Question 1

Why does my roof only leak in heavy rain?

Accepted Answer

Usually because something is being overwhelmed rather than because there is a hole. An undersized or partly blocked valley or gutter backs up above the level of the laps and lets water in sideways, and wind-driven rain gets past a headlap that is perfectly adequate in ordinary weather. An intermittent leak is genuinely informative, because the conditions that trigger it narrow down the cause a long way. The commonest version is capacity. A valley carries the water off two roof slopes at once, and one sized for the roof as originally built can be seriously inadequate once a rear extension has been added and its slope discharges into the same channel. In ordinary rain the water runs down the middle; in a downpour it backs up above the level of the slate laps either side and goes in sideways. Guttering does the same thing when it is blocked or too small — it overflows backwards under the eaves into the roof space. The second version is wind direction, and it is the one that drives people mad. A leak that only appears when the rain is coming from one particular quarter is wind-driven ingress: rain being pushed horizontally up under a lap, past a verge, or behind a flashing that would shed it perfectly well in vertical rain. It is a real defect, but it is a lap or a detail rather than a hole, which is why it never shows in a normal shower. Headlap is often the underlying reason. The distance each slate or tile overlaps the one two courses below is calculated against the pitch and the exposure of the site, and a roof laid to too little headlap — or one whose gauge was set by eye — will let driven rain past even though every unit is intact. It is invisible from the ground and it is why exposure matters when a roof is specified. The third version is stored water. A ponding flat roof, a saturated parapet, or a wall that has been soaked by an overflowing gutter can all release water hours after the rain has stopped, which produces a leak with no rain happening. Timing that carefully is often what identifies it. The practical upshot is that intermittent leaks are diagnosed by pattern as much as by inspection. Which direction was the wind. How hard was it raining. How long after the rain started did it appear. Those three answers narrow it down more than an hour on the roof does.

Question 2

Roof leaks only in a downpour

Accepted Answer

Usually because something is being overwhelmed rather than because there is a hole. An undersized or partly blocked valley or gutter backs up above the level of the laps and lets water in sideways, and wind-driven rain gets past a headlap that is perfectly adequate in ordinary weather. An intermittent leak is genuinely informative, because the conditions that trigger it narrow down the cause a long way. The commonest version is capacity. A valley carries the water off two roof slopes at once, and one sized for the roof as originally built can be seriously inadequate once a rear extension has been added and its slope discharges into the same channel. In ordinary rain the water runs down the middle; in a downpour it backs up above the level of the slate laps either side and goes in sideways. Guttering does the same thing when it is blocked or too small — it overflows backwards under the eaves into the roof space. The second version is wind direction, and it is the one that drives people mad. A leak that only appears when the rain is coming from one particular quarter is wind-driven ingress: rain being pushed horizontally up under a lap, past a verge, or behind a flashing that would shed it perfectly well in vertical rain. It is a real defect, but it is a lap or a detail rather than a hole, which is why it never shows in a normal shower. Headlap is often the underlying reason. The distance each slate or tile overlaps the one two courses below is calculated against the pitch and the exposure of the site, and a roof laid to too little headlap — or one whose gauge was set by eye — will let driven rain past even though every unit is intact. It is invisible from the ground and it is why exposure matters when a roof is specified. The third version is stored water. A ponding flat roof, a saturated parapet, or a wall that has been soaked by an overflowing gutter can all release water hours after the rain has stopped, which produces a leak with no rain happening. Timing that carefully is often what identifies it. The practical upshot is that intermittent leaks are diagnosed by pattern as much as by inspection. Which direction was the wind. How hard was it raining. How long after the rain started did it appear. Those three answers narrow it down more than an hour on the roof does.

Question 3

Leak only when the wind is in one direction

Accepted Answer

Usually because something is being overwhelmed rather than because there is a hole. An undersized or partly blocked valley or gutter backs up above the level of the laps and lets water in sideways, and wind-driven rain gets past a headlap that is perfectly adequate in ordinary weather. An intermittent leak is genuinely informative, because the conditions that trigger it narrow down the cause a long way. The commonest version is capacity. A valley carries the water off two roof slopes at once, and one sized for the roof as originally built can be seriously inadequate once a rear extension has been added and its slope discharges into the same channel. In ordinary rain the water runs down the middle; in a downpour it backs up above the level of the slate laps either side and goes in sideways. Guttering does the same thing when it is blocked or too small — it overflows backwards under the eaves into the roof space. The second version is wind direction, and it is the one that drives people mad. A leak that only appears when the rain is coming from one particular quarter is wind-driven ingress: rain being pushed horizontally up under a lap, past a verge, or behind a flashing that would shed it perfectly well in vertical rain. It is a real defect, but it is a lap or a detail rather than a hole, which is why it never shows in a normal shower. Headlap is often the underlying reason. The distance each slate or tile overlaps the one two courses below is calculated against the pitch and the exposure of the site, and a roof laid to too little headlap — or one whose gauge was set by eye — will let driven rain past even though every unit is intact. It is invisible from the ground and it is why exposure matters when a roof is specified. The third version is stored water. A ponding flat roof, a saturated parapet, or a wall that has been soaked by an overflowing gutter can all release water hours after the rain has stopped, which produces a leak with no rain happening. Timing that carefully is often what identifies it. The practical upshot is that intermittent leaks are diagnosed by pattern as much as by inspection. Which direction was the wind. How hard was it raining. How long after the rain started did it appear. Those three answers narrow it down more than an hour on the roof does.

Why does my roof only leak in heavy rain?

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