Solvent Welding Plastic Pipe
Cut, prime, cement and cure PVC, CPVC and ABS joints that hold, on drain lines and pressure lines.
Table of contents
Solvent cement is not glue. It softens the outer skin of the plastic on both the pipe and the fitting. When you push them together, the softened surfaces fuse into one piece. That is why a good joint is stronger than a glued one, and why a bad one cannot be repaired. Once a joint is made, you can only cut it out. The same process is used on drain lines, irrigation mains, and pressure water lines. The difference is in how long you wait before testing.
What You Will Be Able to Do
- Match cement and primer to PVC, CPVC, and ABS
- Prepare pipe so the joint fuses all the way around
- Make a joint in the right order and hold it long enough
- Read a cure chart before you test or bury a line
- Handle primer and cement safely
Match the Chemistry
Each plastic has its own cement. The cans look alike, so read the label every time.
| Pipe | Cement standard | Primer |
|---|---|---|
| PVC | ASTM D2564 | Required by most codes, usually purple (ASTM F656) |
| CPVC | ASTM F493 | Often required, follow the cement label |
| ABS | ASTM D2235 | Not used |
A cement made for one plastic will not fuse another. Mixing them gives a joint that looks fine and then fails. All-purpose cements exist, but many codes do not accept them, so check before you use one.
Where PVC meets ABS, you usually cannot cement the two together. The common legal transition is a shielded mechanical coupling. Some areas allow a special transition cement in limited places. Ask your inspector.
Cement also comes in bodies, which is how thick it is. Regular body suits small pipe, medium body suits most work up to about 6 inches, and heavy body suits large pipe. Read the size range on the can. Thin cement cannot fill the gap on a large joint. Colors vary by brand, so color alone does not tell you what is in the can.
Discard cement that has gone thick, stringy, or jelly-like. Do not thin it. Keep lids on tight, because cement starts to thicken as soon as solvent escapes.
Warning: Primer and cement are highly flammable, and the vapors are heavier than air. They collect in trenches, pits, and low spots. Keep flames, torches, and sparks away. Work with fresh air moving, keep your face out of the fumes, and wear eye protection and chemical-resistant gloves. Check the Safety Data Sheet for the product you are using.
Measure and Cut
Measure with the fitting in mind. Pipe has to reach the bottom of the socket at both ends.
Worked example. Two fittings need to end up 36 inches apart, face to face, and each socket is 1 inch deep. Cut the pipe 36 + 1 + 1, which is 38 inches.
Cut square. A slanted cut leaves less surface in the socket and can leave a gap at the bottom where the joint never fuses. Use a ratcheting cutter on small pipe, a wheel cutter, or a saw and miter box on larger pipe.
Remove every burr on the inside and outside. Then bevel the outside edge slightly with a deburring tool or file. A sharp outside edge scrapes cement off as the pipe goes in. An inside burr catches solids in a drain line. Wipe the pipe and socket clean and dry. Dirt, water, and oil all keep the surfaces from fusing.
Dry Fit
Before you open a can, push the pipe into the fitting by hand. A good fit starts easily and tightens. The pipe should go in about one-third to two-thirds of the socket depth. If it bottoms out loose, the cement may not fill the gap. Try another fitting.
On a long assembly, dry fit the whole run. Draw a line across each joint with a marker so you can line up the fittings again when you cement them. You get one try at each joint.
Make the Joint
Work fast. Have everything within reach before you start. In hot or windy weather, the cement dries quickly, so make one joint at a time.
- Prime. Apply primer to the inside of the socket and to the pipe end, covering at least the socket's depth. Primer softens the surface so the cement can fuse. A second coat helps in cold weather. Keep the surface wet with primer when the cement goes on.
- Cement the pipe. Apply a full, even coat to the pipe end, covering the same depth as the socket.
- Cement the socket. Apply a lighter coat in the socket. Do not leave puddles, since excess cement can weaken the pipe wall and narrow the inside of a small pipe.
- Cement the pipe again. On larger pipe, add a second coat to the pipe end.
- Assemble. While the cement is wet, push the pipe straight in until it bottoms out, turning it about a quarter turn. The turn spreads the cement. Do not turn it after it bottoms out.
- Hold. Hold the joint firmly for about 30 seconds. The tapered socket wants to push the pipe back out as the solvents flash off. Larger pipe or cold weather may call for longer.
- Check the bead. A continuous ring of cement should show around the joint. Gaps in the ring mean you did not use enough cement. Wipe off extra.
If the pipe does not go all the way in, do not force it. Cut the joint out and start again with a new fitting. A joint with a dry section may hold for a while and then leak.
Why Primer Shows
Purple dye is there for the inspector. A visible stain on the outside of the joint proves that primer was used. Wipe up drips on finished surfaces, but do not wipe away the evidence on the joint. Some jurisdictions allow clear primer or other approved methods. Follow what your inspector accepts.
Set and Cure
There are two stages after a joint is made.
- Set time: How long before you can handle the joint carefully. Often a few minutes in warm weather.
- Cure time: How long before the joint can take pressure. This could be anywhere from minutes for a small drain test to a day or more for a large pressure line in the cold.
Cure time depends on pipe size, temperature, humidity, and test pressure. Each cement has a chart, so use it. Pressure lines need far more time than gravity drains. Cold slows curing a lot, and damp air slows it too.
| Condition | What changes | What to do |
|---|---|---|
| Below about 40 F | Solvents evaporate slowly and joints take much longer to cure | Keep materials warm, use a cold-weather cement, and extend cure time |
| Hot sun | Cement can dry before you assemble the joint | Keep pipe shaded and work one joint at a time |
| Wet trench | Water on the pipe stops fusing | Keep ends dry and clean, and raise the work out of the trench |
| Windy dust | Grit lands in wet cement | Shield the work and wipe the ends again |
Warning: Never test plastic pipe with compressed air or gas unless the manufacturer and your code specifically allow it. A failed joint under air pressure can send fittings and plastic shards flying. Use water.
Handling and Burial
Do not stress a joint while it is curing. Do not bend pipe into place, drop it in a trench, or backfill over it until it has set. On long runs, snake the pipe loosely in the trench so it can shrink when the temperature drops. A hot pipe glued tight on a summer afternoon can pull apart by morning.
Support and backfill the way the job calls for. Keep rocks away from the pipe. Cure time still applies, and testing comes before you cover the work, if your inspector or owner requires it.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Using the wrong cement | A joint that looks good and then fails |
| Skipping the bevel | Cement scraped into a pile instead of filling the joint |
| Letting go too early | The pipe pushes back out and the joint weakens |
| Testing too soon | A blown joint, which then has to be cut out |
| Using gelled cement | Joints that never fuse |
| Working near a flame | Vapor fires |
Check Your Understanding
1. You are about to join a CPVC hot water line. The only can on the truck says PVC cement. What do you do?
Stop and get the right cement. PVC cement is not made for CPVC, and a hot water line is a bad place for a joint to fail.
2. After you push a joint together and let go, the pipe slides back about 1/4 inch. What happened, and what do you do?
You did not hold it long enough. The socket pushed the pipe back out. Cut the joint out and start over. Next time, hold it for about 30 seconds.
3. You finished an irrigation main at 45 F, late in the afternoon. The crew wants to pressurize it right away. What do you check?
The cure chart on the cement for that pipe size, temperature, and pressure. Cold slows curing, so you may need to wait much longer, possibly until the next day.
4. You need to cut a pipe to fit between two fittings that are 24 inches apart, with 3/4-inch sockets. How long is the pipe?
24 + 3/4 + 3/4, which is 25-1/2 inches.
Key Takeaways
- Solvent cement fuses plastic, so a bad joint gets cut out, not patched.
- Match the cement to the pipe. Read the label, because cans look alike.
- Cut square, deburr, bevel, and keep the joint clean and dry.
- Prime, cement, push, turn a quarter turn, and hold for about 30 seconds.
- Follow the cure chart before you test or bury a line.
- Keep vapors away from flames, and do not test with compressed air.