Age and type of house
A king post in the barn roof has been cut to fit a roof window. What original truss action has that cut interrupted, if the survey could see the cut?
The cut interrupts the king post's original action in the truss, the link between the apex and the tie, if that cut was seen. A king post works with the tie and the principals so the truss acts as one frame. Cutting it for a roof window removes that post from the line where the cut is. The survey can describe the cut it could see. It has not calculated what remains, and it has not approved the alteration. Approved Document A is structural guidance. It is not a certificate this inspection has issued.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
In a king-post truss the post runs from the apex area down to the tie beam. That link is part of how the frame holds its shape. A roof window needs a hole, and someone has made the hole by cutting the post. If the survey could see the cut ends, the interrupted action is visible as a post that no longer runs continuously. What the survey cannot do is turn that sight into a calculation of the remaining roof, or into a building-regulations certificate.
Ask a structural engineer to look at the cut king post and the way the roof window is framed, as a separate instruction. Tell the solicitor the survey describes an interrupted post, not an approval. Approved Document A does not become a pass because the cut was photographed. The survey has not produced a certificate for the roof window, and it has not said the alteration is acceptable.
What should you ask your surveyor?
- If you could see the cut, what truss action did it interrupt?
- Did you describe the cut without calculating the remaining roof?
- Have you left any certificate for the roof window unissued?
What are the typical next steps?
- Describe the cut king post as an interrupted truss member.
- Ask a structural engineer to assess the cut and the window framing.
- Do not treat the survey as approval of the alteration.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If the roof window does not leak, is the truss unaffected?
No. A dry opening is a weather note. It does not restore the king post. The cut, if it was seen, still interrupts that post's action in the truss. The survey has not calculated what remains. Approved Document A was not turned into a certificate by a dry frame.
Does a steel plate nearby count as a design the survey has approved?
No. A plate, if one was seen, is something to describe and to pass to an engineer. The survey has not approved it, and Approved Document A is not a certificate this visit issued. Metal beside a cut does not, by itself, restore the king post's original action.