Obviously the best thing to do is to drill through the slab….which will get the most amount of the product where you need it. I am not sure if just spraying against the house will take care of the termites. It all depends on if the spray gets to where it touches some of the termites. There is no way to know until you treat it. Treating through the expansion joint is better than not treating in that area at all, but not as good as drilling. Of course it is your choice. If you know where the termites are in the house you can treat them directly in the wall or wood that they are in as well for good measure. You can drill a hole into the stud or trim and treat directly from a sprayer. That should touch some of the termites as well.
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