BUILDING BOG BRIDGES

The Trails Committee of the Narragansett Chapter of the A.M.C. has developed an efficient method of spanning wet areas on hiking trails. As policy restrictions evolve, there is an aversion to felling trees in the State forests, e.g. Arcadia, to use as bridging material. It should also be noted that native species tend to decay over a relatively short time requiring frequent replacement. Currently we are using pressure treated lumber (PT) as our first choice. We have been made aware that due to the Arsenic content of the preservative, regulations will soon be amended to ban its use and a new product will soon be available. When the regulations change we will adapt the new materials.

The attached description and accompanying photos explain the sequence of activities required to do the job.

Ordering the stock in the appropriate lengths (normally twelve feet) minimizes waste and facilitates the work of moving the material to the work sites. When possible the decking is pre-cut into the appropriate lengths (36 OR 30 inches), and the sleepers into four-foot pieces. The Trails Committee has recently acquired a portable generator so much of the cutting can be done at the work site if necessary.

If the wet areas to be spanned exceed twelve feet, multiple units are connected as required. We have found that twelve foot lengths are ideal. Longer pieces are unwieldy, especially when carrying on narrow, winding trails.

For certain special situations, e.g., where a stream is spanned and the bridge is elevated above the stream, the span length may exceed 14 feet and it is not possible or practical to build a center support. For such unsupported bridges exceeding 14 feet in length, a special design is used to add strength to the stringers so that the bridge can carry hiker/biker loads without having a center support. For example, if an 18 foot span is required, the design is detailed in the section titled “Boardwalk Construction Sequence (18 Foot Type)”.

GLOSSARY and MATERIALS LIST

Sleeper: A four-foot long 4" x 6" timber installed on the ground across the treadway, dug in as necessary to level it. 2 pieces required.

Stringer:
12 Foot boardwalks, Types 1 & 2: 4" x 6" timber, 12 feet long, spiked to the sleepers, 2 pieces required.
18 Foot boardwalk: Field built using (2) 2" x 10" timbers 12 Feet long, AND (2) 2" x 10" timbers 6 feet long. Individual lengths are glued and nailed together to yield the equivalent of a single 4” x 10” timber. 2 pieces required.

Decking:
Type 1: Pieces of 2" x 6" pre-cut, three foot long, installed 1/4 inch apart.
Type 2: Pieces of 2" x 6" pre-cut, 30 inches long, installed 1/4 inch apart. Decking will be nailed to topside of stringers.
26 pieces required for 12 foot boardwalks.
40 pieces required for 18 foot boardwalks

Spikes: 4 to hold stringers to sleepers - each should be about 10 inches long

Lag Scresw: ˝ inch diameter, 6 inches long – used to secure stringers to stringer attachment pieces. 4 required, plus 4 flat washers.

Drill & bit: (hand brace or rechargeable drill): 3/8 inch.

Nails: 16 penny (16D – about 100 for 12 foot boardwalks and about 200 for the specially designed 18 foot boardwalks) - Enough nails to nail decking to stringers, 2 each side of decking pieces, and for the 18 foot boardwalks to additionally nail side by side stringer sections together.

Construction Adhesive (for the 18 foot special design boardwalk) – two tubes needed

TOOL LIST

The following items are needed for construction of any size of boardwalk:

The following additional tools are required for the special design 18 foot boardwalk:


BOARDWALK CONSTRUCTION SEQUENCE (12 Foot, Type 1):

  1. Determine locations for the two 4 ft. sleepers; with spacing determined by overall length of boardwalk and the optimum ground location that is relatively smooth and level and not having major restrictions such as large roots:
    1. For 12 foot boardwalks (the most common size we construct) - sleeper spacing should be approximately 9 feet - to provide an approximate 18 inch overhang of each stringer on each sleeper, with the stringers centered on the sleepers. The overhang can vary from approximately 12 to 24 inches, depending on the optimum placement of the sleepers, based on ground conditions.
  2. Place each sleeper perpendicular to the tread way, with the 6 inch portion of the 4 by 6 resting on the ground. Check that the sleeper is level (use a spirit level). If necessary, grade the soil, re-measuring, until the sleeper is level.
  3. Measure 9 inches in from each end of each sleeper and mark it. (This is to prepare for placement of the stringers on the sleeper, such that the stringers will be properly spaced for three foot wide decking overhanging the two stringers by three inches on each stringer.)
  4. Place the outside edge of each stringer on the 9 inch marking line per above with the 4 inch dimension of the 4 by 6 stringer resting on sleeper.
  5. Using a 3/8 inch bit, drill each stringer at its center point over each sleeper vertically all the way through the stringer only (to prepare for placement of a 10 inch spike).
  6. Using a sledge hammer, pound each of the four spikes into their respective stringers and sleepers. The heads of each of the spikes should be flush with the top of the stringer.
  7. Using 16 penny galvanized common nails, nail one piece of decking across the stringer on one end of the boardwalk. Lightly tack another piece of decking likewise to the stringers on the far end of the boardwalk (The last decking piece may have to be moved into final adjustment when all the other pieces of decking are nailed). Decking is laid perpendicular to and centered on stringers and will have about three inches of overhang on each end. Decking should be placed grain side down for purposes of better long-term wear.
  8. Drive a nail partway into one end of the first & last decking pieces on the same side of the boardwalk. Stretch a string between these nails to act as a guide for the alignment of subsequent decking pieces.
  9. Nail each subsequent piece of decking, allowing about a 1/4 inch space between adjacent pieces of decking. This may be accomplished by using a loose nail between decking pieces.
  10. If multiple sections of boardwalk are being installed in a continuous line, without spacing, then the successive sleepers must be set at a proper height (depth) such that each section of boardwalk is in the same plane.

BOARDWALK CONSTRUCTION SEQUENCE (12 Foot, Type 2):

  1. Determine locations for the two 4 ft. sleepers; with spacing determined by overall length of boardwalk and the optimum ground location that is relatively smooth and level and not having major restrictions such as large roots:
    1. For 12 foot boardwalks (the most common size we construct) - sleeper spacing should be approximately 9 feet - to provide an approximate 18 inch overhang of each stringer on each sleeper, with the stringers centered on the sleepers. The overhang can vary from approximately 12 to 24 inches, depending on the optimum placement of the sleepers, based on ground conditions.
  2. Place each sleeper perpendicular to the tread way, with the 6 inch portion of the 4 by 6 resting on the ground. Check that the sleeper is level (use a spirit level). If necessary, grade the soil, re-measuring, until the sleeper is level.
  3. Measure 11 inches in from each end of each sleeper and mark it. (This is to prepare for placement of the stringers on the sleeper, such that the stringers will be properly spaced for 30 inch wide decking overhanging the two stringers by two inches on each stringer.)
  4. Place the outside edge of each stringer on the 11 inch marking line per above with the 4 inch dimension of the 4 by 6 stringer resting on sleeper.
  5. Using a 3/8 inch bit, drill each stringer at its center point over each sleeper vertically all the way through the stringer only (to prepare for placement of a 10 inch spike).
  6. Using a sledge hammer, pound each of the four spikes into their respective stringers and sleepers. The heads of each of the spikes should be flush with the top of the stringer.
  7. Using 16 penny galvanized common nails, nail one piece of decking across the stringer on one end of the boardwalk. Lightly tack another piece of decking likewise to the stringers on the far end of the boardwalk (The last decking piece may have to be moved into final adjustment when all the other pieces of decking are nailed). Decking is laid perpendicular to and centered on stringers and will have about two inches of overhang on each end. Decking should be placed grain side down for purposes of better long-term wear.
  8. Drive a nail partway into one end of the first & last decking pieces on the same side of the boardwalk. Stretch a string between these nails to act as a guide for the alignment of subsequent decking pieces.
  9. Nail each subsequent piece of decking, allowing about a 1/4 inch space between adjacent pieces of decking. This may be accomplished by using a loose nail between decking pieces.
  10. If multiple sections of boardwalk are being installed in a continuous line, without spacing, then the successive sleepers must be set at a proper height (depth) such that each section of boardwalk is in the same plane.

BOARDWALK CONSTRUCTION SEQUENCE (18 Foot Type):

  1. Determine locations for the two 4 ft. sleepers; with spacing determined by overall length of boardwalk and the optimum ground location that is relatively smooth and level and not having major restrictions such as large roots:
    1. Sleeper spacing should be approximately 15 feet - to provide an approximate 18 inch overhang of each stringer on each sleeper, with the stringers centered on the sleepers. The overhang can vary from approximately 12 to 24 inches, depending on the optimum placement of the sleepers, based on ground conditions.
  2. Place each sleeper perpendicular to the tread way, with the 6 inch portion of the 4 by 6 resting on the ground. Check that the sleeper is level (use a spirit level). If necessary, grade the soil, re-measuring, until the sleeper is level.
  3. Center the 20 inch long 6 x 6 (stringer attachment piece) on top of  the sleeper at each end of the bridge and then using a 3/8 inch bit, drill vertically all the way through each 20 inch piece in two places (centered 5 inches from each end of the 20 piece) to prepare for placement of spikes.  Then using a sledge hammer, pound two 10 inch spikes per each 20 inch piece, to attach the 20 inch piece to the sleeper.
  4. Construction of each stringer:   A stringer consists of four 2 x 10’s glued and nailed together (12 footer butted to a six footer side by side with a 6 footer butted to a 12 footer, such that the inside ends of the 2 x 10’s are overlapped).  Since lumber is rarely dead straight, sight along each of the four 2x 10’s held on edge and “crown” the pieces so that the convex edge of each of the four pieces is installed up.   Prior to gluing, prepare the stock for nailing by aligning the four pieces and snapping two chalk lines on each outside face of the intended stringer , each one inch in from the top and bottom of each butted 12 foot and 6 foot piece.  Thus, snap four chalk lines total.  To prepare for gluing, using a caulking gun containing a cartridge of construction adhesive, run a continuous bead of adhesive about one inch in from each of the four edges on the inside face of one 12 footer and one  6 footer.  Also zigzag a bead down the center lengthwise on the inside face of the same 12 footer and 6 footer.  Then glue the four 2x10’s together, taking care that the edges of the side by side pieces are in alignment with each other.  A bar clamp may be needed at the end of the pieces to achieve alignment.  To nail, on the upper chalk line, insert the first 16d nail 2 inches in from the end of the board and then insert nails at approximately every 24 inch intervals.  On the lower chalk line, insert a 16d nail 2 inches from the end, the next nail at approximately a 12 inch interval and then the succeeding nails at approximately 24 inch intervals, such that the nails on the top and bottom are staggered. The nails should be driven straight in and then bent over to lie flat against the opposite face in the direction of the wood grain  Similarly, on the opposite outside faces, perform nailing in a similar manner on the upper and lower chalk lines, except that the nailing pattern for the upper chalk line is the same as the nailing pattern for the lower chalk line of the opposite outside face and the nailing pattern for the lower chalk line is the same as the nailing pattern for the upper chalk line of the opposite outside face.  This guarantees that nails are staggered on opposite outside faces. 
  5. Placement of the two stringers:   Place them on edge and lay them across the bridge sleepers, with the 10 inch faces resting against the outside edges of the 20 inch 6 x 6 attachment pieces.  Then square these stringers, by adjusting one or both of their positions on the sleepers such that diagonal distances from the ends of each stringer to the opposite ends of the other stringer are equal. 
  6. Attachment of the two stringers:  To prepare to attach the stringers to the 20 inch 6 x 6 attachment piece at each end of the bridge, drill a horizontal 3/8 inch pilot hole through each stringer and three inches into the 20 inch 6 x 6 at the four corners of the bridge.  Then with two ˝ inch diameter by 6 inch long lag screws/washers per stringer (four screws and four washers total for the entire bridge) and an appropriate wrench, attach each stringer to the 20 inch 6 x 6, at each end of the bridge.
  7. Using 16 penny galvanized common nails, nail one piece of decking across the stringer on one end of the boardwalk.
    1. If decking will be installed starting from only one end and working to the other end, then lightly tack another piece of decking likewise to the stringers on the far end of the boardwalk (the last decking piece may have to be moved into final adjustment when all the other pieces of decking are nailed).
    2. If decking will be installed working from both ends towards the center, then nail the other piece of decking likewise to the stringers on the far end of the boardwalk When all decking has been installed, except for the last four or five pieces in the center, position these last four or five pieces accordingly, to give proper spacing, prior to nailing them.
    3. Decking is laid perpendicular to and centered on the stringers and will have about two inches of overhang on each end (when using 30 inch long decking).  Decking should be placed grain side down for purposes of better long-term wear.
  8. Drive a nail partway into one end of the first & last decking pieces on the same side of the boardwalk. Stretch a string between these nails to act as a guide for the alignment of subsequent decking pieces.
  9. Nail each subsequent piece of decking, allowing about a 1/4 inch space between adjacent pieces of decking. This may be accomplished by using a loose nail between decking pieces.
  10. If multiple sections of boardwalk are being installed in a continuous line, without spacing, then the successive sleepers must be set at a proper height (depth) such that each section of boardwalk is in the same plane.
Prepared by: Stephen F. Imms August 2007

View Photographs of the Construction of a Boardwalk

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