Next, once this is done use a regular nailing schedule (I’m so glad you are not using flooring staples) but 6″ would be better and make sure that each board has at least one nail about 4″ from each end, but no closer. Keep the pressure on your nailer so that the nails just nicely tuck into the little groove on top of the tongue, check often. Take your time so that you don’t get glue everywhere. The glue sets up in about 30 minutes, so if you have some stubborn boards, get them in fast, because once the adhesive has set, it’s almost impossible to remove.
Oh, and that’s another reason for the 1/4″ underlay. If this floor fails you will be able to remove the plank from the underlay (destroying the underlay, not the subfloor). After the job is done tell the customer that this type of installation, only works when they keep their indoor humidity levels, within a narrow 20% range. Try to find the MEAN (half the time the RH is higher than this figure and half the time lower) indoor RH that this house experiences, and have them keep the house + and – 10% of this figure. And strongly suggest that they buy a wet/dry bulb hygrometer to accurately measure this. The cheap metal ones at the hardware stores are not accurate. A cheap but good one is available though:
This assures only minimal seasonal movement of the planks. Less gaping, crowning and cupping. I hope this wood has a satin finish on it. A prefinished floor with a high gloss or semi gloss finish, the home owners will see the crowning effects right away once the spring hits, as the wood gets slightly damp from below. Especially if there is only a crawl space under the floor. I would prefer not to install such floor over an unheated crawl space, this is just asking for trouble.
I do not advocate this last type of installation with prefinished wood. Basically the wood becomes trapped in it’s own glue. And when the home owners subject the wood to extremes in RH changes (believe me they will) they will blame the store, or you, or the manufacture of the wood when this floor starts bucking in humid weather or gaping in the winter.
And then I will finally hear about it when they (or someone like them) write to me complaining of shoddy workmanship and material. What they need to understand is the hygroscopic nature of wood, but I doubt they will. I never tackle jobs like this.