Water ingress! Weather sleuthing...

Despite the extensive work we had done on the roof recently, we saw some water ingress during some recent rainfall. It wasn't exactly biblical downpour territory, so what could have been the issue?

This is what was done:

• Refit various loose slates.
• Repair the lead lining on one of the valleys.
• Replace the chimney pots, saddle stone and repoint the stack.
• Replace all gutters and downpipes.
• Fit new facias - treated wooden boards and black uPVC facias.
• Repoint and seal the battlements (the tops of the bay windows).
• Reroof the small bay roof at the office French door, including reflashing.
• Strip all slates, battens and felt on the workshop roof, lay in additional joists (to give us reasonably "flat" surfaces on the 2 sloped roofs, compared to the current moonscape feature, and cure the minor leaks around some of the skylights.
• Refit new felt, battens and slates. Reuse salvaged slates on the neighbour's side where possible.
• Reflash where the roof meets the wall of the house.
• Fit new facias, gutters and downpipes on the workshop.
• This required covering the entire workshop with scaffolding and shuttering to make it weatherproof while the work is ongoing. And scaffolding the main house when tackling the facias and chimney.

Here's the point of ingress. I've never seen this before, so what could have caused it? Seems unlikely to be a result of the work we had done:

Tissue provides a useful witness, showing where the water was landing.

Why did it come in only on this occasion? It only happened on that day (late morning, 11am - noon on 24th March) and hasn't happened since.

• Was the prevailing wind coming from an unusual direction?
• Was there some sort of blockage in the drains?
• Did something get damaged during the recent work?

There's a website called Weather Spark historical data that logs local weather data hour by hour and is easily searchable. We are about a mile or so from the nearest weather station which is at Blackpool Airport, so we are sort of in luck. 

Generally, the prevailing wind at our home is Westerly (from the West, coming off the Irish Sea), although the airport data doesn't seem to reflect that quite as much as I'd expect. I think it's fair to say that the wind had a bit of a Southerly direction on that day.

Note that it hadn't rained for quite a few days before this incident:

So, the best I can conclude is that there was a combination of driving rain and a Southerly component to the wind.

The roofers returned (14th April)and inspected the brickwork above the windows and just below the parapets. The problem seems fairly clear - missing pointing on the facing brickwork. Any water running down the wall is liable to seep into the cracks. Once in there, the only way for it to drain is into the interior. You can see why it would tend to happen under limited circumstances.

The cure seems to be reasonably straightforward - rake out and repoint the flaky pointing.

So - Weather Spark is a pretty handy resource that is work knowing about!