Attic Completion Work - Course: Timberwork Techniques. Instruction Examples for Practical Vocational Training
 (introduction...) Preliminary Remarks Instruction Example 2.1.: Making Openings for Roof Superstructures in Existing Roofing Instruction Example 2.2.: Making a Rafter Trimming Instruction Example 2.3.: Making the Opening for the Roof Window to be Built in Instruction Example 2.4.: Joining of the Parapet Transom Instruction Example 2.5.: Joining of the Window Posts Instruction Example 2.6.: Joining of the Lintel Transom Instruction Example 2.7.: Joining of the String Transoms and Roof Rafters

### Instruction Example 2.7.: Joining of the String Transoms and Roof Rafters

This instruction example teaches how to join the string transoms and roof rafters for an upright roof window with lean-to dormer roof to be built in.

Constructional details

- String transoms are to be supported on main roof rafters and cogged in the lintel transom.
- Roof rafters are to be supported on rafters of the main roof and on the string transoms.
- Cross sections of timbers are shown in the working drawing of Instruction Example 2.4.
- String transoms are cut to a length of 1750 mm and roof rafters are cut to a length of 2750 mm.

Figure

Hand tools

Hammer

Measuring and testing means

Folding rule

Auxiliary accessories

Workshop and detail drawings, trestles (one ell), pencil, hand sketch, engineer's square

Necessary previous knowledge

Measuring, scribing, angling, use of marking-out marks, reading of drawings, reading of tabular values

 Sequence of operations Comments 1. Study workshop and detail drawings and draw hand sketch for determination of timber lengths. See working drawings 3 and 7 and calculation method below! 2. Calculate the timber lengths for string transoms and roof rafters and the recess sizes for scribing. Calculation method:Length of string transom (V)V= (250 + 1134 + 120) mm = 1710 mmV chosen = 1750 mmLength of roof rafter (VI0) VI0 = X2 + V2= 8402 mm2 + (250 + 1340 + 960)2 mm2 VI0 = 2684.8 mm, chosen = 2750 mm Recess sizes (VI1, VI2) VI1 : V = X : VII alpha2 = 180 degrees - (135 degrees + 18.23 degrees) alpha2 = 26.77 degrees tan alpha = tan 26.77 degrees = 0.5045 VI2 = 237.9 mm 3. Prepare working place, erect trestles with correct distance. Check trestles for firm standing and stability! 4. Put string transoms on trestles, decide on joining side and mark with marking-out mark. Joining side "I" shall have an even face!Use any camber of timbers for top face of string transoms and mark with "IV"! 5. Put string transoms closely together on trestles. Place string transoms so that face marked with "II" is visible from above!The left ends of string transoms to be about flush, hammer with gentle hammer blows to make them flush, if necessary! 6. Scribe section line at left ends of string transoms and mark with marking-out mark. Go only as far to the right as to permit a right-angle section to be produced! 7. Angle section line at both joining sides, scribe and mark with marking-out mark. Place engineer's square exactly! 8. Measure-in maximum length at right ends of string transoms, scribe section line and mark with marking-out mark. For maximum length of string transoms see calculation method!Measure from section line 6. towards the right, place folding rule in parallel with timber edge, place exactly at section. Let folding rule tongues engage correctly! 9. Angle section line at the joining sides at the right ends of string transoms, scribe and mark with marking-out mark. Place engineer's square exactly! 10. Measure-in recess sizes from the section lines of the left and right ends of string transoms towards the centre, scribe and mark with marking-out marks. Do not mix up the recess sizes!The string transoms cannot be interchanged because they are to be cogged in the lintel transom! 11. Measure-in the inclination of the rafters of the main roof at the left ends of the string transoms, scribe and mark with marking-out mark. Use leg of square for scribing, place it exactly!Scribe inclination on both joining sides! For recess sizes see working drawing 4! 12. Measure-in inclination of cornice at the right ends of the string transoms, scribe and mark with marking-out mark. Use leg of square for scribing, place it exactly!Sizes see working drawing 4! 13. Measuring-in and scribe the front edge of the lintel transom from the right section line of the string transoms towards the left. Scribe thin line and use engineer's square. 14. Measure-in to the left and scribe the lintel transom width. For width see working drawing 4! 15. Angle scribed lines of 13. and 14. at the joining sides and scribe. Do not scribe the lines over the total timber width! 16. Measure-in and scribe cog depth. For cog depth see working drawing 4! 17. Mark wood to be removed for cog recess. Mark with diagonal lines! 18. Put string transoms apart, scribe on inner faces and mark. Scribe all lines as on the joining side! 19. Work string transoms, take them off the trestles and store them outside the working area. Use supports when storing them! 20. Put roof rafters on trestles, decide on joining side and mark with marking-out mark. Joining side "I" to have an even face!In the event of any slight camber, mark such faces with "IV" and put them on the trestles with the faces down! 21. Put roof rafters closely together on trestles. Shown on page 12 but not identified by figures.Place timbers so that faces marked "II" are visible from above and left ends of roof rafters are about flush, to be made flush by gentle hammer blows, if necessary. 22. Scribe section line at left ends of roof rafters and mark with marking-out mark. Go only as far to the right as necessary to produce a right-angle section! 23. Angle section lines at the joining sides, scribe and mark with marking-out mark. Use engineer's square, place it exactly and scribe thin line! 24. Measure-in, scribe and mark with marking-out mark the roof rafter length. 25. Angle right-end section line of roof rafter length, scribe and mark with marking-out mark. 26. Measure-in the recess size (VI1) at the left ends of the roof rafters, scribe and mark with marking-out mark. Place folding rule exactly at section line and scribe exact size VI1, otherwise the roof rafter of the lean-to dormer roof will not be supported on the rafter of the main roof! 27. Scribe roof pitch (inclination) of main roof on joining sides and mark with marking-out marks. Use leg of square for scribing and place it exactly at the lines! 28. Measure-in the recess size at the right ends of the roof rafters, scribe and mark with marking-out mark. Place folding rule exactly, otherwise the foot bevel will not fit on the string transom! 29. Scribe rafter foot bevels and mark with marking-out mark. Use leg of square for scribing and scribe exactly! 30. Put roof rafters apart and scribe on inner faces (III). Scribe all lines as on the joining sides!The roof rafters may be placed with the joining sides on the trestles when scribing! 31. Work the roof rafters, take them off the trestles and store them outside the working area. Use supports for storing them! 32. Clear the working place.

Joining of the string transoms and roof rafters