Why a leak can come and go
An intermittent roof leak can look straightforward, but the visible sign does not always sit directly over the fault. On this page the important question is a leak that appears only on certain days or in particular weather. For an intermittent roof leak, the position of the problem, the weather when it appears and whether it is changing are more useful than guessing from one photograph.
Pay particular attention to rain direction, duration and strength when the stain appears, whether snow, frost or a sudden thaw has occurred, and whether the leak follows blocked gutters or a particular season. Those observations are worth passing on because they describe how an intermittent roof leak behaves at your property, not just how it looks on one day.
Weather-sensitive faults to consider
The main possibilities are wind-driven rain entering beneath a small gap, a gutter or valley that overflows only when partly blocked, and a flashing joint that opens slightly with temperature movement.
Other causes worth considering are condensation that is mistaken for rain penetration, and a loose tile that shifts in strong wind and then settles back. With an intermittent roof leak, the important thing is to match the cause to a leak that appears only on certain days or in particular weather, rather than treating it as a general roofing fault. For an intermittent roof leak, that distinction can prevent work being carried out on a nearby detail that happens to look worn but is not responsible for what you are seeing.
Keep a simple record of when it happens
From a safe position, you can keep a short note of the weather whenever the leak appears. You can also photograph the same stain so you can compare its size.
It is also useful to check the loft soon after the event for a fresh moisture trail. For an intermittent roof leak, write down what you can see and, where relevant, what the weather was doing at the time. A short sequence of photographs of an intermittent roof leak from the same safe viewpoint can show whether the condition is changing. When checking an intermittent roof leak, avoid walking on the roof or leaning out of windows; the aim is to collect useful information without creating another problem.
Intermittent does not always mean harmless
This becomes more urgent if the intervals between leaks are getting shorter, each episode produces more water than the last, and hidden timbers remain damp between events. With an intermittent roof leak, those signs suggest that waiting for a convenient time could allow more water entry, falling material or further damage.
If those warning signs are absent, an intermittent roof leak may not need an emergency call-out, but it is still worth arranging a proper look before the condition gets worse. In this case, change over time matters: a leak that appears only on certain days or in particular weather that has been stable for a long period is a different situation from something that has appeared recently or is becoming more obvious after each spell of bad weather.
How a roofer can use your observations
A roofer looking at an intermittent roof leak should use your weather notes to narrow down the likely entry route, then inspect parts of the roof that are sensitive to wind direction and flow.
They may also need to check whether more than one small defect is combining to cause the leak. Because this page is about an intermittent roof leak, the inspection should stay focused on that problem and the nearby details that could realistically produce it. For an intermittent roof leak, ask what they think has failed, what evidence supports that view, and whether the proposed work is local or part of a wider defect. A clear explanation of an intermittent roof leak is more useful than a quotation that simply lists materials.
Aim for a repeatable cure, not a lucky patch
Depending on what is found, a suitable repair might fix the specific weather-sensitive detail, improve drainage where water only builds up in heavy conditions, or replace loose or failed components rather than relying on temporary sealant.
For an intermittent roof leak, the best option is the one that deals with a leak that appears only on certain days or in particular weather and leaves the surrounding roof working as it should. If wider work is proposed for an intermittent roof leak, ask why a local repair would not be reliable, what condition the adjoining materials are in, and whether access costs make it sensible to combine closely related repairs. The answer should make sense for an intermittent roof leak, not read like a standard reroofing recommendation.
This guide is general information about an intermittent roof leak. The cause of an intermittent roof leak at an individual property can only be confirmed by inspecting the relevant roof area.