A frame anchor — also called a frame cramp — is a metal strap or angle that ties a window or door frame to the surrounding masonry. One leg is screwed to the frame jamb, the other is bedded into a bed joint of the inner leaf as the wall goes up, or fixed with a plug or resin anchor into existing masonry.
What it is for
A frame is not simply wedged into the opening; it has to be held. The anchors take two kinds of load:
- Wind load — wind pressure and, more critically, wind suction on the sash or leaf is carried through the frame into the wall. Under suction the wind pulls the frame outwards, and only the anchors resist it.
- Service load — the weight of the opening leaf, the slamming of a door, and for external doors the requirements for security.
Sizes and spacing
- Length — frame anchors are commonly supplied in lengths of roughly 140 to 250 mm, depending on the depth of the jamb and the distance to the inner leaf.
- Spacing — a common arrangement is around 60 cm centres along each jamb, with an anchor fairly close to the corners — but not right in the corner, or the masonry will break out.
- Types — the plain strap for timber frames, the wide L-shaped anchor for PVC frames and concrete walls, and adhesive anchors for aerated concrete and thin-joint blockwork.
- Material — galvanised or stainless steel. In the cavity, where the anchor can see moisture, a corrosion-resistant grade is not a luxury.
Common mistakes
- Anchoring only at the threshold or the head. The jambs carry the wind load, and that is where anchors are needed.
- Bedding the anchor into a perpend instead of the continuous bed joint, so it is not embedded in mortar over its full length.
- Using the anchor to line the frame up. Levelling is done with packers and wedges; the anchor only fixes the frame afterwards.
- Forgetting that an anchor is also a thermal bridge. In an insulated cavity, prefer a thin corrosion-resistant strap over a heavy continuous angle.
Related terms
- Window frame
- Window frame finishing
- Cavity wall
- Bed joint
- Thermal bridge
- Masonry
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