In April 2004 I built a 12ft x 8ft shed in my garden to a plan I found in an American book. I modified the plan where necessary to suit the materials that are available locally (I live in SW Scotland).
The main modification was the type of exterior cladding for the walls. In the book they used 10" x 1" planks laid vertically with battens over the joins - I couldn't find a timber merchant selling boards in these dimensions so I used 15mm x 150mm treated t&g weatherboarding from my local timber merchant. I applied 2 coats of Ronseal or something similar after it was finished.
During persistent heavy rain when the wind is blowing from the same direction, the water seeps through the t&g joins & gathers at the base of the interior wall. I first noticed it in December - it was a very wet month here, probably much wetter than average for the time of year, & windy too.
I'm a little worried that if this happens every winter I'll get wet rot developing in the vertical studs, the bottom plate where it joins the base of the shed, and the floor (18mm ply).
Anyone got any suggestions? I'm thinking of applying a 'second skin' of more weatherboarding, probably just on the 2 sides facing the prevailing wind, as I don't think applying more Ronseal/whatever will make a difference.