Short answer. A metal roof is assembled from the bottom up, each layer overlapping the one below so that water always runs on the outside. But the expensive mistakes are made before the first sheet goes on: picking a profile that will not hold the pitch, and counting sheets by overall width instead of working width. Below is the order of operations and the arithmetic of a slope.
The order of assembly
The sequence is not decorative: every item exists because the one before it does not work otherwise.
| Step | What goes on | What breaks if you swap the order |
|---|---|---|
| 1 | Roof pitch → profile choice | below the minimum pitch water climbs into the lap by capillary action, and no amount of careful installation helps |
| 2 | Lower flashing on the fascia board | fitted after the membrane — condensation off the membrane runs onto the board |
| 3 | Underlay membrane, bottom up, over the lower flashing | tucked under the flashing — water off the membrane misses the gutter |
| 4 | Counter-battens along the rafters | without them there is no vent gap, and the steel dries from one side only |
| 5 | Battens spaced for the wave height | spacing by eye — the sheet deflects under snow |
| 6 | Eaves flashing over the battens | — |
| 7 | Sheets bottom up, starting from the leeward gable | a side lap facing into the prevailing wind gets driven full of water |
| 8 | Barge and ridge flashings, filler | — |
Steps 2-6 have their own articles: the two flashings at the eaves and batten spacing by profile.
How many sheets per slope
Count by working width — what is left after the one-wave side lap. Overall sheet width is not needed for the calculation at all: count with it and you will come up short.
A slope 4.0 m from ridge to eaves and 9.0 m along the ridge is 36 m² of plane. Number of sheets = 9,000 mm divided by the working width, rounded up.
| Profile | Working width | Sheets across 9 m | Area purchased | Overrun |
|---|---|---|---|---|
| T35 | 1,080 mm | 9 | 38.88 m² | +8% |
| T45 | 1,020 mm | 9 | 36.72 m² | +2% |
| T57 | 960 mm | 10 | 38.40 m² | +6.7% |
The conclusion is counter-intuitive: the overrun depends on the remainder of the division, not on whether the profile is wide or narrow. T45 wins here even though it is narrower than T35. So check the slope width against several profiles before choosing on price per metre.
The last sheet in the row is trimmed lengthwise. Its cut edge ends up at the gable and is covered by the barge flashing — that is normal, and it does not need touching up with paint.
Minimum pitch by profile
| Profile | Wave height | Minimum pitch |
|---|---|---|
| T20r | 20 mm | from 12° |
| T35 | 35 mm | from 9° |
| T45 | 45 mm | from 8° |
| T57 | 57 mm | from 6° |
T10 and T20f are wall profiles and do not go on a roof. More on shallow slopes in the mono-pitch roof article.
The transverse joint is the main risk
If the sheet is shorter than the slope, the roof goes on in two rows with a transverse lap. That is three things at once: an extra row of battens under the joint, a place where water stands, and a line you can see from the ground.
We roll the sheet to the full length of the slope, so on our steel there is usually no transverse joint at all. Length is set when you order — the limit here is not the line but the fact that a very long sheet still has to be delivered and lifted onto the roof without bending it.
What we do not do
We do not install roofs, design rafters or visit sites to measure. We roll the sheet to your dimension and fold flashings from the same coil so the colour matches.
How to order
Step 1. Measure the slope
Length from ridge to eaves, width along the ridge, and the pitch angle. For each slope separately.
Step 2. Match the profile to the pitch
Use the table above, then check the overrun by width — it may change your choice.
Step 3. Order sheets and flashings together
Call (067) 077-56-77, Mon-Fri 08:00-17:00. Production 1-2 days, delivery another 1-5 days.
FAQ
Which end of the row do you start from? The leeward gable, so that the side lap faces away from the prevailing wind rather than into it.
Can you walk on the roof? In soft shoes, stepping into the trough of the wave and only where a batten runs underneath.
How many screws should I count? Typically 6-8 per m², into the lower wave. Lengths by profile are in a separate article.
What do I trim the last sheet with? Tin snips or nibblers. An angle grinder with an abrasive disc burns the coating off — here is why.
Is a membrane needed on a cold roof? Condensation forms on the underside of the steel whether the roof is heated or not. The question is not whether, but which type — and that is a design decision, not a steel supplier’s.