Upgrading from shingle roofing to sleek and modern metal roofing can be labor-intensive and costly, so some consider just putting a metal roof over old shingles to cut back costs and labor hours.
Most roofers would tell you that the best method to do this is by adding furring strips over your existing shingles and under your new metal roof. But what exactly are furring strips, and why are they so important when installing a metal roof?
In this guide, we’ll discuss everything you need to know about furring strips, from why they should be added to your roof to what kind of wood you should use. We’ll also share a step-by-step guide on how to install furring strips for those who want to DIY.
What are furring strips?
Furring strips, also called wood stripping, are thin strips of wood used to create a framework on the roof or other construction projects, like insulation installation or soundproofing.
They’re usually installed parallel or perpendicular to the eaves or the edge of the roof. They are mostly used to create a level surface when attaching new roof materials, like installing a metal roof over shingles.
Should I add furring strips over existing shingles if I want to install a metal roof?
It’s recommended to install furring strips if you plan on installing a metal roof, although it’s not always necessary. The strips provide an even base for the metal panels and prevent condensation, which can compromise the roof’s quality.
Some local authorities require furring strips to be over the shingles before installing metal roofs, so make sure to check your local building codes before going on with the project.
Some types of metal roofs, like standing seam metal roofs and metal tile roofs, also require furring strips before they can be installed. Always check the metal roof’s manufacturer’s instructions to make sure you’re doing the project right.
Benefits of Adding Furring Strips Under Metal Roofs
Adding furring strips to the roof can offer several benefits. Hence, many roofers and handymen advise adding them before installing a metal roof.
Here are some potential benefits that you can get from adding furring strips over your existing shingles before installing a metal roof:
- Improved Ventilation
Furring strips create a gap between the metal roof and the current shingles, which allows air to circulate better. This prevents moisture buildup and reduces the risk of mold growing under the roof.
- Condensation Control
Better ventilation and adequate airflow helps control condensation on the underside of the metal roof. This is particularly beneficial for those who live in areas with varying temperatures as they’re more at risk of condensation.
- Thermal Break
Furring strips serve as thermal breaks that create a barrier between the roof and the shingles.
The barrier reduces the heat transfer between the two roofing materials, preventing excessive heat absorption in warm weather or heat loss in cold weather.
- Reduced Noise
Furring strips give another level of sound absorption, which helps dampen the noise from things like hail or rain that sometimes strike the metal roof.
- Level Surface
Shingles, like other roofing materials, can warp or get damaged over time. Furring strips can compensate for any irregularities or unevenness caused by the damages in the shingles.
- Easier Metal Roof Installation
Furring strips offer a convenient and secure surface for attaching the metal roofing panels. This can simplify the installation process, making it more efficient for those who want to install their metal roof by themselves.
Should I use treated or untreated wood furring strips when installing a metal roof?
It’s generally better to use untreated wood for furring strips if you’re installing a metal roof on top of them. Chemicals used to treat wood can react to metal roofs over time, potentially causing galvanic corrosion.
Most treated woods have copper in them since chromated copper arsenate (CCA) is often used to treat them. Copper is a different metal from steel used in metal roofs, so galvanic corrosion can happen if moisture forms under the roof.
Galvanic corrosion happens when two dissimilar metals get in direct contact with each other in the presence of an electrolyte, like water or moisture. This can degrade the quality of the metal roof and the fasteners used to attach it to the wood.
What is the spacing required for furring strips when installing a metal roof?
The spacing of furring strips for metal roofs depends on the type of metal roofing you’ll use, the local building code, and the slope of the roof. However, most roofers typically space the wooden strips 16 to 24 inches away from each other.
For roofs with a 2:12 slope or less, it’s best to space out the strips no more than 16 inches apart from each other. If the roof is steeper, the furring strips can be spaced up to 24 inches away from each other.
However, if you’re using standing seam metal roofs, make sure not to install them more than 18 inches apart from each other, even if the roof’s slope is greater than 2:12. If you’re using corrugated metal roofing, they can be spaced up to 24 inches apart.
How to Add Furring Strips over Shingles for Metal Roof Installation
Step 1: Trim the edges of the current shingles
Using a razor blade, trim the old shingles all the way to the edge of the fascia board. We’re doing this so we won’t see any shingle overhanging the edge.
Step 2: Open the ridge
Remove any shingles that are covering the ridge, and using a circular saw or handsaw, cut the roof’s decking about two inches on either side. This is to let the hot air coming from the attic escape and improve the house’s ventilation.
Don’t forget to install a window screen over the opened ridge to avoid any debris, bugs, or insects that can get inside the house.
Step 3: Plan the layout of the furring strips
Now that the roof is prepared to install furring strips, it’s time to plan how the wooden strips will be installed. Measure the length and width of the roof to determine the layout and spacing of the furring strips.
Step 4: Locate the rafters and mark the location
Rafters are the framing structure that supports the roof. This is where you’d want to attach your furring strips to make sure that they’re securely fastened on the roof.
Locate the rafters and mark the locations using chalk. Create a chalk line from the ridge all the way to the edge of the roof.
Step 5: Cut the furring strips
After planning the layout of your furring strips, cut them according to the size you need. Make sure that the ends of the furring strips are cut at the right angles so they can fit with each other properly.
You can use 1×4 inches or 2×4 inches of wood strips. Either one will work as long as the wood is not pressure-treated.
Step 6: Install furring strips along the edges of the roof
Start at one edge of the roof and position the first furring strip along the eave or the roof’s edges. Make sure that they sit right at the edge because we don’t want to see them every time we look up.
Attach the furring strip to the roof using screws or nails. Make sure the screws or nails you’re using are long enough to penetrate through the furring strip and into the underlying roof sheathing and rafters.
Repeat this for all four edges of the roof so that the entire roof is framed with wood strips. Place a level on each strip to make sure that they are straight.
Step 7: Install the subsequent strips
Install additional furring strips along the roof, maintaining the desired spacing and alignment. Use the level periodically to check for straightness.
If some of the wood strips are uneven due to some damage in the shingles, you can add small pieces of wood underneath the strips to create an even level.
Step 8: Proceed to install the metal roof
Once you secure the furring strips to their place, you can now proceed to install a metal roofing according to the manufacturer’s guidelines.
