Firstly, let us say thanks for your business. Purchasing a playhouse is a big investment on your part and we want to ensure that it makes your grandkids squeal with delight when they play school/house/dog grooming parlour in it in years to come, as much as your children do now.
Our playhouses are constructed with wood and love; and the Irish climate can take its toll on both. All of our timber is pressure treated against rot but wood contracts in the heat of our normal 3 day summer in April as it dries out, and when our other season of winter comes, it will absorb the moisture in the air and swell. Your playhouse needs room to expand and contract so don't be concerned about small gaps in the floor and walls during the dry season.
As mentioned, the wood is all treated, but the colour will begin to fade and turn slightly grey in time. To make the house watertight, it will need to be painted or stained at some stage; whether you want us to do it or you choose to paint it yourself after delivery. For our painted houses, we use Dulux Weathershield for Wood or Cuprinol Colour Range. If you want to keep the natural wood look, we recommend an external wood stain, but NOT yacht varnish as it makes the wood watertight and unable to breathe and expand and contract.
Roofs are a whole different ballgame though, and we don't want to see any gaps in those. We use a waterptoof membrane liner underneath similar to that used in roofing normal houses but the shingle/shiplap will need some love over the years to preserve it.
When you first purchase your playhouse with a shingle roof, the tiles are dark as the shingles are kept wet so that they don't crack when nailed. Within a few weeks, (less in the heat), the tiles will begin to dry, lighten and curl at the edges. This is when they're thirsty and looking for a refreshing lick of treatment. We personally like Sikkens (slightly oilier than others but easy to apply and gives a good satin finish), but Ronseal or Cuprinol work just as well. Once you've generously applied this, you are free for another 5 summers before it has to be done again.
Given the playhouses interest in expanding/contracting, you made find in time that the doors/hinges/handles etc are looser and a quick tighten with a screwdriver will have it back to square one.