Depth, sill slope & squareness.

Build a depth chain

Choose a reference you can locate again, such as the face of a retained interior jamb. Record the distances from that plane to the proposed frame seat, exterior finish, stops, anchors and trim interfaces. Mark which measurements are horizontal depths and which are vertical heights.

Compare that chain to the incoming product’s cross-section. Include frame depth, fins or flanges, jamb extensions, sill adapters, receptors, accessory grooves, handles and screen locations. Confirm that the frame can be supported and fastened without interfering with a retained return.

A jamb extension covers a depth difference; it does not automatically solve a support, anchorage or flashing problem. Likewise, a sill expander may be a finish component rather than a structural support. Use the manufacturer’s role for each accessory.

Specify the sill reference

Record sill angle and the location where height is taken whenever the product requires them. Confirm whether the new frame bottom is flat, beveled or used with a particular adapter. Compare its section with the existing sill and planned supports.

A sloped existing sill can support the new frame at different points depending on its profile. Moving the frame forward or backward changes that relationship. Verify the seat before treating a height reduction as an adequate remedy.

Use a sound, relevant surface for the angle finder. A metal cap, loose trim strip or built-up sealant bead may not represent the underlying sill. If the surface to remain is concealed, arrange an investigation.

Check shape as well as size

Plumb describes a vertical line. Level describes a horizontal line. Square describes right-angle geometry. Bow is curvature along a member. Twist means the corners or surfaces do not lie in one plane. They are different conditions.

Take diagonals between consistent corners in the same plane. Unequal diagonals can reveal racking. Equal diagonals alone do not prove every side is straight or every corner is coplanar. Use the level and straightedge to locate high points and assess the space for a plumb, level frame.

Use the actual product’s acceptance criteria. Published manufacturer guides use different checks and thresholds. Do not turn a threshold found in one insert guide into a universal industry tolerance.

Establish the usable rectangle

When an opening is racked, the minimum horizontal width and minimum vertical height may still fail to describe the largest square frame that fits. Establish plumb and level reference lines inside the opening, check the limiting corners and determine a defensible installation envelope using the approved method.

Allow for the planned sill support and corrections. A low corner that needs to be raised consumes height; a protruding jamb can consume width. Identify what will be repaired before ordering and what is accommodated by the product’s permitted clearances.

Check the entire perimeter

After selecting a candidate size, compare it against the opening at every limiting location. Record both the tightest fit and the largest expected gap. A smaller frame can solve one tight corner but create an excessive unsupported or unsealable gap elsewhere.

If the opening cannot provide the product’s permitted geometry, gap range or anchorage, change the preparation or seek a different approved assembly. Do not keep subtracting until the frame passes through and assume trim will solve the rest.