25C-144 (2) Existing Roof Structure
35 -37 Orchard Street
Northampton, MA
November 27, 2009
of snow load on the roof. The most efficient position for this tie is at the rafter bottoms, and is
provided by the attic floor joists, which span parallel to the rafters. However, in the span
between the top of the rafter at the peak and the bottom of the rafter at the eave, the rafter will
tend to sag under the dead weight and snow. The smaller the cross - section of the rafter, the
greater the sag, hence the Code requirements. Deeper rafters also permit more thermal
insulation.
The kneewalls can provide vertical support to reduce the rafter span. This will place some load
on the floor joists, the analysis of which is beyond the scope of this report. The ceiling joists,
which are colloquially referred to as "collar ties ", do not perform as tension "ties" to resist thrust,
since that is provided more capably by the floor joists. The ceiling joists can act as braces, in
compression, to help reduce the sagging of the rafters by keeping them spread apart a that
location.
Considering the little amount of institution afforded by the old rafters with vent channels, as well
as the tendency of slate roofing to shed snow, it is likely that most of the wintertime, there would
be little or no snow accumulated on this roof, except perhaps in the valleys around the dormer.
However, if the full snow load were to occur, particularly in the unbalanced condition, then there
could be excessive sag of the rafters, and possible cracking of ceiling drywall joints, in the areas
where the ceiling joists were removed.
In place of the ceiling joists, discrete collar beams could be introduced to provide the bracing
effect. Assuming that the roof sheathing boards and the ceiling drywall panels lock adjacent
rafters together, the spacing of these collar beams could be at every other rafter, which is roughly
48" on center. They could be exposed 3x4 dimension lumber, in any species and grade that is
aesthetically satisfactory, or two plies of dimensional 2x4's that are wrapped with wood trim or a
drywall finish. Connection details would depend oft the chosen beam material, but should
include positive mechanical fastening, such as with lag screws or TimberLOK's.
I hope that you find this satisfactory. Please let me know if you have any questions.
Respectfully,
Ryan S. Hellwig
Massachusetts Professional Engineer #37300 - STRUCTURAL
Page 2 of 2
r
• RYAN S. HELLWIG, PE • STRUCTURAL ENGINEER •
November 27, 2009
Dean Acheson
Acheson Company �.
6 North Main Street
Williamsburg, MA 01096
1+ c is" ;' l, : 3 1 ?)ncom
Re: Existing Roof Structure
35 -37 Orchard Street
Northampton, MA
The following is a summary report of my review of the above - mentioned structure. On
November 23, 2009 I personally conducted an inspection of the attic in this building. The
following conditions were noted:
The roof is a conventional gable shape with a gable dormer on the south side. It is approximately
22 feet wide, and has a slope of about 35 degrees. The roofing is slate, and the sheathing consists
of wood boards with 1" nominal thickness. The rafters are full dimension softwood lumber 2x4
spaced at 24 inches on center. The rafter bottoms bear on the attic floor. The age of original
construction is assumed to be approximately 100 years.
Ceiling joists in the attic were removed in the front and back rooms of the attic as part of the
current renovation work. These joists were left in place in the center section at the dormer. Also
as part of this work, new flatwise dimensional 2x4's were installed parallel with the rafters,
nailed and screwed to the undersides of the rafters. There are existing kneewalls crossing under
the rafters, located between two feet (back room) and four feet (front room) from the eaves.
Per the Massachusetts State Building Code, for a mixed -use building (not a 1 +2 family
residence) in Northampton, the ground snow load, p is 55 psf. Because the rafter bays are
vented, the roof temperature factor, C is considered to 1.1; for this site, the exposure factor, C
is 1.0; with a slope of 35 degrees, the slope factor, C is taken as 0.6 for a slippery, unobstructed
roof (i.e. no snow guards). This results in a sloped roof snow load, p of almost 26 psf. If snow
guards or other obstructions are present, then p would be more than 42 psf. Also per Code,
unbalanced snow loads must be taken into account, because wind effects on gable roofs during a
snowstorm often result in snow loads greater than p on the leeward side, and less on the
windward side. If this were for new construction of a 1 +2 family residence, the Code would
require these rafters to be 2x10. For commercial construction, the 2x10's would have to be at 16"
on center spacing.
Sloped rafters such as these require horizontal restraint to tie the bottoms of the rafters from
sliding outward. The force in the tie is proportional to the height of the tie, as well as the amount
• 28 ALDRICH STREET • NORTHAMPTON, MA 01060 •
• VOICE 413 - 584 -HLWG (4594) • FAX 4I3- 584 - HLWFax (4593) •