The Half-Size Print That Moved the Floor Drain

Why this matters

Anything cast into a slab is located once. A floor drain set in the wrong spot before a pour is moved with a saw, a breaker and a second inspection, at several times the hours it took to set it. This case is a drain that landed 2'-7-1/2" from where the drawings put it, on a job where every drawing was right, the field layout was done carefully, and the plumber measured exactly what they meant to measure. The error was in the paper, and specifically in an assumption about the paper that most shops make every day.

What turned up after the pour

A plumbing shop had the underground on a small commercial building: slab on grade, a mechanical room at one end. The underground was roughed, tested, inspected and backfilled, and the slab was poured. Two weeks later the general contractor (GC) laid out the housekeeping pad for the boiler, a 4 in raised concrete base, and found the mechanical room's floor drain inside the pad's footprint.

The architect's enlarged mechanical room plan placed the pad from 15'-0" to 19'-0" east of column grid line 3, and the floor drain on grid line B at 21'-0" east of grid 3, 2 ft clear of the pad's east edge, where the boiler's relief and drain lines could discharge to it. The installed drain sat on grid line B at 18'-4-1/2" east of grid 3: inside the pad, 2'-7-1/2" west of design.

The candidates, and how each was tested

The shop's first instinct was to blame the field layout, and its second was to blame the drawings. Each was tested before anyone accepted the next.

The layout was taped from the wrong grid line or with a bad hook. The foreman re-taped from the grid 3 column line, which was still marked on the slab edge, along grid line B to the drain's center: 18'-4-1/2". The plumber's layout notes for that day read "FD-1 at 18'-4 1/2 E of 3." The field put the drain exactly where the plumber intended. The layout was not the error; the intended number was.

The drawings put the drain there. The plumbing plan P-101 showed the floor drain as a symbol with a fixture tag and no dimension. The architect's enlarged plan A-402 dimensioned it: 21'-0" east of grid 3. Scaled off a full-size P-101 at its stated quarter-inch scale, the symbol sat at 21'-0" too. The two sheets agreed with each other. The drawings were not the error either.

The plumber scaled the wrong sheet or the wrong scale. The plumber had worked from an 11x17 print of P-101 that lived in the truck, called "the half-size" by everyone in the shop. A half-size print of a quarter-inch sheet reads at eighth-inch scale, so the plumber laid the eighth-inch side of an architect's scale on the print and read 18'-4-1/2". To test whether that was the error, the foreman measured something on the same print that had a figured dimension: the column bay between grids 3 and 4, dimensioned 30'-0". On the 11x17 with the eighth-inch scale, it read 26'-3".

That was the number that lied, and it settled it. A print that turns 30'-0" into 26'-3" turns every distance into 26.25 / 30 = 0.875 of itself.

Why the print was not half size

P-101 was issued on a 24 by 36 inch sheet. Exactly half of that is 12 by 18 inches. The common 11 by 17 inch paper is not half of anything in the set: it has a different shape (17 / 11 = 1.55 against 36 / 24 = 1.5) and it is smaller on both sides. Printing a 24 by 36 sheet onto it with fit-to-page shrinks the drawing by whichever direction is tighter, and then shrinks it again for the printer's unprintable margin.

REDUCTION OF A 24 x 36 SHEET ONTO 11 x 17
printable area at 1/4 in margins: 16.5 x 10.5
long side:  16.5 / 36 = 0.4583
short side: 10.5 / 24 = 0.4375  <- tighter, used
print factor = 0.4375 (true half size = 0.5000)

scale on print = 0.25 in/ft x 0.4375
               = 0.1094 in per ft
eighth-inch scale assumes 0.1250 in per ft
reading factor = 0.1094 / 0.1250 = 0.875

design drain 21.0 ft -> prints 21.0 x 0.1094
             = 2.297 in on paper
read at 1/8: 2.297 / 0.125 = 18.38 ft
  (exactly 21.0 x 0.875 = 18.375 ft)
           = 18 ft 4-1/2 in
error = 21.000 - 18.375 = 2.625 ft
      = 2 ft 7-1/2 in, toward grid 3

check: column bay 30.0 x 0.875 = 26.25 ft
       = 26 ft 3 in, as read on the print

The margins in that block are typical of an office printer set to a quarter inch on each side. A different printer, a different margin setting, or a print made at "actual size" and cropped would give a different factor. That is the point: the shop could not know the factor of a print it had not measured. Even with no margin at all, the tighter side gives 11 / 24 = 0.458, and the print reads at 0.458 / 0.5 = 0.917 of true, still wrong by about 1'-9" over 21 ft.

Why the error ran toward grid 3. The plumber measured from grid 3 eastward. Every scaled distance came out short, so the drain landed short of its design location, on the grid 3 side. A distance taken from grid 4 back toward the drain would have read short as well, putting the drain too close to grid 4. Which side the error lands on depends on where you measure from, which is why a shop cannot catch it by noticing that drains "tend to land west."

Why the print gave no warning. P-101 had no graphic scale, the drawn bar of marked feet that shrinks with the print and stays true at any reduction. It had only the written note "SCALE: 1/4" = 1'-0"", which was true on the original and false on every reduction. A graphic scale on the sheet would have measured the 11x17 correctly, and the plumber could have scaled against it.

The same trap on a screen. The shop's tablets carry PDF viewers with a measuring tool, and the plumber's first question afterward was whether that would have been safer. Only if it is calibrated. A viewer's measure tool set to "1/4 in = 1 ft" assumes the PDF is the original sheet at full size; a PDF exported from a reduced print, or a scan of an 11x17, carries the reduction inside it and the tool reads short by the same factor. Calibrating the tool against a figured dimension on that same PDF (most viewers let you set the scale by drawing over a known length) fixes it for that file. The calibration does not travel to the next file, so it is redone per sheet.

The repair, and what it cost in hours

The drain had to move 2'-7-1/2" east along grid line B, with its branch re-sloped to the new location. The GC's concrete subcontractor scanned the slab for embedded conduit before cutting, because a saw through a live conduit in a slab is an electrical hazard and a second repair; the one conduit found ran clear of the cut. They saw-cut and broke out a section of slab around the old and new locations. Cutting and breaking concrete releases respirable crystalline silica, which 29 CFR 1926.1153 regulates for construction; the subcontractor cut wet under that standard's Table 1 controls for the saw used. The plumbing shop re-excavated, cut in the new drain location, re-sloped the branch, and the plumbing inspector re-witnessed the test before backfill and patch.

Setting the drain originally had taken about 4 labor hours. The repair took the plumbing shop 2 technicians for 10 hours each, 20 labor hours, five times the original, plus the concrete subcontractor's time and the patch, which the GC back-charged. The boiler pad pour slipped 4 working days.

The shop rule now

The shop wrote one rule and three supports for it.

The rule: locate from figured dimensions or from the architect's grid, never from a scaled small print. A figured dimension is a number written on the drawing; it is true at any print size. If the location you need has no figured dimension on any sheet (check the architectural enlarged plans, which commonly carry what the trade plans omit), send an RFI asking for one. The answer usually takes a few days, which is cheap against a saw-cut. See related: How to Write an RFI That Gets a Usable Answer.

Support one: every reduced print in the shop says so. Reduced prints carry "REDUCED - DO NOT SCALE" written across the title block, and the shop stopped calling 11x17 prints "half-size," because the word is the assumption that caused the error.

Support two: if anyone must scale for planning, calibrate first. Scaling is still useful for estimating pipe lengths or finding roughly where something is. Before scaling any print, measure a figured dimension on it and write the factor beside the title block: on this print, 30'-0" read 26'-3", so a factor of 30 / 26.25 = 1.143 corrects any eighth-inch reading to true. A reading of 18'-4-1/2" times 1.143 is 21.0 ft. A calibrated print is good for an estimate and nothing cast in place. See related: How to Scale a Dimension Off a Print.

Support three: anything cast in concrete gets a second check from a different source before the pour. The foreman checks each embedded item's location against the architectural enlarged plan's figured dimensions, not against the plumber's layout notes, so the check does not inherit the original error.

What would have looked different

A full-size print on the truck. Read at its stated quarter-inch scale, the full-size P-101 places the drain at 21'-0", the same as the figured dimension. Scaling a full-size sheet at its stated scale is unreliable for other reasons (paper stretch, a drafter who did not draw to scale), but it would not have produced this error.

A drain located from the architect's figured 21'-0". The plumber would never have touched a scale. This is the whole rule.

A pour delayed for any reason before the pad layout. Found before the pour, the same error costs a few hours to dig and re-set rather than 20 hours plus a saw-cut, which is why support three is timed before the pour rather than after.

References

• See related: How to Scale a Dimension Off a Print
• See related: Architectural and Engineering Scales on One Ruler
• See related: How to Write an RFI That Gets a Usable Answer
• See related: How to Find Your Place by Column Grid and Key Plan
• 29 CFR 1926.1153, respirable crystalline silica in construction