Well actually, first we should start with the floor. Scrape any protrusions off the floor (Photo 1). This is important; because you are gluing the pad to the floor and the carpet to the pad, any imperfections in the subfloor will telegraph through the finished product.
Not just any pad can be used for double glue. Rubber double-glue pads are either made of a closed-cell construction or coated so as to not allow the adhesive to soak into them like normal pad designed for stretch-in installation would. They are also much thinner, usually no thicker than 1/4-inch, and very dense with rebonds starting at 10-lb and sponge rubber pads at 20-lb.
In a traditional double-glue installation, the pad is glued to the floor with a pressure sensitive adhesive. These adhesives are quite often mistakenly called "release adhesive." Not true; what they do is remain tacky when dry, and how well they grab depends the amount used and how much pressure is applied. A small amount of this glue goes a long way. It's applied with a VCT notch trowel or a roller, sometimes the flat side of the trowel; flat troweling is risky and not recommended. Allow it to dry to its translucent tacky state with no transfer when touched; duct tape could be considered a pressure-sensitive tape. After laying the pad into the glue, use a stiff push broom, a carpet tube, or light roller to smooth out the bubbles and press the pad into the glue.
Think of it as installing a subfloor made of rubber; the whole idea is to use just enough adhesive to hold the pad in place, making removal easy when replacing the carpet. Having cussed many a double glue off the floor with elbow grease and scrapers, I can attest it doesn't always work. Going into it even just a little wet and you've got a permanent and I do mean permanent bond, like putting two pieces of duct tape together. Or the pad can be glued with a multi-purpose adhesive using a 3/32 V notch trowel for a permanent bond.
There are many different types of double-glue pads on the market at this time, requiring different adhesives and installation techniques. It's important to read carefully and follow the manufacturer's installation instructions.
Once the pad has been installed, it's easy to think, "OK, just do a direct glue, bing bang boom, and we're outta here. Don't let yourself be fooled; this is where double glue gets tricky.
2. Amount of Glue: The recommended trowel sizes for double glue are: Smooth-back carpet, 1/8(width)-by-1/8(depth)-by-1/8(space); rough- backed carpet, 1/8(width)-by- 1/8(depth)-by-1/16(space) 1/8(width)-by-3/16 (depth)-by-1/8 (space) U notch.
You look at these trowels and say, "Holy mud! I'm installing carpet not doing yard work! Let's see if I've got this straight; use the best glue I can get and put it on with a rake! OK, see ya, I've got work to do."
Wait! There is more. Let's stay with the trowels a bit longer. First, new trowels will chatter and jump on the pad because of the burrs left from the notches being stamped out, not something you would ever notice doing direct glues. A quick and easy solution is to buff them off with a wire wheel on a bench grinder or a piece of sand paper (Photo 4).
You have just applied an enormous amount of adhesive to the floor. It's important to get the excess moisture out of the glue before the carpet is laid into it for a couple of reasons. First the carpet will absorb the moisture causing the carpet to expand. The job is installed (cut to fit), looks great, and you're on your way.
A few days or weeks later the adhesive cures, the carpet gives up all the moisture it's absorbed, and it shrinks! Well, not really, unless it was wool or jute backed. What it has done is contract, just like anything does with heat and humidity changes. "So, what do I do, just stand around and watch the glue dry?" No, use fans, and I don't mean bring one box fan on the job. Invest in at least two high-power fans (Photo 8). What you are looking for are fans that move between 4,000 and 12,000 CFM (cubic feet per minute) of air.
A good illustration to explain the other reason for letting the glue set up is; spread some glue on a scrap piece of pad, drop a small piece of carpet on it and step on it immediately. When you pull back the carpet you will see the imprint of your foot with most of the glue forced into the carpet back, not enough left on the pad for a good bond.
Rollers: For the same reasons mentioned above, a light roller, no heavier than 35LB, or a carpet tube, is recommended to press the carpet into the glue.
Seams: Here's a tricky deal. There are two ways of making double glue seams, and these are hotly contested, I might add. 1. Seal the seam just like a regular glue down. Make sure to cover both the primary and secondary backs, not just putting the bead of sealer on the pad at the base of the carpet. If you do this right, having properly acclimatized the carpet, used the right amount and kind of glue allowing it to set up properly, used the right roller, and restrict traffic for the recommended 24 hrs., you will probably be fine.
Remember earlier when we spoke about the carpet contracting when it gave up the moisture it absorbed from the glue? If it does that and your seams aren't properly sealed, they could pull apart.
This brings us to the second method of making seams in double glue. Hot tape the seams. There are two methods of doing this; 1. Preseam 2. Seam it in place.
Preseaming is not practical for large areas. The only time I would recommend this is for adding fills to rooms. You can use regular seam tape for this but, and this is a very big but, after the seam has cooled you must peal off the excess paper (Photo 9) and scuff the silicone off the back of the tape (Photo 10); use heavy sandpaper i.e. 50 grit. If you do not, the glue will not stick to the silicone-treated paper. It's probably a good idea to run your vacuum over the tape to pick up the dust from sanding the back of the tape (Photo 11).
Seaming in place. Lay out the carpet, seams trimmed and sealed with latex seam sealer; fold back one piece. Position the tape under the remaining piece, depending on which type of tape; there are two types, with paper and without. The ones with a paper backing have a pressure sensitive adhesive to hold them in place. The ones without paper, designed to stick to the pad when the seam is made, will need to be "tacked" in place, with a quick smear from a hot glue gun. Once the tape is in place, fold back the remaining piece of carpet and spread glue to the edges of the seam tape; when the glue has tacked up properly, lay the carpet into the glue, make your seam and then roll the carpet.
I recommend starting, whenever possible, in the center of the room at a seam. Once the first seam is done, you can work in both directions, using your fans to tack up the glue on one side while spreading adhesive on the other side.
A couple more things; acclimatize the carpet and pad to the job site before installation. While it is important to restrict traffic from any glued installation for at least 24 hrs. after installation while the glue is setting, because of the large amount of adhesive applied in a double-glue, restricting traffic is even more important in these installations. And never, never, cover the newly installed carpet with plastic. This will trap moisture, not allowing the adhesive to cure properly, and create a multitude of problems.
This has been a quick outline of what's needed for a successful double-glue installation. Always make sure you follow the manufacturer's installation instructions; make sure the carpet, pad, and glue you intend to use are recommended for double glue. Some carpet backings, such as Unitary, are not suited for double-glue installations, and some pads have different installation requirements than mentioned above.
When I was younger, my usual approach was, when all else fails read the instructions. Save yourself some aggravation; learn the lessons the easy (and cheaper) way; read the instructions first.