Trim Frames: Accurate Moulding Installation

Trim Frame CornerFaster, neater trim and molding installation with joints that never open up. These are the advantages of trim frames, and it’s an approach that’s easier and better. Instead of custom-cutting pieces of trim individually before nailing them to the wall one by one, trim frames are cut and assembled ahead of time into three or four-sided frames, then mounted as one unit on the wall. This does demand a completely different way of working, but the advantages are worth it in many situations. There’s no beating trim frames for stain-grade work or any kind of composite trim designs with multiple moulding elements.

There are three main problems with traditional cut-and-nail trim installation:

  1. The process is inefficient because you’re always running back and forth between chopsaw and wall.
  2. The deck is also stacked against tight trim joints because door and window casings typically aren’t flat. A
  3. When you do manage to get tight trim joints, there’s no guarantee they’ll stay that way because unconnected wood joints move seasonally.

Trim frames solve all of these problems while offering a special advantage for stain-grade work. When you can’t hide your sins under a layer of latex caulk, you’ve got no choice but to be good.

Trim frame construction starts with a tape measure. Carefully measure the size of each door and window jamb you’re trimming, note the results, then add an allowance for a reveal along the inside edge of the jamb before making the frames to fit.

BiscuitIn most cases the trim frame approach is only possible because of biscuit joinery. Biscuits are ovals of compressed, manufactured hardwood that fit into slots cut with a hand-held power tool. The result is an ultra-strong and hidden joint, but the way you use biscuit joints for trim frames depends on the kind of trim you’re working with. I use #20 biscuits for most trim frames, though the smaller #10 size is also useful.

The easiest trim for making into frames is flat, wide casings that have no profile. This is the basis for traditional, extra-wide trim and moulding designs, with smaller trim and molding elements added afterwards piece-by-piece to enhance the look. Flat trim is usually thick enough that it’s easy to cut biscuit slots on the ends and edges of adjoining pieces of butt-joined trim. Cut the pieces of wood to length that you need for the top and sides, mill biscuit slots into adjoining ends and edges, then swab glue into the joints with a plumber’s flux brush before bringing parts together with biscuits and clamps. A sheet of plywood or particleboard on sawhorses makes a perfect jobsite workbench, and you only need to leave the joints clamped for 10 minutes before carefully taking the frame out, setting it aside to dry completely, then assembling another frame with the same clamps. Sand, stain and urethane your trim frames when they’re completely dry and before they go on the wall. Since door frames only have three sides, temporarily fasten a brace to the back of the trim frame at the bottom. You’ll remove it just before installation on the wall.

Since trim frame joints are held together securely at the corners, you don’t need nearly as many fasteners to secure a trim frame to the wall. A few 2-inch long, 23-gauge pins every foot or so holds the frame nicely. If you want to use even fewer fasteners, daub construction adhesive on the back of the trim frame before it goes down.

Plunged Biscuit MitreTrim with moulded profiles is more challenging to join into a frame with biscuits ahead of time because you don’t have the chance to sand exposed surfaces flat and level after assembly. Depending on the complexity of the profile you’re working with, and how accurately you find your biscuit joinery turns out, you may want to try plunged biscuits. Assemble the mitre joints with glue, aligning the surface profile perfectly. Let the glue dry completely, flip the trim frame over gently, then plunge biscuit slots in from behind, making sure they don’t come out the front. Swab glue in the slots, add biscuits, then trim off the excess biscuits after the glue is dry. Plunged biscuit joinery isn’t nearly as fast because you need to wait for glue to dry completely, but it is a lifesaving technique when all else fails.

The further along you progress on a project, the slower and more finicky the work gets. Trim installation proves it for sure. You might start off as a lean, mean framing machine, but eventually you’ve got to pretend they’re a cabinetmaker. Putting up trim demands that you get pretty close to that challenge, and trim frames can certainly make you better than you might otherwise be.

Tip: Humbling Proud Drywall

Misaligned Drywall FixIt’s not unusual for drywall to stick out proud of the interior edges of door and window jambs, and even a little bit of this nonsense is a big problem because it stops trim from contacting the jamb tightly. The traditional fix involves the dirty proposition of using a Surform plane to work the excess drywall back, but this is too messy, slow and inaccurate for anything more than occasional situations. An alternative is to install the main part of the trim on the existing drywall, but back a little further from the edges of the jamb than usual. This extra space allows room for an inner run of small bullnose trim or beading that can extend down to touch the edge of the jamb, spanning the gap caused by a jamb that doesn’t extend far enough. This is faster than the Surform and looks terrific.

Tip: When Frames Won’t Work

Trim frames depend on square window and door frames, and prehung doors and modern windows almost always deliver the kind of accuracy required. But just to be sure, check your doors and windows with a 24” carpenters square before you build frames. You may need to enlarge the inside measurements of your trim frames a bit to allow for a small out-of-square condition.

Tip: Pocket Screws for Thicker Stock

Pocket ScrewBiscuits are great for joining trim frames, but pocket screws are faster when you can use them. Trim thickness and shape are deciding factors. If you’re dealing with wood that’s at least 5/8”-thick and flat, pocket screws are the way to go. Parts for fireplace mantels, box newels, built-in cabinets and bars are all applications where pocket screws shine. The fastest approach uses a stationary machine for drilling the required angled pocket screw holes.

Tip: Toasting Biscuits

A snug fit within slots is one reason biscuit joints are so strong, but sometimes humidity can swell biscuits so they’re too tight to go into their slots. Twenty minutes in a toaster oven at 300ºF dries out the wood and shrinks the biscuits so they slip right in.


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Posted on January 17th, 2013

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