Addressing drainage issues in steel-structure factory buildings requires a systematic approach, focusing on roof slope, gutter & downpipe capacity, and critical node sealing. 

Here is a practical guide to diagnose and fix common problems:

1. Roof Surface Drainage (The First Defense)

The goal is to ensure rainwater flows quickly off the roof into the gutters.

Check the Slope: The minimum design slope for metal roofs is typically 5%. If the slope is too flat (or sagging due to deflection), water will pond, accelerating corrosion and leakage.

Fix: For existing buildings, it's difficult to change the structure. Focus on ensuring the gutters and scuppers are clear. If redesigning, use steeper slopes (≥8% in snowy areas to prevent snow accumulation).

Clear Obstructions: Inspect for debris, fallen leaves, or dust buildup that blocks water paths, especially near parapets or equipment curbs.

2. Gutter & Downpipe System (The Main Path)

Over 70% of drainage failures occur here due to blockage or undersizing.

Gutter Capacity:

Problem: Water overflows the gutter edge during heavy rain.

Solution: Clean gutters regularly (especially before rainy season). If overflow is chronic, consider deepening the gutter or adding overflow scuppers. Ensure gutter longitudinal slope is ≥0.5%–1% so water flows toward downpipes.

Downpipe Sizing & Layout:

Rule of Thumb: One downpipe (DN100–150) per 150–200 m² of roof area. Downpipes should be spaced ≤24m apart.

Fix: If downpipes are too few or too small, add more downpipes or replace with larger diameters. For large-span roofs (>60m), consider upgrading to a siphonic drainage system for faster evacuation.

Winter Special (for Cangzhou): Install gutter heaters or steam pipes to prevent ice dams from blocking downpipes, which can cause water to back up under the roof sheets.

3. Leakage at Critical Details

Water often enters through poorly sealed joints, not through the roof sheets themselves.

Roof Penetrations (Skylights, Ventilators, Pipes):

Ensure upstands are high enough (≥150mm). Use EPDM rubber gaskets and high-quality neutral silicone sealant (not acidic) to seal joints. Avoid direct penetration of roof panels if possible.

Gutter Joints & Endplates:

Check for cracks at welded seams. Repair by grinding clean, re-welding, and applying a high-elasticity waterproof coating (e.g., polyurethane) over the seam.

Roof Sheet Lap Joints:

Ensure laps are in the direction of water flow. Use 360° lock seams instead of simple lap joints with screws. Replace aged or cracked sealant washers on screws.

4. Emergency & Retrofitting Measures

If you are facing immediate problems and cannot do major reconstruction:

Add Overflow Systems: Install overflow scuppers (holes through the parapet) at a level below the roof seam but above the normal water level in the gutter. This prevents water from backing up into the building during a storm.

Local Reinforcement: For leaking seams, use a polyester fabric + elastic waterproof membrane system over the affected area instead of just applying sealant.

Ground Drainage: Don't forget the ground! Ensure the stormwater from downpipes is directed away from the building foundation via adequate yard drainage to prevent basement flooding or soil erosion.

Quick Maintenance Checklist

Bi-annual Cleaning: Clear gutters and roof valleys of leaves and debris.

Pre-Rainy Season Inspection: Check all sealants, gaskets, and gutter slopes.

Post-Storm Check: Look for new ponding areas or rust streaks indicating leaks.

For severe structural issues (significant roof deflection, widespread corrosion), consult a professional structural engineer to assess the integrity before applying waterproofing fixes.