Is it normal for old window sashes to pull on a timber frame like this?

Shifting in a timber frame is usually a sign that your joinery isn't tight enought to resist racking or you’re missing essential triangulation. Since you're using heavy sash windows, that dead weight is constantly pushing down and out. If your posts are shifting at the base, the first thing to check is how they're anchored to that reclaimed PT lumber. Reused deck wood is often compromised by old bolt holes or internal rot that makes it soft. If the wood is yielding under the weight, your fasteners might just be chewing through the fibers instead of holding firm. You definately need to add **diagonal knee braces** to every corner post. In a traditional timber frame, those braces are what keep the rectangle from becoming a parallelogram. Since you didn't follow a set of plans, aim for braces that cover at least the top third of the post height. Use structural screws like TimberLoks instead of standard deck screws; they have the shear strength to acutally handle those heavy windows and the lateral pressure. Don't ignore that movement before winter hits. Once you get a foot of wet snow on the roof, that vertical load increases exponentially. If the frame is allready leaning, the snow will find the path of least resistance and push it right over. You might also want to look at your rafters. If you don't have a solid ridge beam or collar ties, the weight of the roof will try to push the walls outward, wich puts even more stress on those shifting posts.

Getting some **2x4 or 2x6 kickers** tucked into the interior corners will make a massive difference. It might feel like you're cluttering up the interior space, but it's better than watching the glass shatter in January. Even if the wood is mismatched, geometry doesn't care—triangles are your best friend when you're worried about a structure buckling.

Spot on about those knee braces—they're non-negotiable with that much glass weight. One thing though, since you're using that old rough-sawn lumber, make sure you pre-drill every single hole for those structural screws. That salvaged stuff is usally bone-dry and brittle; if you just drive them in, you'll split the grain right at the most critical joint.

Also, double-check that your sashes have a tiny bit of 'wiggle room' in the openings. If the frame settles even a hair and the glass is tight against the wood, those old panes will crack the second the temperture drops and things contract.

I'm acutally planning a small shed soon, and I don't know much about these 'structural screws' people keep mentioning. Are TimberLoks somethig I can just find at a normal hardware store, or do I need to go to a specialty shop? Also, you mentioned the wood might be brittle—is there a way to tell if it's allready too far gone to support a roof? I’d hate to start a project like this and have it crack immediatly just becasue the wood is old. I'm still learning the basics of how much weight diferent boards can actually hold.

You can grab TimberLoks at Home Depot or Lowe's in the hardware aisle, usually near the joist hangers. When I built my shed with scrap, I didn't trust my shaky notched joints, so I slapped some 12-gauge steel mending plates over the main connections. It’s a bit of a 'cheat,' but it stops the racking instantly when you're dealing with heavy glass. Also, check if your base is sinking. I had to jack up one corner of my project and slide a concrete paver underneath because the weight was pushing the lumber straight into the dirt. If it’s leaning, the ground might be the culprit, not just the frame.

Quick update—I grabbed some TimberLoks and added knee braces like you guys suggested. It definately feels stiffer now, but I'm still seeing that slight tilt at the pressure-treated base. Since the salvaged wood is so dry, I'm worried about it splitting further. Should I try those steel mending plates at the bottom too, or is that just a band-aid for a deeper foundation issue?