How do roof falls and standing water link in Qardho?

A low roof bid in Qardho can leave out the work that makes water move. A roof fall is the slope built into the roof surface so water runs to a gutter, outlet or scupper instead of sitting on the deck. When a tender drops the setting out, the tapered screed and the outlet detailing, the finished roof can hold standing water after rain. That ponding then works on joints, flashings and coatings until a leak appears.

Why do roof falls underbidding risks start in the tender?

Most of the gap opens before anyone pours, because a fall is created by thickness, not by a single product. A roof that needs to drain one way needs a screed or slab that is thicker at the high side and thinner at the outlet, and that difference has to be set out, formed and checked. A bidder who prices a uniform layer can look cheaper and still meet the drawing on paper. Ask the estimator to show the setting out method, the finished level at the high point, the finished level at the outlet and the minimum thickness at the low point. General practice is to confirm those levels and the minimum cover over insulation or membrane against the project specification and the structural engineer, because the acceptable minimum varies with the build up. If the answer is a single average thickness, the slope has not been priced.

What does omitted drainage work do to a finished roof?

Once the slope is missing, water finds the flat spots first. Walk the surface after rain and watch whether water remains after the rest of the surface has dried, and record how long it takes to clear on that roof. Ponding adds load, softens some coatings and pushes water against laps, upstands and outlet seals. Small outlets and undersized gutters make it worse because water backs up before it can leave the roof. Ask the project designer what drainage records exist for the site, then walk the finished surface and look for the patch that stays wet after the rest has dried. Waterproofing alone does not fix the geometry. A membrane laid on a flat deck still holds water, and trapped moisture under it can lift the coating at the edges. Ask whether the outlet sits at the lowest point, whether the gutter has a fall to the downpipe, and whether the roof edge has a raised upstand to stop water running back under the covering.

How do you detect a missing fall before the roof is covered?

Catch it while the surface is still open and levels can be corrected. Ask the site team for the setting out record and a level survey at the high point, the outlet and two or three points between them. A straightedge or a laser level laid along the fall shows whether the surface drops steadily or dips in the middle. A flood test, done before the final covering where the specification allows it, shows where water collects and how fast it drains. Walk any roof surface after rain and note where the patches of water sit and how long they take to clear. Keep the readings, photographs and dates with the payment claim, because a slope that is short by a few millimetres per metre is hard to argue about later without numbers.

What to do next

Ask each bidder for a one page roof drainage method statement before you compare prices. It should list the high point level, the outlet level, the fall direction, the outlet size and the flood test plan. Send the same sheet to two bidders and ask them to price it as written, then compare the totals. If a bidder cannot describe the setting out, treat the price as incomplete rather than cheap.