Question 1

How long is a common rafter for a 6/12 roof with 12 feet of run?

Accepted Answer

Common rafter length is the run multiplied by √(12² + rise²) per foot. At 6 inches of rise per foot that unit length is 13.42 inches, so 12 feet of run gives 13 ft 5 in and 0/12 — 13.42 ft — before adding the overhang and subtracting half the ridge thickness. Gerald Warren’s notebook carries this as a table for 1 to 24 inches of rise and 1 to 19 feet of run, written as feet, inches, twelfths; the table on the transcription page is regenerated from the same formula and checked against his cells. Trusses replaced hand-cut rafters on most new Ottawa houses after the 1970s, so today this is the maths for additions, dormers and repairs.

Question 2

What is the hip run, and why is it 17 inches?

Accepted Answer

A hip rafter runs diagonally across the square corner of the roof, so for every 12 inches the common rafter runs, the hip runs 12√2 = 16.97 inches — rounded to 17 on the square. Hip length per foot of common run is therefore √(288 + rise²): at 6 inches of rise that is 18.0 inches exactly, which is why a 6/12 hip works out so cleanly. The notebook’s cuts table says it plainly: ‘hip run for a square angled roof is always 17″.’ Nothing about this has changed.

Question 3

How do I get every roof cut off a steel square?

Accepted Answer

Set two numbers, one on the tongue and one on the blade, and mark the cut. From Gerald Warren’s cuts table: rafter plumb and seat — rise on the tongue, 12 on the blade. Hip plumb and seat — rise on the tongue, 17 on the blade. Jack splay — 12 on the tongue, rafter length on the blade, mark on the blade end. Hip splay — 17 on the tongue, hip length on the blade. Hip backing bevel — rise on the tongue, hip length on the blade, mark on the blade end. Side purlin mitre — rise and rafter length, mark on the tongue end; top purlin mitre — run and rafter length. Every rafter square sold today carries the same numbers.

Question 4

What pitch in degrees is a 6/12 roof, and what is 40 degrees in inches per foot?

Accepted Answer

Rise per foot = 12 × tan(pitch). 6/12 is 26.6°; 8/12 is 33.7°; 12/12 is 45°. Going the other way, 40° is 10.07 inches per foot — call it a 10/12. The notebook carries a degrees-rise table for every degree and a degree-based rafter table from 12° to 45°, because his drawings were sometimes in degrees and his square was in inches. Speed squares still show both.

Question 5

How do I lay out a stair with a story rod?

Accepted Answer

Take the total rise from finished lower floor to finished upper floor on a rod, divide it into equal risers, then choose a tread so that riser plus tread lands in a comfortable sum. Gerald Warren’s rule was ‘not more than eight inches’ per riser and 17 to 18 inches for riser plus tread, with 7½″ × 10″ as his example. The story-rod method is still how a careful carpenter does it. The numbers have moved: the Ontario Building Code (Part 9, 9.8.4) now limits a private stair to a maximum rise of 200 mm (7⅞″) and a minimum run of 255 mm (10 1/16″), so his 7½″ × 10″ example passes and his 8″ × 9½″ example does not.

Question 6

How do I step off stair strings with a steel square?

Accepted Answer

Exactly like a rafter: rise on the tongue, going on the blade, and step the square along the string marking plumb and seat cuts — the notebook calls the square a ‘pitch board’ for this. His worked example is a 105-inch total rise over 14 risers, giving 7½″ risers on a 9″ going, with the string board at least 9″ × 1½″ and a ½″ sinking for wedged treads. Stair gauges for the square are still sold for this. That 9″ going would fail today’s 255 mm minimum run; use 10″ or more.

Question 7

How is a floor opening for a stairwell or chimney framed?

Accepted Answer

The joists either side of the opening are the trimming joists; the short one closing the opening is the trimmer, and the shortened joists it carries are tail joists. Gerald Warren’s method was a tusk tenon wedged into a haunched mortice, with the trimmer sized by adding ⅛″ of thickness for every joist it carried, set 14 to 18 inches back from a chimney breast with a 1″ air gap. Today the joinery is a metal joist hanger, and the Ontario Building Code (9.23.9) sizes headers and trimmers by span: a header over 1.2 m is doubled, and beyond roughly 3.2 m it is engineered. Clearance to a masonry chimney is now 50 mm, not 1 inch. His 14-to-18-inch setback is still good practice.

Question 8

How far apart should floor bridging be, and how do I cut it without measuring?

Accepted Answer

Lay a length of strutting material diagonally across the joist space at the marked line, hold it flat, and scribe both joist faces onto it — length and both splays in one go. Gerald Warren’s rule was lines of strutting not more than 6 feet apart, set 6½″ apart on a 7″ joist, with a ⅜″ space at the bottom. Current practice puts bridging or solid blocking at intervals not exceeding 2.1 m (6′ 11″), so his 6 feet sits inside it. Metal bridging and solid blocking have largely replaced cut struts, and engineered I-joists use whatever blocking the joist maker specifies.

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