We get asked "how to form a concrete trench drain" quite often. People often think that they will save a lot of money forming a poured in place concrete trench drain. A good old fashioned hand formed concrete trench drain is labor intensive but very flexible in terms of design. The contractor can easily adjust width, depth, slope, and add almost any feature desired. The down side to hand forming a concrete trench drain is that it takes skill and time to get it right. On this page I will give some of the basics of how to form a concrete trench drain. We also sell the frames and grates to make a hand formed trench drain. If you are not sure you have the skill to form a concrete trench drain consider one of our precast trench drains or trench drain forming systems. These prefabricated trench drain systems make it easy to get a quality trench drain.
Since you made it this far, we will assume you have the carpentry skills to form and pour your own poured in place concrete trench drain. Lets start the meat of how to form a concrete trench drain. Start with the concrete trench drain design. Select the frame and the trench drain grate based on your design criteria. This needs to be done before you form the drain. In fact, we have a lot of calls from contractors who have formed a cast in place trench drain and now can't find a grate to fit the trench drain dimensions they have formed. In the end this will cost you more because you will need a custom fabricated trench drain grate. Also, it may be difficult to reach the required load rating due an improperly sized trench seat size.
A hand formed concrete trench drain can have a formed concrete grate seat or an embedded frame. We recommend an embedded frame. Without a frame the grates sit directly on the concrete. This can put excessive stress on the concrete and it will cause the concrete ledge to shear off over time. The upper lip next to the grate is also unsupported leaving the upper edge of the trench drain susceptible to spalling and shear failures. A good trench embed frame will give your grates a proper ledge to sit on that will ensure the hand formed concrete trench drain has a long life.
Once your trench drain frame and grate have been selected, you must determine the width to form the drain. The grate seat is the ledge where the grate sits to keep it from falling into the trench. This ledge must be the right width and it must be co-planar (meaning the same elevation - more on this when we discuss the actual forming). The width of the grate seat is typically the grate width plus 1/4". This should govern the width of your trench drain form work. The forms will be set such that the inside face of the trench drain frame is 1/4" wider than the grate. If the opening is too narrow the trench grates will not fit in the opening and will be difficult to remove for cleaning. If the opening is too large the grates will slide, look unsightly, or fall into the opening.
Now let's discuss the basics of hand forming a concrete trench drain. First excavate the ditch to the depth of the drain plus the thickness of the concrete. Next, install any required rebar. Note for a poured in place concrete trench drain you should have U shaped rebar to hold the trench drain wal