Timber shed – water ingress during heavy rain (photos attached)

[DesperateDan754]

I have a timber potting shed built approx. 19 months ago. Construction is horizontal shiplap cladding fixed to a timber frame. Internally it is lined with OSB, except on the wall with the large window, where the lower part is OSB and the upper part is internal shiplap. There is rigid Unilin insulation in the cavity between the external cladding and internal lining. The shed sits on large paving blocks with gravel around it.

Since the first autumn/winter, during heavy wind-driven rain, water has been getting inside around/through the timber frame built for a fixed plastic window (non-opening, no weep holes). I initially applied TEC-7 generously around the window perimeter (all joints), which reduced the initial severity but did not resolve the issue.

I suspect the same is happening with the other, smaller window on the front side, but splashback was an early suspicion, and I’m still not fully convinced whether it is contributing to the issue. The areas where the floor gets wet are the sides with windows; however, these are also the most exposed sides of the shed, while the other two sides are sheltered by a fence/wall.

Internally, moisture can be seen:

• appearing near an upper window corner
• then either tracking down internally to the bottom corner and/or appearing there directly
• in heavy rain the bottom corner can be wet to the touch
• that water has been strongly suspected to go down behind the OSB, and on a few occasions as small drops or trails on the front face of the OSB
• this ultimately wets the floor edge, leaving visible staining and dampness

I plan to remove some internal OSB panels to trace the exact water path, but would appreciate advice on correct external detailing first. I also fitted a few rain deflectors externally as a test. If splashback is contributing, my next idea was to add DPC hanging down over the rain deflectors.


Specific question:

• In a timber shed of this type, should the bottom of the window frame be left open to allow drainage?
  I originally sealed the bottom with TEC-7 as well. More recently I’ve read that the bottom should not be fully sealed, to allow any water that gets in to escape. I’ve since removed some sealant at the bottom corners, just in case this can act as a drainage path — though I remain unsure whether this is correct.

Any thoughts or advice would be very welcome.


Photos attached:
01–04: affected sides and corners
05–07: window where the issue is confirmed
08: impact on the shed floor

 

[Why Not Indeed]

Have you got gutters? Eaves? Don't see any pictures that include any part of the roof.

 

[big-all]

general comments
you should only ever have vertical surface and these should start protected at the top and finnish overlapping all timber so it can only shed on the ground
you have 3 ledges, one above and at the bottom off the glass the plastic strip at the bottom is also a trap point a small around 7 to 10% slope on all ledges and sills you have no choice on also the bottom off any timber like the corner bits should both be 12mm longer and angled forward to stop the water tracking back

solutions try something like 95mm chamfered skirting added to or replacing the flat surface above extended perhaps 50mm each side and at the bottom off the window siliconed at the glass and bottom off the shiplap at the frame top this is not ideal but will give you a year or so to work things out

instead off the bottom plastic trim or as well as a roll off dpm tucked up the back off the cladding around 50-80mm[at least 25mm] and behind the corners at the bottom to shed the water down more than track back

will suggest other ideas as i think off them and as said 100% gutters gutters gutters

 

[freddiemercurystwin]

I plan to remove some internal OSB panels to trace the exact water path, but would appreciate advice on correct external detailing first.

The correct external detail is, timber studs + breathable membrane + vertical timber battens to create vented cavity + horizontal cladding. This allows any rainwater that gets through the cladding to run down the rear face of the cladding and drain out the bottom of the cavity. This has been the right (and only) way to do timber cladding for decades. Timber sheds simply aren't waterproof if they are built as per the instructions, timber cladding is a rainscreen it is not rainproof.

Anything else will allow rain/moisture to leach through. So you can faff about with gutters, sealant, whatever but doubtful it will ever get solved without the aforementioned construction method.

 

[DesperateDan754]

Have you got gutters? Eaves? Don't see any pictures that include any part of the roof.

You are right, I should have included some photos, as that is surely contributing to my issues - there are basically no proper eaves (it's pent roof) and there is a gutter, on the short side.

I'll attach what I have handy now, it's not great photo, but you can see what it looks like.

 

[DesperateDan754]

The correct external detail is, timber studs + breathable membrane + vertical timber battens to create vented cavity + horizontal cladding. This allows any rainwater that gets through the cladding to run down the rear face of the cladding and drain out the bottom of the cavity. This has been the right (and only) way to do timber cladding for decades. Timber sheds simply aren't waterproof if they are built as per the instructions, timber cladding is a rainscreen it is not rainproof.

Thanks - I wish I knew this from the start... I had wooden shed, not custom made like this one, and it lasted for more than 20 years, would have still fine if I set it up right from the start (it was too low). And I still never had such water ingress issues. It was 'normal', gable roof though, with proper eaves on both sides.

 

[DesperateDan754]

general comments
you should only ever have vertical surface and these should start protected at the top and finnish overlapping all timber so it can only shed on the ground
...

Not being an experienced handyman, took me some time to understand all the points that you made. I was already thinking of putting DPC to hang at the bottom so that more water goes down and away. I was even considering a door canopy above the large window on a side, to protect that area from rain.

I'll check further your suggestion about chamfered skirting - I was already planning to something about window sill as it is flat and water accumulates and sits there. (Not sure that it gets inside from there, as I put TEC-7 on the plastic window edges)

 

[DesperateDan754]

Thank you all for taking the time to have a look and share your thoughts - much appreciated.