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How to calculate a roof drainage system: step-by-step instructions with formulas and examples
Most mistakes in installing a drainage system occur before the master climbs onto the roof. The owner orders “gutters for the house” — and it often turns out that something is missing: a few meters of gutter, a couple of elbows, a funnel. Even worse, when, due to the wrong diameter selection, the entire system cannot cope with the flow during a downpour, and the water simply overflows over the edge.
This article is a step-by-step calculation method from the factory Aqueduct, which will help you draw up an accurate specification even before the first contact with the manager. All formulas have been tested on real objects. At the end is a ready-made example of a full calculation for a gable house 8×10 m.
Aqueduct produces systems in two sizes: standard (125 mm gutter, 87 mm pipe) with elements 2 m and 4 m long; and enlarged (150 mm gutter, 100 mm pipe) with elements 3 m long. All formulas and tables in this article are based on these parameters.
Step 1. We take off the required roof dimensions
Before you can calculate anything, you need to gather the initial data. You will need four parameters: length of each overhang (horizontal projection of the lower edge of the roof), area of each slope, height from overhang to blind area і number of corners (external and internal), where the gutter changes direction.
If the house is still in the project, take the numbers from the architectural plan. If the roof is already finished, measure with a tape measure along the edge of the eaves. It is necessary to measure the length of the overhang, not the length of the slope along the slope. The slope area is calculated using the formula:
Formula 1. Roof slope area
For example, for an 8×10 m house with a gable roof and a slope width of 4,5 m: S = 10 × 4,5 = 45 m² per slope. Both slopes together give 90 m² - and this is exactly the figure on which the choice of system size depends.
Pay attention: slope area — this is not the roof area. The roof is calculated based on the actual surface, including the slope. For hydraulic calculation of the gutter, a horizontal projection is required — that is, a view of the house from above.
Step 2. Choose the system size
This is probably the most important decision in the entire calculation. The choice between a standard or an enlarged system determines whether the drain can handle the actual flow or not.
Basic principle: One drainpipe with a diameter of 87 mm is capable of draining water from a slope area of no more than 100–120 m². A 100 mm pipe covers up to 150–200 m². But this is the maximum. In practice, especially in regions with heavy rainfall, it is better to lay a reserve and divide the load between several drain points.
Refer to the following table:
Table 1. Selection of system size depending on slope area
| Slope area (m²) | Number of drainage points | Distance between pipes (m) | Min. chute diameter (mm) | System size |
| to 100 | 1 | to 10 | 125 | Standard |
| 100–150 | 1–2 | to 10 | 125–150 | Both |
| 150–250 | 2 | to 10 | 150 | Enlarged |
| over 250 | 3+ | to 10 | 150 | Enlarged |
If your house falls into the border zone (for example, a slope of 130 m²), it is better to choose enlarged system 150/100 — excess stock never hurts, and the hydraulic resistance in a larger pipe is significantly lower. Read more about the possibilities of an enlarged system in our article "A revolutionary solution for large facilities: the metal drainage system Aqueduct 150/100".
Step 3. Count the gutters
So, the size is determined. Now let's move on to the number of elements. There is an important nuance here: systems 125 / 87 mm і 150 / 100 mm have different lengths of gutter elements, so we calculate differently.
For 125/87 mm systems Gutters are available in two lengths - 2 m and 4 m. For 150/100 mm systems 3 mThe calculation algorithm is the same in both cases: first we determine the total need in linear meters, and then we select the optimal combination of segments.
Formula 2. Number of gutters - System 125/87 mm
Total need in linear meters: L_total = L_overhang × 1,1, where the coefficient 1,1 is a 10% margin for trimming and unforeseen situations. Next, from the resulting figure, we select a combination of segments. For example, for an overhang of 10 m, 10 × 1,1 = 11 m is required. A convenient combination: 2 segments of 4 m + 2 segments of 2 m = 12 m — and the margin is covered.
In addition to the quantity, you need to take into account the angle of inclination of the gutter. The gutter is mounted with a slope of 2–3 mm for each linear meter towards the funnel. On a 10 m section, this is a difference of 2–3 cm between the beginning and end of the gutter. If the slope is smaller, the water will stagnate and ice plugs will form in winter.
Table 2. Selection of combinations gutter — 125/87 mm system (2 m and 4 m elements)
| Overhang | Combination of elements | PCS | Total (m) | Note |
| 6 m | 1×4 m + 1×2 m | 2 | 6 m | |
| 8 m | 2 × 4 m | 2 | 8 m | |
| 10 m | 2×4 m + 1×2 m | 3 | 10 m | out of stock |
| 10 m | 2×4 m + 2×2 m | 4 | 12 m | +10% margin (recommended) |
| 12 m | 3 × 4 m | 3 | 12 m | |
| 12 m | 2×4 m + 2×2 m | 4 | 12 m | alternative combination |
⚠ Maximum distance between two funnels in one gutter contour: 10 m.
If the overhang is longer than 10 linear meters, it is recommended to install two funnels - one at each end, with the gutter sloping from the edges to the center. This reduces the hydraulic load on the pipe and allows you to do without increasing the diameter of the system.
Important: The distance between two funnels in one gutter circuit should not exceed 10 meters - for both system sizes. If the overhang is longer - add an additional drain point or divide the gutter into two independent circuits.
Step 4. Determine the number and location of drainpipes
The number of pipes directly depends on how many funnels you placed in the previous step: one funnel = one pipe. But there is an important nuance here - not only the number, but also the location of the pipes.
General rule: pipes should be placed no further than 50 cm from the corners of the house to avoid overflow in the areas where water flows merge. The number of pipe sections is calculated as follows:
Formula 3. Number of pipe sections
For example, for a house with a wall height of 6 meters: N = (6/3) × 1,1 = 2,2. Round to 3 segments for one pipe.
Count separately knees 67°To bypass a cornice overhang, two elbows are usually required for each pipe—one directly under the funnel and one to bring the pipe closer to the wall plane.
If the cornice is wide or there are architectural protrusions, the number of knees can increase to 4.
In Aqueduct systems, it is used single standard angle 67°, which is suitable for most eaves overhang configurations.
Step 5. Counting small accessories
This is the part of the calculation that is most often underestimated. Owners order gutters and pipes, but forget about holders, connectors, and plugs - and then buy them separately, wasting unnecessary time and money.
Table 3. Hardware configuration standards
| Element | Norm per 1 linear meter of gutter | Norm for 1 pipe | Note |
| Gutter holder (bracket) | 1 pc / 0,6 m | step 50–60 cm | |
| Gutter connector | 1 piece per joint | with corrugated gasket | |
| Gutter plug | 2 pieces for each segment | universal | |
| Gutter angle external / internal | behind the corners of the roof | don't forget about the corner connectors | |
| Drain funnel | 1 pc per pipe | junction of gutter and pipe | |
| Pipe to wall clamp | 1 pc / 1,5 m | step 1,5–2 m in height | |
| Elbow 67° | 2–4 pcs per pipe | bypassing the cornice and approaching the wall | |
| Spider letter holder | around the perimeter of the tree crown | details — in the article about "Spider" |
Pay special attention to gutter connectorsUnlike plastic systems, where the joint is simply glued, in metal Aqueduct systems each connector is equipped with corrugated rubber gasketIt performs a dual function: it seals the joint and at the same time serves as thermal compensator, because steel expands when heated. The gutter is fixed rigidly, but there is a minimum allowance for thermal movement in the connection area - all this happens imperceptibly and without deformation.
Okremo varto zgadati pro letter holdersIf there are trees growing near your house, leaves will inevitably clog your gutters in the fall. A special leaf catcher is the solution. "Spider" — it installs in the funnel and traps debris without impeding the flow of water. It's a small investment that saves an hour of work every spring and fall.
Step 6. Finished example - 8×10 m house, gable roof
Now let's put everything together and see what the actual calculation looks like.
Output data: The house is 8×10 m, with a gable roof, the width of the slope is 4,5 m, the height of the wall from the eaves to the blind area is 6 m. The roof is symmetrical, with two independent gutter contours of 10 m each.
Area of one slope: S = 10 × 4,5 = 45 m². Both slopes: 90 m². This is the area of a standard 125/87 mm system with two drainage points.
Gutters: each overhang is 10 m. For one wing: 2×4 m + 1×2 m = 10 m (minimum) or 2×4 m + 2×2 m = 12 m (with a 10% margin). We choose the option with a margin.
Table 4. System specification for a house 8×10 m (gable roof, system 125/87 mm)
| Element | Calculation | Number | System |
| Gutter 4 m | 2 pc × 2 slopes | 4 pc | 125 mm |
| Gutter 2 m | 2 pcs × 2 slopes (+stock) | 4 pc | 125 mm |
| Gutter holder | 20 m × 1,67 pcs/m | 34 pc | |
| Gutter connector (with gasket) | 2 joints × 2 slopes | 4 pc | |
| Gutter plug (set of left + right) | 2 ends × 2 slopes | 4 pc | |
| Drain funnel | 1 pc × 2 slopes | 2 pc | |
| Pipe 3 m | (6 m / 3) × 1,1 × 2 pipes | 5 pc | 87 mm |
| Pipe clamp | 6 m / 1,5 m × 2 pipes | 8 pc | |
| Elbow 67° | 2 pcs × 2 pipes (bypassing the cornice) | 4 pc | |
| Drain tip/outlet | 1 pc per pipe | 2 pc |
Please note: the specification states “1 pc / 0,6 m” for the holders – that is, one bracket for every 60 cm of gutter. This is not reinsurance, but a norm that ensures the gutter maintains its shape properly under the weight of snow in winter.
A few important nuances that are often overlooked
Thermal expansion and connectors with corrugated gasket
Steel, like any metal, changes length when temperature fluctuates. Many expect this to lead to deformations, but in Aqueduct systems this issue is solved constructively. Gutter firmly attached to the brackets, and no "floating" fastening is required. Instead, each gutter connector has corrugated rubber gasket, which absorbs thermal expansion directly in the joint area - without transferring forces to the brackets and without visible deformation of the profile. Therefore, with the correctly selected number of connectors, the system remains airtight and stable regardless of seasonal temperature changes.
Read more about preparing the system for winter and the behavior of metal in frost in the article "Caring for Metal Gutters: A Complete Maintenance Guide".
System color and facade matching
Technical calculation and color selection are two independent tasks. But it is better to solve them simultaneously so that together with the fittings you do not have to ask about the availability of the desired shade. Aqueduct offers a wide palette, including the popular terracotta RAL 8004 and other architectural colors. The article tells how to choose a color for the facade and roof "How to choose the color of the gutter system".
Steel quality is the basis for the system's durability
When the specification is ready and the order is almost placed, it makes sense to ask another question: What steel is the system made of? The aqueduct uses SSAB (Sweden) steel with a Z275 zinc coating and a polymer finish. This directly affects how long your system will last — 15 or 50 years. For more information about steel grades, coating thickness, and what the numbers in the technical documentation mean, read the article "Aqueduct: European quality from the official partner of SSAB".
Visnovok
Correct calculation of a drainage system is not a complicated math, but it is systematic work with four steps: measuring the roof, choosing the system size, calculating the main elements and fittings. Skip any of them and you will have unpleasant surprises on the construction site.
If you have a non-standard roof geometry, a complex facade relief or a large commercial facility, contact an Aqueduct technologist. We will provide a free consultation and help you draw up a specification for your specific facility. Order from 50 linear meters are supplied with a specification from our technologist free of charge.
Please note: Akveduk is a manufacturing plant, not an installation organization. We sell the system directly from the factory, without intermediaries. This means a fair price, stable quality, and a package assembled for your facility — not what is on the shelf of the nearest hardware store.