A roof jack is a temporary metal support that holds up roof sheathing or decking while a roofer works underneath or replaces structural beams
A roof jack is a hydraulic or mechanical post that a roofer places under the roof from inside the attic or crawl space. It transfers the weight of the roof to the floor or foundation below, so the roofer can safely remove, repair, or replace the beams and trusses that hold the roof up. Without a roof jack, removing a structural beam would cause the roof to sag or collapse onto the work area.
Roof jacks are not permanent fixtures. They stay in place only as long as the repair work takes — usually a few days to a few weeks. Once the new beam is installed and secured, the jack is removed and the roof rests on its new support.
The two main types are hydraulic jacks, which use fluid pressure and can be adjusted smoothly, and mechanical screw jacks, which use a threaded rod turned by a handle. Hydraulic jacks are faster to raise and lower; screw jacks are slower but hold their position without power.
Key Takeaways
- A roof jack temporarily supports the roof's weight while a roofer works on or replaces the beams and trusses underneath.
- Hydraulic jacks raise and lower quickly with a pump handle; screw jacks use a threaded rod and are slower but more stable.
- Roofers place jacks from inside the attic or crawl space, positioning them on a solid base like a floor joist or foundation beam.
- The jack must be removed once the new structural support is installed and the roof is resting safely on it.
How a roof jack is positioned and what it rests on
A roofer positions a roof jack directly under the area where work is happening, usually in the attic or crawl space. The base of the jack sits on a solid, level surface — typically a floor joist, a beam, or the foundation itself. If the base is not solid, the jack can sink or tip, which defeats its purpose and creates a safety hazard.
The top of the jack has a plate or saddle that contacts the roof sheathing or the beam being supported. The roofer adjusts the height slowly and carefully, raising the roof just enough to take pressure off the damaged beam. Raising it too much can crack the roof or damage walls and ceilings below.
Most roof jacks have a maximum load capacity — often between 10,000 and 30,000 pounds, depending on the model. A roofer calculates how many jacks are needed by dividing the weight of the roof section by the capacity of each jack, then adds extra jacks for safety.
Hydraulic versus screw jacks: the difference in operation
A hydraulic roof jack uses a hand pump connected to a cylinder filled with hydraulic fluid. Pumping the handle pressurizes the fluid, which pushes a piston upward. The roofer can raise or lower the jack smoothly and stop at any height. Hydraulic jacks are faster and require less physical effort, which matters when you are working in a cramped attic.
A screw jack has a threaded rod running through the center. The roofer turns a handle or crank at the top, which rotates the rod and raises the lifting plate. Lowering it means turning the handle in the opposite direction. Screw jacks move more slowly and require more turning, but they hold their position without any power source and do not leak.
Roofers often choose hydraulic jacks for speed when the job is straightforward, and screw jacks when they need the jack to stay in one place for hours without adjustment or when working in areas where a hydraulic leak could damage insulation or finishes.
Why roofers use roof jacks instead of removing the beam outright
Removing a structural beam without support would cause the roof to drop suddenly. This can crack drywall, break windows, damage plumbing and electrical lines, and injure anyone in the space below. A roof jack prevents that by holding the roof at its normal height while the old beam is cut away and a new one is installed.
The jack also protects the roofer. Working under a roof that is sagging or unstable is dangerous. The jack keeps the workspace level and predictable, so the roofer can focus on the repair without worrying about the roof shifting.
In some cases, a roofer will use multiple jacks across the repair area, spacing them evenly so the weight is distributed and no single point bears too much load. This is especially common when replacing a long beam or when the roof has already sagged from age or damage.
When a roof jack is needed versus when it is not
A roof jack is necessary whenever a roofer needs to remove or significantly weaken a structural beam — the main posts, trusses, or collar ties that hold the roof up. It is also used when replacing a rotted or cracked beam, or when installing a new opening like a skylight that requires cutting into the framing.
A roof jack is not needed for work that does not touch the structure, such as replacing shingles, flashing, or gutters. It is also not needed if the roofer is only inspecting the framing or making minor repairs that do not remove any load-bearing wood.
The roofer or structural engineer decides whether a jack is required based on what work is being done. If there is any doubt, the safer choice is to use one.
What happens if a roof jack fails or is not used when needed
If a roof jack fails — because it was overloaded, positioned on an unstable base, or had a mechanical defect — the roof can drop suddenly. This can cause serious injury to anyone below, crack the structure, and damage the interior of the home. A failed jack is a structural emergency.
If a roofer removes a beam without using a jack, the roof will sag or collapse. This is not only dangerous but also expensive to repair, because the damage spreads to walls, ceilings, and the interior. Insurance may not cover the damage if the roofer did not follow safe practices.
For this reason, using a roof jack is not optional when structural work is being done. It is a required safety step, and a licensed roofer will never skip it.
Frequently Asked Questions
How long does a roof jack stay in place?
A roof jack stays in place only while the structural repair is happening — usually a few days to a few weeks. Once the new beam is installed, secured, and the roof is resting safely on it, the jack is removed. Leaving a jack in place longer than necessary can cause the wood it contacts to compress or weaken.
Can a homeowner rent or buy a roof jack?
Yes, roof jacks are available from equipment rental companies and home improvement suppliers. However, using one requires understanding load capacity, proper positioning, and safety procedures. Most homeowners hire a licensed roofer or structural contractor to do the work, because improper use can cause serious injury or structural damage.
What is the difference between a roof jack and a house jack?
A roof jack is designed for work inside the attic or crawl space and has a smaller footprint. A house jack (or foundation jack) is larger and is used to lift an entire house or section of it for foundation repair. House jacks are heavier and have higher load capacities, but both work on the same hydraulic or screw principle.
Do I need a permit to use a roof jack?
Most structural repairs that require a roof jack also require a building permit. The permit ensures the work is inspected and meets code. Your local building department can tell you whether your specific repair needs a permit and whether a licensed contractor must do the work.