Getting ready to run some tile for some downspouts, is it OK to run trench close to outside wall? To keep proper slope and not get any low spots I’m planning to use white PVC sewer pipe, with glued joints. How much slope is necessary, total distance is around 100′ ?
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Any slope will do. In theory, for a 3" pipe, if your outlet were 2" higher than the beginning, it would still drain. Your pipe is essentially a 1" pipe now. However, you would always 2" of standing water at the beginning.
We go overkill and use a laser transit to figure out the total change in elevation, then divide the distance by that = linear feet/inch change of elevation. Then make the bottom of the trench 1" higher after travelling that linear foot distance.
We usually tie the downspouts directly into the drain lines. This prevents any surface erosion around the downspout. We use 3" ADS, the black corrugated stuff, that then ties into PVC underground. Because the ADS is somewhat flexible, you can remove it from the bottom of the downspout if you ever need to remove the downspout.
You can't glue this ADS/PVC connection together. So the simplest is 3"ADS pipe, 3" ADS coupler, slides over PVC pipe. This ADS coupler/PVC connection is then taped with pipe wrap tape. The best is ADS coupler into PVC. But now your PVC is 4" and so are your fittings and your budget just got bigger too. Remember this is only for landscape drainage. A little leak won't flood your yard.
And put a drain box at the end. It has a basin about 2" deeper than the inlet. The basin collects debris and gets cleaned out every year or two. Ever better put a double inlet box (inlet and outlet) after the downspouts. This gives you a chance to intercept debris before it gets further down the line.