Thanks for reaching out across the ocean for help with your problem. I’m fairly confident I can help you.
First, let’s talk about the characteristics of masonry and concrete. Based on the enormous volume of email I receive from homeowners and builders, I have come to believe that many believe concrete, brick, stone, mortar and other similar materials are waterproof.
The truth is water can pass through many, if not all these materials, albeit at radically different speeds. It’s child’s play for water to pass through tiny hairline cracks, and I see those in the high-resolution photo you sent to me.
The steps that form the sloped roof for your garden shed are poured against the poured concrete foundation of the villa. This is a very common practice, as it would be prohibitively expensive to cast the steps as part of the poured foundation. It’s possible to do this but not practical.
Think of the dynamics that are happening in a moderate to heavy rainstorm that has wind as a component. Gallons of water are hitting the stone walls of the villa and cascading down the walls to the hairline cracks where the steps meet the foundation wall. Wind blowing against the wall and steps adds more energy forcing water into the tiniest of openings or cracks.
Since you can’t stop the rain and wind, you’ll need to do your best to stop the water from entering the cracks. It appears to me that someone has already made an attempt to do this by caulking the cracks on the flat stair treads. However, they did not caulk the cracks at the vertical interface where the stair risers touch the foundation walls.
The repair job doesn’t look good to me, and I would seriously consider chipping off the caulk or whatever material it is. I’d then clean the foundation wall and steps well using a pressure washer. Yes, this is also going to cause water to enter the garden shed, but with luck it will be the last. You need the surfaces clean so the repair material bonds tenaciously.
I’d be very tempted to chisel carefully along the joint between the steps and foundation to open up the gap to at least 3 mm or about 1/8 inch. This crack should be about 6 mm or 1/4-inch deep. Be sure to remove all loose material and dust. I’d then caulk it with a concrete epoxy that’s extremely strong and waterproof.
I’ve used an epoxy with amazing results. It’s made by a company called PC Products and it’s available on Amazon.com. This epoxy comes many ways, but I prefer for jobs like this to use the caulk tube dispenser that automagically mixes the A and B components.
Before you start to do this repair, I want you to do a test with the epoxy on some small concrete bricks that you can buy. Place one against the other so one is taller, recreating the relationship you have between the stair tread and foundation wall. The purpose of the test is to refine your caulking skills and to make the caulk match the existing concrete.
It’s imperative that you study the concrete steps and take a close-up photo. You need to look at the size, color and makeup of the exposed sand grains in the concrete. Your job is to visit a local gravel and sand supplier to find similar, if not exact, sand. When you look at the sand, and I can see it in the photo, you’ll notice black, orange, brown, white and other grains.
You’ll use this sand to hide the stark gray epoxy. The epoxy I prefer comes out somewhat wet, and fresh sand will stick to it with no problem. Carefully apply a bead of epoxy, and then cover it immediately and completely with the sand. Be careful not to tamp the sand into the epoxy because you could smear it.
Allow the epoxy to harden. This usually takes a few hours. Then sweep off the excess sand from your test bricks. See how it looks and hold it next to the existing steps. You may have to make several attempts to get the exact look and texture you want.
Once you have all the cracks repaired where water is entering, then you need to apply a top quality silane/siloxane clear water repellent on the concrete steps, landing, foundation wall and the masonry walls of the villa above the garden shed. This final step will really help stop water from rusting your shovels, rakes and whatever else is the storage area. Good luck to you, and please report back your results.
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