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I have purchased a house which had plans and engineering diagrams for a loft extension in its cut and pitched roof. The back of the house is to have a full-width dormer, the front staying as a sloping roof.

The plans seem to suggest that the purlin on the sloping roof is removed, and the rafters supported on a knee wall which rests on an RSJ embedded in the load-bearing end walls.

The knee wall will bear the weight of the roof vertically downwards onto the RSJ, and the rafter spans conform to the relevant building regs tables.

What is worrying me is that the purlin is currently supported by wooden struts that are at 45 degrees, giving support to the rafters vertically and horizontally. The knee wall will have little or no support in the horizontal plane. The engineers plans are sketchy to say the least, and the plans give no hint that horizontal support is required,
Does it need any?

jaamiller,
Start by getting the plans checked by an architectural technician, he will then refer any dubious bits to a structural engineer.
end

Hi, the main reasons that struts are added to purlins are to either shorten the effective length or to prevent them bowing down the length of the rafters. The main reason the purlin bows is if it's at an angle rather than vertical. When your new wall is built it will have a wall plate on top and ideally this should be birdsmouthed into the rafters, that way the imposed load will be vertical.

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