Wall Trim · Metal building trim guide
Metal Building Base Trim (Rat Guard): Measure, Order and Install
Base trim runs along the bottom of metal wall panels at the slab, keeping water, rodents and bugs out of the wall.
Also called: rat guard, base angle, base flashing, bottom trim, drip base trim
Metal building base trim is the first trim that goes on the walls and the last line of defense at the ground. It runs along the bottom edge of the wall panels where they meet the slab or base rail. Crews also call it rat guard or base flashing, and its job is simple: keep water, rodents and bugs from getting under your walls.
If the bottom of a metal wall is left open, every rib is a doorway for mice, snakes and wasps, and rain splashes right in. This guide covers what base trim is, where it goes, how to measure it with a worked example, and how to install it right.
What is metal building base trim?
Base trim is bent sheet metal that covers the joint between the bottom of the wall panels and the foundation. Most profiles have a short drip kick at the bottom so water runs off the panel and away from the slab edge, not under it.
Don't confuse it with a structural base angle. On many red-iron buildings, the base angle is a heavier steel angle anchored to the slab that the wall panels screw to. Base trim is the finish flashing that covers it. On tube-frame carports and barns, the panels usually fasten to a square-tube base rail, and the base trim covers that.
Where base trim goes on a metal building
It runs around the full perimeter at the bottom of every wall panel and stops at each door opening. Doors need a clear threshold, so no base trim goes across them.
- Enclosed garage or shop on a slab: all four walls, broken at each door.
- Barn or carport on dirt with a base rail: along the base rail under the panels.
- Lean-to with walls: the lean-to walls too.
- Open carport: none, unless you add walls or partial panels.
Base trim sizes, gauge, profile and colors
Most base trim is 26 gauge steel to match R-panel and PBR walls. 29 gauge versions exist for lighter systems. Leg sizes and the drip kick vary by panel maker and by whether you have a notched slab, so ask us what fits your building.
It comes in Galvalume or painted colors. Many people match the trim color, while others match the wall or wainscot color so it blends in. Pieces are typically 10 ft long. Custom bends are available for odd slab edges, brick ledges or retrofit jobs.
How to measure base trim and how many pieces to order
Formula: building perimeter minus the width of every door opening = total feet. Add about 10% for laps and waste. Divide by the piece length and round UP to whole pieces. Each lap joint eats about 2 to 4 in.
Worked example: a 30 x 40 gable building with 12 ft walls and a 3:12 pitch, with one 10 ft roll-up door and one 3 ft walk door. Perimeter = 30 + 40 + 30 + 40 = 140 ft. Subtract the doors: 140 - 10 - 3 = 127 ft. Add 10%: 127 x 1.10 = 139.7 ft. With 10 ft pieces, 139.7 / 10 = 13.97, which rounds UP to 14 pieces. If you plan to add a door later, measure it out now.
Wall height and roof pitch don't change the base trim count. Only the perimeter and the door widths matter.
How to install metal building base trim
- Snap a level chalk line along the slab edge or base rail. A level base trim makes the whole wall look straight.
- Install base trim BEFORE the wall panels. Start at a corner and work along the wall.
- Fasten it to the base angle or base rail with #12 tek screws about 12 to 24 in. apart. On bare concrete with no base member, use concrete fasteners rated for the job.
- Lap each piece over the next by 2 to 4 in. Lap so the exposed edge of each joint faces away from the prevailing wind. Add a dab of sealant in the lap.
- Set inside foam closures on the base trim, under the panel ribs, so the ribs are filled from below.
- Hang the wall panels so they lap OVER the base trim. Water running down the panel drips past the trim, not behind it.
- Screw the panel bottoms through the closure into the base member, using the screw spacing your panel maker calls for.
- Stop the trim cleanly at each door jamb, and brush off all shavings.
Common base trim mistakes to avoid
- Hanging the wall panels first and trying to slip base trim behind them later.
- Skipping the inside foam closures. The trim alone leaves the rib gaps open to rodents.
- Running base trim across door openings.
- Letting panels sit in standing water on the slab. Keep the panel bottom just above the concrete and let the trim kick water out.
- Installing it out of level, which makes every wall panel look crooked.
- Mixing up base trim with a structural base angle on a red-iron building.
What to order with base trim
- Inside foam closures matched to your panel profile
- Outside and inside corner trim that covers the trim ends at each corner
- Jamb trim for the door openings where the base trim stops
- Color-matched tek screws and lap screws
- Butyl tape or sealant
- Touch-up paint
Get metal building base trim in Houston from Ironrod Steel
Ironrod Steel rolls its own R-panel and PBR and supplies matching metal building base trim, inside foam closures, corner trim, screws and butyl tape. Send us your building size, door sizes and a few photos, and we'll help you build your full trim list. Pick up at 8122 Hillsboro St, Houston, TX 77029, or ask about delivery across Houston and Southeast Texas. Call or text (281) 902-7619.
Base Trim (Rat Guard) FAQ
What is a rat guard on a metal building?
Rat guard is another name for base trim. It covers the bottom of the wall panels where they meet the slab. Paired with inside foam closures, it keeps rodents from crawling up inside the panel ribs.
Does base trim go on before or after the wall panels?
Before. The panels lap over the base trim so water runs past it and drips off. If you hang the panels first, the trim ends up behind the water path or has to be forced in.
Do I need base trim on a carport?
Only where there are wall panels. An open carport has nothing to trim at the base. If you close in sides or ends, add base trim and closures along those walls.
Is base trim the same as base angle?
Not always. On many red-iron buildings, the base angle is a structural piece bolted to the slab that panels screw to. Base trim is the finish flashing. Some smaller buildings use one bent piece for both jobs, so it depends on your building.
Should base trim go across doorways?
No. Stop it at each door jamb so the threshold is clear. Jamb trim covers the panel edges at the sides of the door.