Building a Greenhouse From Old Windows

A rewarding reuse project, provided you design around the windows you have rather than the greenhouse you imagined.

Salvaged windows make excellent greenhouse glazing, but they impose their dimensions on the whole build. The workable method is to collect all the windows first, lay them out on the ground, and design the frame to fit them. Attempting the reverse, designing a structure and then finding windows to fit, produces endless awkward infill panels and a great deal of wasted timber. Sort by height, group the matching ones for each wall, and accept that the walls will not be symmetrical.

Frame, Base and Drainage

Build the frame from pressure-treated timber, since it will be in permanent contact with damp air and often with the ground. The base needs to be level and to drain: a gravel bed or a slab with fall is far better than bare soil, which turns into mud and rots the bottom rail within a couple of seasons. Set the timber on a damp-proof course or on concrete pads rather than directly on the ground.

Fix the windows so they can be removed individually. Old glass breaks, and a design where replacing a single pane means dismantling a wall is a design you will come to resent. Screwed timber beads let you take one window out and put another in.

Ventilation and Old Paint

Greenhouses overheat far faster than people expect. Plan at least two openable windows, ideally one low on one side and one high on the opposite side, so warm air can leave while cool air is drawn in. Retaining some of the original opening casements is the easiest way to achieve this.

One genuine caution: windows from buildings painted before the late 1970s may carry lead paint. Do not dry-sand them. Test if the age is uncertain, and if in doubt, strip chemically or leave the paint intact and overcoat it.

Summary

Collect first, design second, and build on a base that drains. Make the glazing individually removable, plan cross ventilation, and treat pre-1980 paint with caution. Our Board Foot Calculator helps price the framing timber once your layout is fixed.