking.
Sometimes, wind will also drive water over the upstream ends of the corrugated roof sheets – even though they are turned up.
The upstream ends of ALL roof sheets are supposed to be ‘turned up’ to form dams that guard against overflowing. Heavy rain will cause overflowing if there are not done or not done properly.
Some roofers don’t have the proper ‘turn-up’ tool for the low pitched ‘Trimdek’ and ‘Kliplok’ profiled roof sheets and the ‘dam’ becomes ineffective when heavy rain, leaf debris and wind combine to create the overflowing pond.
Low pitched valleys or valleys blocked by debris will overflow in heavy rain. Valleys on flat ‘kliplok’ roofs need custom designs to provide adequate water carrying capacity. Otherwise, they will overflow frequently.
Other roof elements that can only leak during heavy rain
If you have a box gutter, then it must be cleaned regularly and have some overflow provisions. Otherwise, it will suddenly send a flood of water into your living space during heavy rain.
Most box gutters discharge water via downpipes. It is vital that the openings into the downpipes are not blocked. Some people put mesh covers over the downpipe inlets – only for them to choke up with debris when heavy rain comes around.
Normally, if you have good eaves, you will not get water into the house (even if the gutters themselves are overflowing). If you have no eaves or you have the deep fascia gutter, then the perimeter gutter will overflow in heavy rain and get into the wall cavity. From there, it appears in the house at some ‘funny’ places.
Most gutter cleaners don’t clean around the base of the skylights. This makes skylight bases the worst maintained element on a roof. Years of gradual trapping and accumulation of leaves between the roof and the sides of the skylight suddenly forms a dam during heavy rain. Water will shoot over the outer edge of the skylight base and into the ceiling below in a big rush of water.
How does knowing the intensity of the rain when a roof leaks help in finding the cause?
Some roofs only leak during heavy rain and there is usually only one reason why this happens.
• Water flow exceeds the capacity of the roof elements.
When we go out tracking the cause of roof leaks, It helps a lot if we know ‘WHEN’ the roof starts leaking. If a roof only leaks in heavy rain, then the above reasons come to mind first and It gives us an easy starting point to successfully track down the cause(s) of the leak(s).
If a roof leaks whether it has rained heavily or not – then there is a good chance that there is an easily identifiable (eg Broken roof tile or hole in roof) cause. Every roofer should be able to find and fix this problem easily.
The most challenging category is ‘leaks only during prolonged heavy rain’. What this means is that the water entry point is quite small (and therefore needs prolonged heavy rain to cause a leak) and perhaps there are elements under the roof that hold the water leak for a while.
This makes the cause of the leak very difficult to identify at first glance. Most roofers will get ‘stumped’ by this category of leaks.
Over the years, I have discovered many reasons for these types of leaks, and I will cover these on a future post.