Introduction #

A tall window jamb extension becomes more demanding when the top of the opening sits higher than standard reach. This is a supplemental guide for installing window jamb extensions on windows where the top of the opening is 90″ to 119″ from the finished floor (roughly 7′6″ to 9′11″). These measurements describe how high you need to reach — not the size of the window itself. A small window mounted high on a stairwell wall is in this range the same as a large picture window at the same height.

The complete tall window jamb extension process is covered in our core guide: How to Install a Window Jamb Extension Like a Pro. Read that guide first. Everything below assumes you already know how to measure, rip, assemble, shim, and nail the extension box, and focuses only on what changes when the top of the window sits higher than you can comfortably reach from the floor.

Windows in this range are common in homes with 9-foot or 10-foot ceilings, great rooms, stairwell landings, and formal areas. The process is identical — same box assembly, same shimming and nailing technique — but the higher working position creates challenges in measurement, material handling, and working from a ladder that don’t exist when the top of the window is within standard reach.

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 from the finished floor. You cannot measure the upper gap, shim the top piece, or nail the upper portion of the side pieces from the ground.
• The side pieces are 7.5 to 10 feet long. For any window in this height range, standard 1× stock often comes in 8-foot lengths, which may be barely enough or 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 — especially 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 a minimum of four to five points on each side piece — not just three.
• More shim points on the sides. Versus a standard-height window, three shim points per side keep things straight. On a 90-119″ window, plan for four to five shim points per side to prevent bowing between them.
• A helper is useful but not required. Solo installs are fully manageable with the techniques described 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 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 in your material, you may need to join two shorter pieces with a scarf joint — an angled splice where two shorter pieces overlap. This Old House has useful resources on wood joinery for trim work. This is workable but adds complexity. The joint should be positioned where casing or a window treatment will conceal 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 top of the opening is at this height, 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. The only difference is supporting long pieces during the table saw cut. A 9-foot piece of 1×6 extending off one side of the table saw will droop, affecting cut accuracy and potentially creating a safety hazard. Use a roller stand, outfeed table, or have a helper support the piece as it exits the saw. Keep it level with the saw table throughout the 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 amplify any out-of-square error. A 1/16″ diagonal difference on a 36″-wide box is barely noticeable; on a box that’s 36″ × 100″, it means the frame has a perceptible twist. Measure diagonals, adjust, and let the glue grab before moving.

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 without racking the frame or popping a corner joint. Tilt it at a slight angle, feed the top edge in first, then rotate and press flat. If working solo, support one end on your knee or a temporary cleat while positioning 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 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 addition: check flush along the entire length of each side piece, not just at working height. It’s easy to get the bottom half perfect and miss a proud spot 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 biggest challenge is holding the assembled box in position while shimming the top. Two approaches:

• 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 requires the box to stay roughly positioned 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 screw holes in the drywall are easily patched — much easier than trying to hold a large box and shim 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 ladder repositioning, more shim points, more measurement verification, and handling longer material — not from any difference in technique.

Window Jamb Extension Quality Check #

Complete the full core guide quality checklist, then verify:

• Side pieces are straight along the full length — Sight down each side piece from the bottom looking up. Longer pieces have more room to bow between shim points. If you see a bow, add a shim point at the bow and re-nail.
• Upper section is flush — Climb the ladder and check flush with a straight edge at the top of each side piece and across the full header. The upper area is above eye level and 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 be out at 8 feet if a shim pushed it off alignment during nailing.

Tall Window Jamb Extension Troubleshooting #

Side Piece Bows Outward Between Shim Points #

This is the most common tall window jamb extension issue. Long extension pieces can bow away from the framing between shim points, especially if the material has any natural curve. Add an additional shim point at the bow, push the extension flush, and nail through. If you’re using MDF, which is generally flat, the bow is almost always caused by insufficient shim points — shim every 12-16″ instead of 16-20″.

Box Racks During Insertion #

A larger box is more prone to twisting or racking as you maneuver it into the opening, especially 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 in this size range, consider adding a third staple at each corner for extra insurance.

Upper Shims Slip Before Nailing #

Working overhead means gravity is working 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, apply a small dab of hot glue to the shim before inserting it. A helper holding shims from below makes overhead shimming significantly easier.

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