Moldings Skill Level: Beginner (Level 1)

How Height Affects a Tall Window Jamb Extension #

• You need a step stool or stepladder. The top of the opening sits 8 to 10+ feet above the finished floor. You cannot reach that high from the ground. You need something to stand on to measure the top gap, to shim the top piece (drive in a thin wood wedge, called a shim, to hold the piece tight and straight), and to nail the top part of the side pieces.
• The side pieces are 7.5 to 10 feet long. Standard 1× stock often comes in 8-foot lengths. For this height range, that is barely enough, or sometimes too short. You’ll need to source 10-foot or 12-foot stock for the sides.
• The assembled box is larger and heavier. A standard-height extension box is light and easy to maneuver. In this range, the box is big enough that sliding it into the opening takes more coordination. Be careful getting the top corners past the drywall edges without damaging the corner joints.
• More measurement points are needed. Taller side pieces mean more length where the gap can vary. Take gap measurements at four to five points on each side piece, not just three.
• More shim points on the sides. On a standard-height window, three shim points per side keep it straight. On a 90-119″ window, use four to five shim points per side. That keeps it from bowing between them.
• A helper is useful but not required. You can do this solo with the techniques below.

Additional Tools for Window Jamb Extension #

Everything in the core guide tool list, plus:

• Step stool or 4-foot stepladder - Sturdy enough to stand on while shimming and nailing at the top of the opening. A step stool works for windows up to about 100″. Taller windows in this range need a proper stepladder.
• 6-foot level (optional but helpful) - A 4-foot level covers less than half the side piece length. A 6-foot level gives you a better read on plumb across the full side of a 90-119″ opening. If you don’t have one, the 4-foot level from the core guide still works. Just check plumb at the top half and the bottom half separately.

Tall Window Jamb Extension Material Planning #

Before starting, verify that your stock is long enough for the side pieces:

• Measure your tallest side piece, add 2-3″ for waste at each end, and confirm your stock covers it.
• 10-foot or 12-foot lengths are the right stock for this window size. Most lumber yards carry 1×6 MDF, poplar, and pine in these lengths. If 8-foot is all that’s available, the material won’t be long enough for the side pieces.
• If long stock isn’t available, you may need to join two shorter pieces with a scarf joint (an angled splice where the two pieces overlap). This Old House has useful resources on wood joinery for trim work. This works, but it adds a step. Put the joint where casing or a window treatment will hide it.

Window Jamb Extension: What’s Different Step-by-Step #

Follow the core guide for the complete process. These are the modifications for 90-119″ windows:

Step 1: Tall Window Jamb Extension Measurements #

Same measurement process, but take more readings on the side pieces. When the opening is this tall, measure the gap at four to five points per side instead of three: bottom, lower-middle, center, upper-middle, and top. On a 90″+ side, the gap can shift by 1/8″ or more between the bottom, where you can easily see it, and the top, where you need a ladder. Don’t skip the upper measurements. That’s where surprises hide.

Step 2: Rip and Cut Material #

Same ripping process (cutting the board lengthwise, with the grain, to bring it down to width). The only difference is supporting long pieces during the table saw cut. A 9-foot piece of 1×6 hanging off one side of the table saw will droop. That throws off the cut and can be a safety hazard. Use a roller stand, an outfeed table (a support set at saw height to catch long stock as it comes off the blade), or have a helper hold the piece level as it exits the saw. Keep it level with the saw table through the whole cut.

Step 3: Assemble the Box #

Same assembly technique - glue and staple on a flat surface. Two things change:

• You need more surface area. A box this size doesn’t fit on a standard workbench. Assemble on the subfloor or on two sawhorses with a sheet of plywood laid across them.
• Check squareness more carefully. Longer sides make any out-of-square error more obvious. A 1/16″ difference between the two corner-to-corner diagonals is barely noticeable on a 36″-wide box. On a box that’s 36″ × 100″, that same 1/16″ difference means the frame has a visible twist. Measure both diagonals corner to corner. If they match, the box is square. Adjust as needed, then let the glue grab before you move it.

Step 4: Install the Tall Window Jamb Extension Box #

Same shimming and nailing process, with these adjustments:

• Sliding the box in takes more care. A box this size is harder to maneuver through the room and into the opening. Go slow so you do not rack the frame (twist it out of square) or pop a corner joint apart. Tilt the box at a slight angle, feed the top edge in first, then rotate it and press it flat. If you are working solo, rest one end on your knee or on a temporary cleat while you position the other end.
• Shim top and sides from the ladder. Position the ladder off to one side so you can reach the upper corners and the top piece. Shim the top first (while you’re up there), then come down and shim the bottom. Work the sides from top to bottom, repositioning the ladder as you go.
• Four to five shim points per side piece, spaced every 16″ to 20″. Three shim points aren’t enough on a 90″+ side. The extension will bow between them, and you won’t catch it until the casing goes on.
• Nail from the top down. Secure the top piece first, then the upper portions of the sides, then work downward. This locks the top of the box in position while you deal with the lower sections from a more comfortable height.

Step 5: Plane, Sand, and Prepare #

Same planing and sanding process. The only new thing: check that it is flush along the entire length of each side piece, not just at working height. Flush means sitting flat and even, with no gaps or high spots. It’s easy to get the bottom half perfect and miss a proud spot (a small high point sticking up above the surrounding wood) at 8 feet, because you didn’t climb the ladder to check. Run your straight edge along the full side from top to bottom.

Solo Tall Window Jamb Extension Technique #

If you’re working alone, the hardest part is holding the box in place while you shim the top. Here are two ways to do it:

• Bottom shim first: Slide the box in, insert two shims at the bottom to keep it from dropping, then climb the ladder and shim the top. This is the faster method, but the box has to stay roughly in place while you climb.
• Temporary cleat: Screw a scrap piece of 1×2 or 1×3 to the wall just below the bottom of the opening to create a ledge. Rest the box on the ledge while you shim the top and sides. Remove the cleat after nailing. Two small screw holes in the drywall are easy to patch. That is much easier than trying to hold a large box and shim it at the same time.

Time Estimate #

The core guide applies to standard windows (under 89″), where a typical window takes 15-25 minutes. For a window in the 90-119″ range, plan for 25-40 minutes per window. The extra time comes from repositioning the ladder, adding more shim points, double-checking more measurements, and handling longer material. The technique itself does not change.

Window Jamb Extension Quality Check #

Complete the full core guide quality checklist, then verify:

• Side pieces are straight along the full length - Look down the length of each side piece from the bottom, sighting up toward the top. Longer pieces have more room to bow between shim points. If you see a bow, add a shim point right there and re-nail.
• Upper section is flush - Climb the ladder. Use a straight edge to check for flush at the top of each side piece and across the full header (the horizontal piece at the top of the opening). This area sits above eye level, so it is easy to skip.
• Plumb check covers the full height - Hold your level against the full side piece, not just the lower half. A side that reads plumb at 4 feet can still be out of plumb at 8 feet if a shim pushed it off during nailing.

Tall Window Jamb Extension Troubleshooting #

Side Piece Bows Outward Between Shim Points #

This is the most common issue with a tall window jamb extension. Long pieces can bow away from the framing between shim points, especially if the material has any natural curve. Add another shim point right at the bow, push the extension flush, and nail through it. MDF is usually flat, so if you’re using MDF and still see a bow, it is almost always because you do not have enough shim points. Shim every 12-16″ instead of 16-20″.

Box Racks During Insertion #

A larger box is more likely to twist or rack as you maneuver it into the opening, especially when you are working solo. If a corner joint pops open, pull the box back out, re-glue and re-staple the corner, and let it set before trying again. For future boxes this size, consider adding a third staple at each corner for extra hold.

Upper Shims Slip Before Nailing #

When you are working overhead, gravity works against your shims. If shims slip out of position while you reach for the nailer, try wetting the shim pairs slightly. The moisture creates just enough friction to hold them in place for the few seconds you need. Alternatively, put a small dab of hot glue on the shim before you insert it. A helper holding shims from below makes overhead shimming a lot easier.

Get the shimming and flush-checking right on a tall window jamb extension, and the casing that follows will go on without a fight. It is the same discipline as the standard-height process, just repeated a few more times, from a ladder, before you climb down.

Related Guides #

• Core Guide: How to Install a Window Jamb Extension Like a Pro - Complete step-by-step process for standard-height windows (under 89″)
• Window Jamb Extension for Tall Windows (120″ and Above) - Windows with tops at 120″+: full stepladder, two-person recommended, possible section assembly, and managing oversized frames
• How to Install a Door Jamb Extension - The same concept for door openings
• How to Case a Window - The next step after the extension is installed