Heat Resistant Cement/Board Water-Sealant?

[creative1000]

We had an additional storage room (10 sq meters) built using the cement wall/fencing as the lower half of the storage wall, and board on the top half (The board isn't wood, I think its some synthetic/cement like substance, snaps really easy). It appears the sealant between the board+cement wall has weakened a lot after 1 year. Water came in from the recent rain storms, and we can see sunlight coming through some cracks. I'm guessing it's because of the heat, as some of our stored items had their plastic parts disintegrate (like some of the plastic clips on luggage crumbled). There is a 4-inch gap between the roof and top of the wall, so airflow is possible, but perhaps needs an additional exhaust fan...

 

Don't want to fix this every single year. Just once and forget about it. Is there a particular sealant recommended or other thing I should try?

 

 

[Kwasaki]

I would say you're get more help showing some pix. 

[sometimewoodworker]

13 minutes ago, Kwasaki said:

I would say you're get more help showing some pix. 

I agree completely.
 

The disintegration of items, usually plastic but not exclusively, here has various reasons. Heat is one of them along with humidity and UV. Ambient temperatures are enough, so unless you’re going to have air conditioned space for your items susceptible ones will continue to fail. As to which ones will fail? Only time will tell.

 

with the construction failure it’s probably due to the cheapest materials being used.