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I currently have a large patio which has brick paving. unfortunately the paving has started to sink in spots and gathers rain water.

I will be removing the brick and laying a slab of concrete, in preparation to place floor tiles on the patio.

Please advise how I could prepare the site so that i know that the cement slab has the correct slope (to drain water).. The only thing I can think of is to use string and a spirit level.

Is this the common practice, or are there better ways to define your slope and angles of the slab?

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2 Answers 2

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I use a hose level. To make one obtain at least six feet (2 m) of 5/8 inch clear flexible tubing which has an outside diameter of 3/4 inch which is exactly the inner diameter of a standard garden hose. Cut into two lengths of 2–3 feet and slide the tubing inside each end of the hose 4–6 inches. It should form a good enough seal without any additional fuss. (If it doesn't, a wrap or two of Teflon, duct, or electrical tape should be enough.)

Lay out the hose assembly on the patio with each end held vertically with a stake into the ground (or chair, etc.) so it will hold water. (I find it convenient to use one inch brads through the tubing into the stake, but tape or string would work too.) Fill it full so the water level is about half way into the clear ends. Now you can easily determine the same level with extreme precision—perhaps 1/32 inch/0.8 mm at distances apart of 50+ ft depending on the length of the hose. Such accuracy is challenging (at best) with string and a carpenter's level.

To use it, place one end at a reference level and the other at a point in question. Raise or lower either end to cause the water height to increase or decrease, respectively, as doing so decreases or increases—respectively—the volume of the hose level. Once the reference end water level exactly matches the target height, mark the other end where the water is. Be sure to be consistent about using the top edge of the water or the bottom of the meniscus. It is entirely possible the hose has weeped a little water or it has relaxed or tightened due to temperature, sunlight, external moisture evaporation, etc. so recheck the reference end and re-level both ends if necessary and re-mark.

Arrange reference markers (such as more stakes) firmly into the ground at the corners and maybe some points along the sides of the patio and chose a reference height. It doesn't need to be at patio level. In fact, it is better to have it 12–24 inches above the patio level. Mark all the stakes at the reference level using the hose level.

From the reference marks all around, you can measure down to the actual height of the concrete pad or the cement forms. Do whatever slope calculation you desire, like 2.5 inches per 120 inches (a standard recommendation corresponding to 1/4 inch per foot) by measuring the distance from the house to the stake and applying the slope.

For example, if the marker stakes along one side are at 5', 10', and 15' from the house, the slope for each would be

5': (60 inches) is 2.5 / 120 * 60 = 1.25 inch drop
10': (120 inches) is 2.5 / 120 * 120 = 2.5 inch drop
15': (180 inches) is 2.5 / 120 * 180 = 3.75 inch drop

If the reference mark is 17.5 inches above the ground, then the final target marks should be at

5': 1.25" lower than 17.5" or 18.75 inches below the reference mark
10': 2.5" should be 2.5+17.5" or 20 inches below ref mark
15': 3.75" should be 2.5+17.5" or 21.25 inches below ref mark

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Excellent answer, my only concern is that I dont really have too much clearance off the ground to work with (since i need to get the end pieces vertical)... Would it work if i terminate the hose with 90deg elbows then put the clear pieces onto the vertical end of each elbow - otherwise i will just add a few inces to my stakes and then subtract the extra height later?? –  Hightower Nov 21 '13 at 9:22
    
Awesome answer !!! –  Michael Karas Nov 21 '13 at 15:16
1  
@Hightower -- You could add elbows to the ends of the hose but use care that anything you add does not leak. The best bet is to just use the clear tubing and bring that up along the vertical stakes and mark your references about a foot above where your working patio height is located. The clear tubing does not all need to be vertical. Part of it can be turned and laying on the ground where it joins to the garden hose. –  Michael Karas Nov 21 '13 at 15:20

Good discussion of what you want to do WRT reference marks from @wallyk. In my personal experience, see if you can borrow/rent a laser level, or be very, very careful not to have any bubbles in the hose - I have had generally poor experiences with hose levels (one of those better in theory than practice things - you move the end of the hose, the water gets moving, it comes out the end of the hose, the water level at the other end has now changed, they are easily affected by bubbles, which you cannot see if you use an opaque garden hose with those end-kits they sell for this, etc...)

One reason to keep the reference line fairly high is that if you do, you can use a calibrated (put a line on it) rake handle (or stick, but you're probably going to be using a rake to fix the grade) in combination with tight strings on the stakes to spot grade all over the site - you just need tight strings on the reference marks, and then you line your eye up with two strings (thus, a plane) and set your rake upright - the mark on the rake handle tells you where the spot under your rake-head is at with respect to grade. Rather than needing a lot of reference stakes, this can cut you down to only needing 6 or so (both ends and the middle) so you can always spot two strings and your rake at the same time. If you make the reference too low, this is difficult to do, or requires an extra step like using a mirror, which can be awkward and/or confusing. I usually put it somewhere in the 26-36" range so it's easy to bend down and look at it, without needing absurdly long stakes.

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Thanks for the compliment. Bubbles in the hose make no difference as long as they aren't in the top few inches where they would interfere with sighting the top of the water. Having the water come out is no big deal—it is easily replaced. As I mentioned in my description, both ends should be doubly checked for several reasons, including water loss. –  wallyk Nov 25 '13 at 4:40

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