How to Install Joists for a Flat Roof

The roofing surface is installed over the joists, while insulation is placed between them, and the ceiling beneath is suspended from them. To build a roof joist for a flat roof, you need some carpentry skills and a few basic tools.

You Will Need:

Tape measure

Pencil

Steel joist hangers

Power drill

Screw gun

Wood screws

2-by-4 inch lumber

Chalk line

Furring strips

Work gloves

Eye goggles

The Job:

Mark the installation location for each joist along the wall plate. Start at one end of the wall so that the first joist is flush with the walls which run parallel to the joists. Mark off a joist every 16 inches along the length of the wall, with the last joist lining up against the opposite verge wall.

Install steel joist hangers at each mark. Place the hanger into position so that it is centered over your pencil mark, and use your power drill to make pilot holes into the wall plate through the hanger screw holes. Drive wood screws into each pilot hole to fasten the hanger in place. Repeat the process at each pencil mark along both wall plates.

Measure and cut 2-by-4 joists to span the distance between each set of joist hangers. Use your power drill to make pilot holes through the hangers and into the joist. Drive screws into each pilot hole to fasten the joists in place. Repeat the process at both ends of each joist.

Install braces or blocking between each set of joists for extra support and to prevent movement. Measure and mark the joists at each end of the roof about 4 feet in from the edge. Lay your chalk line across your two pencil marks, and snap a line across all of the joists in between. Measure and cut 2-by-4 inch lumber to fit the gaps between each set of joists.

Install one cut 2-by-4 inch brace between the first and second joists, so that it lines up on the left edge of the chalk line. Use your power drill to make pilot holes through the joist and into the end of the brace. Use your screw gun and wood screws to fasten the brace in place. Line up the next brace on the right edge of the chalk line, and repeat the installation process.

Repeat the process between every set of joists. Make a set of marks 4 feet further from the edge than the first marks and repeat. Repeat again 4 feet past that and continue until the roof structure has been completely braced.

Install furring strips to the top of each joist to create a surface onto which the roof decking can be installed.

Idea:

Slope your flat roof surface about 1/8 inch every 12 inches to avoid standing water and future leaks.

Safety:

Use caution when working with dangerous tools like saws, hammers and nails. Serious injury can occur. Wear work gloves and eye goggles for protection.

SOURCES:

This Old House: Building a Flat Roof Right

http://www.thisoldhouse.com/toh/article/0,,1127447,00.html

This Old House: The Four Kinds of Flat Roofs

http://www.thisoldhouse.com/toh/article/0,,1110914,00.html

Realtor: Roll Roofing: A DIY Installation Guide

http://www.realtor.com/home-garden/do-it-yourself/roofing/installing-roll-roofing.aspx?source=web

Medford Truss: Roof Truss Span Tables

http://medfordtruss.com/RoofSpan.pdf

Worth Design: Construction of a Flat Roof

http://www.worthdesign.co.uk/crc/vle%20project/Assignment%203/firring%20peices.pdf

City College Coventry: Build Flat Roof