Decking is the standard way to provide a flat surface for laying roofing shingles, tiles, etc. Specifically, OSB and plywood provide a stable and strong surface that lasts many years with few repairs. Roofing felt underlayment over the deck completes the set for decades of a reliable roof over your head.
But did you know you can also turn to wood strips to accomplish the same? Roofing strips are less strong and durable than solid sheathing materials. However, they’re perfect for certain roofing and ceiling projects.
Read on to learn how they work, common roofing applications, and standard furring strip spacing requirements for metal roofs.
What’s the Required Spacing of Furring Strips on Metal Roofing?
The standard furring stip spacing for metal roofs is twenty-four inches on-center. However, you should consider closer spacing for thinner furring strips. The strip material also matters.
For instance, metallic furring strips are stronger and thus can be placed closer. Arrange the strips downward to encourage moisture to run down the end caps.
What are Furring Strips?
Furring strips, also known as wood strapping or furring boards, are thin metal or wood strips used to prepare level surfaces for various projects. Some contractors also use furring strips to raise surfaces to prevent moisture creep.
If you’ve used common boards before, furring strips are similar. The main difference is that common boards are softwoods, typically cypress. Meanwhile, furring strips come in various materials, especially metal and wood.
Types of Furring Strips
Furring strips can be categorized by material and style. The two main types of furring strip materials are wood and metal.
The most common styles are hat-channel and z-channel. Here’s what you need to know about each;
Wood vs. Metal Strapping
Wooden furring strips are made from natural or processed wood. Original strips from millers are sawn from solid timber, typically softwoods like spruce and pine. Alternatively, contractors rip plywood to form furring strips on-site.
Meanwhile, metal furring strips are mainly constructed in factories from aluminum or steel metal sheets. Many manufacturers are happy to customize the straps to suit your project needs.
Hat Channel vs. Z-Channel
Hat-channel furring strips resemble hats, thus the name. They’re the most common metal furring strip design and are popular in real estate development. Hat-channel strips are heat-resistant, providing solar heat protection.
On the other hand, z-channel furring strips take the shape of the alphabet Z. They are versatile construction materials most commonly used in construction projects.
Applications of Furring Strips in Roofing
Furring strips have a host of applications in the construction industry. However, the three main uses in roofing projects are as follows;
Installing a Metal Roof Over an Existing Roof
Furring strips are commonly used when installing a layover (a new metal or asphalt shingles over an old roof). The strips serve as spacers between the old and new roofs, providing additional insulation and ventilation while forming a level surface for the new metal panels.
In addition, it provides soundproofing. The gap between the two roofs cancels sound from outdoors, keeping your indoors quiet and comfortable.
In this case, the firring strips replace the decking. So, you need wider planks, like 1×4 strips. Pre-drill a hole into all four corners of each furring strip. Then position the stips on the roof and drive nails into the pre-drilled holes to secure the strips.
After installing the firring, set and attach the underlayment or slip sheet, then install the roofing panels.
Installing Roof Insulation
Furring strips are popular for holding rigid insulation materials in place, especially on masonry walls. However, you can also use it to hold insulation on the roof.
Different manufacturers provide different insulation installation guidelines. However, in most cases, you first install 1×2 furring strips to the bottom and perpendicular to the Z purlins on 22″ centers.
Then, use 1 1/4-inch self-taping sheet metal screws to secure the strips. Butt the furring strips together, tape them, and staple the tape for additional reinforcement.
From there, you can staple the insulation product at 4-inch intervals to the furring strips using 3/4-inch tabs, then tape the seams for an air-tight seal.
Furring for Ceilings
Finally, you can use furring strips to support the ceiling. In this case, the main advantage is cost savings. The thin furring strips are much cheaper than traditional framing. Moreover, they’re easier to install.
In addition, furring provides a stronger framework for fastening the ceiling. A strongly attached ceiling is critical for durability and function. For instance, a strong ceiling can support more lights and ceiling fans. It’s also less likely to sag as it ages.
Above all, furring strips provide surfaces on which to fasten electrical wiring. Most wires follow the main framing members. However, furring provides more wiring routes for more complex wiring projects.
Ceiling furring strips are nailed to the main framing members, such as joists. Again, follow the manufacturer’s guidelines for the best results.
Do you need to Put Wood Strapping Over a Metal Roof?
Whether you need wood strapping over a metal roof depends on many factors. Indeed, some roofs must be installed with furring, whereas others must NEVER be installed with the stripping. Meanwhile, others may or may not be installed with furring strips.
Our advice is to first check with your local code. Does it mandate furring straps? If so, install them. However, avoid straps if the code says NO.
But if the code is silent on the topic, we recommend installing single wood battens as it improves roofing convenience. In addition, it creates a vital gap between the roof sheathing and metal roofing panels for greater ventilation.
Why Put Furring Strips Under a Metal Roof?
We’ve discovered that furring strips are optional in most metal roofing applications. So you can avoid them, and the roof inspector will have no issues.
So, why install them? The following are three reasons why we recommend wood strips for metal roofs;
1. Provide a Level Surface for Roof Installation
This is especially true when installing a metal roof over an existing roof or if the decking needs to be more balanced. Furring strips provide a level surface for roof installation to ensure a structurally sound roof.
Consider a metal-over-metal re-roofing project, for instance. The old shingles may be worn and torn in some sections. Also, some sections may be raised due to patches and surface debris. Furring is the best way to overcome surface imperfections for a uniform new roof layer.
2. Provide Critical Ventilation Space
Shingle and metal roofs are the hottest roofing materials. Indeed, unpainted metal roof panels can reach 145 degrees in the summer.
Fortunately, most homes have air conditioning systems for indoor air quality. But two issues arise – higher energy bills and potential roof damage.
Furring creates pockets of space between the roofing panels and the deck beneath for increased airflow, reducing heat transfer to the rooms below. Also, better ventilation can prolong the life of the roofing panels.
3. Minimize Condensation
Improved ventilation naturally eliminates or minimizes roof condensation problems. To understand this, we need first to understand how roof condensation forms.
Condensation is essentially frozen moisture from everyday air. The moisture freezes at the dew point, typically at night.
However, poor ventilation is a key contributing factor. The moisture is trapped under the roof because it has nowhere to go. Furring eases the problem by providing exit routes for the moisture, eliminating condensation.
4. It’s More Practical than Cross-Battening
Cross battens are the alternative to furring strips, especially when installing a second layer of shingles or metal panels over existing shingles. They comprise two sets of wood planks.
The first set of battens run vertically, attaching to the roof deck and, ideally, the purlins. Then the second set of battens is attached horizontally.
Cross-battens provide excellent ventilation and soundproofing. They also minimize ice damming. However, they’re impractical in many applications because they raise the roof significantly.
For instance, a new roof height often means reinstalling the gutters and the fascia. Otherwise, you’re left with a large gap between the new shingles and the fascia board.
In addition, you may need to adjust the ridge vent, skylights, chimneys, dormer, and other roof features.
5. It’s Cost-Effective
The benefits of installing furring strips far outweigh the cost. For instance, installing furring on a standard roof only costs $2.46 to $2.94 per square foot. However, it can save you hundreds of dollars years in air conditioning costs.
More importantly, consider the impact on the roof’s life. Without furring, roofs trap tons of heat and moisture every year, increasing the rate of sheathing and roof panel damage.
Thus, you need to repair the roof more often and replace the decking and panels sooner. You can avoid these issues by installing furring strips.
Furring Strip Sizes: What Size Furring Strips should I Use?
Standard furring strips are one inch thick, though the actual thickness is just under an inch. Two-inch thick strips are available but are rare. The width varies depending on the application. However, the standard length is 8.0 feet.
The following is a summary of the most common sizes (thickness x width x length);
- 5/4-inch x 3-inch x 8-foot
- 1-inch x 2-inch x 8-foot
- 1-inch x 3-inch x 8-foot
- 1-inch x 4-inch x 8-foot
- 2-inch x 2-inch x 8-foot
- 2-inch x 4-inch x 8-foot
So, what size furring strips should you use for a metal roofing project? It depends on many factors.
For instance, use lighter strips for lighter metal roofing material and vice versa. Thus, 2x4s make sense for heavy-duty 26-gauge steel roofs. Meanwhile, 2x2s are ideal for 32-gauge aluminum roofs. Putting too much weight on lightweight furring strips can cause buckling.
Additionally, consider the spacing. If you wish to space the strips further apart, consider thicker planks. Otherwise, you end with a weak furring structure.
Furring Spacing Requirements: How Far Apart do you Put Furring Strips?
The best spacing for furring strips when installing a metal roof is 24 inches on-center. However, the exact spacing varies, depending on the sizes of the furring strips, the metal gauge (thickness), and the roof load, among other factors.
Nevertheless, the standard range is 4.0 inches to 4.0 feet (48 inches) between furring strips. Lightweight aluminum strips require the smallest spacing, typically 4.0 inches on-center, due to a low load-bearing capacity. Meanwhile, heavy gauge steel furring strips can be spaced as much as 48 inches apart on-center without any issues.
The maximum allowable gap between furring strips depends on the roof’s ability to resist buckling under live and dead load. Live loads are temporary weight factors, like snow, wind, or a roofer making repairs on the roof. Meanwhile, the dead load is permanent, like the weight of the shingles.
Also, note that furring spacing is measured on-center and not from the edge of one strip to the edge of the next. Thus it’s as much about screw spacing as furring strip spacing.
For this reason, you must also consider whether the screws go into the sheathing only or through the decking into the purlins. For instance, the pullout resistance is very low if the screws only penetrate the sheathing compared to screws that reach the underlying purlins.
Finally, you must consider the individual size of each furring strip. Most strips are 1×2 or 1×4 inches. Unfortunately, 1×2 strips split too easily.
Moreover, they provide low pullout resistance. Therefore, you must space them closer. On the other hand, 1×4-inch strips perform much better and thus can be spaced further apart.
However, even 1×4 strips cannot withstand repeated tornados, high winds, and severe storms. Therefore, you should consider thicker strips if you live in an area prone to extreme weather. In this case, 2x4s make the most sense.
How to Install Furring Strips for New Metal Roof Over Asphalt Shingle Applications
Installing furring strips on a metal roof is easy once you have the right strips. Here’s a step-by-step guide to follow;
- Inspect the old shingles: How’s the old roof? Do you have a healthy rood deck? Can you spot roof penetrations? If so, repair the damaged shingles and deck section before installing the new metal roof.
- Prepare the boards: Cut the strips to size and pre-drill holes for easy installation. Then treat the boards if you wish.
- Mark rafters or truss chords using chalk lines: It’s best to nail the strips through the decking into the trusses or rafters for the strongest hold.
- Install the furring boards: Lay out the strips and screw them to the surface using drills. Alternatively, drive nails through the pre-drilled holes using a nail gun.
Can You Use Metal Roofing Directly Over Shingles Without Furring Strips?
Yes. You can put metal roofing directly over the existing roof without furring strips. However, we strongly discourage it if you want a quality metal roof. Doing so increases heating, moisture, and noise issues. Moreover, installing a metal roof on a bumpy, uneven surface can cause faster roof wear.
Does a Metal-Over-Shingle Roof Structure Need a Moisture Barrier?
Yes, all roofs, including metal-over-shingle structures, require a moisture barrier. Otherwise, you’re left with a barrage of problems, including trapped moisture, condensation issues, and roof leaks. In addition, moisture damage accelerates shingle and metal deterioration. It also attracts pests, mold, and mildew.
FAQs
Do You Need Furring Strips for Metal Roof?
Yes. You need furring strips for metal roofs. It’s optional in many states. However, furring is critical for optimal ventilation and a structurally sound roof. It also provides a flat, even surface for roof installation. Thus, installing a metal roof without furring strips is inadvisable.
Can I Use Plywood for Furring Strips?
Yes, you can make furring strips from plywood. Many roofers use 1/4-inch and 1/2-inch screws as they’re flexible but sufficiently strong for furring applications. However, consider 3/4-inch pine plywood sheets if purchasing the sheets purposely for furring strips. The only challenge is that plywood doesn’t hold screws as strongly as solid wood.
What are the Best Furring Strips for Corrugated Metal Roofs?
Wooden strips are the best furring strips for corrugated metal roofs. They’re affordable, eco-friendly, and easy to work with. Moreover, wooden strips can last many years with good maintenance. Meanwhile, metal furring works best for drywall insulation. Speak with your roofer if you’re concerned.
Can I Use Furring Strips Outside?
Yes, you can use certain types of furring strips outside. The best choices are redwood and pressure-treated lumber. However, be warned that treated wood contains chemicals. So, you must be careful where you use it. This leaves redwood as the best chemical-free furring material for outdoor use.
Summary
Most furring strips are spaced 24 inches on-center in metal roofing projects. However, the actual spacing varies depending on multiple factors, including the size of the strips, strip material, and type of roofing project.
Hopefully, you’ve learned enough from this guide to choose the right furring spaces for your next metal roofing project.