The short answer
Not for every new roof. A like-for-like covering of similar weight does not normally need one. The Planning Portal's advice is to get a structural engineer's or surveyor's opinion ahead of the work if a new covering will be heavier, and Building Regulations approval is needed once the new covering is 15 per cent or more heavier than the old. Signs of movement in the roof timbers are another reason to call one in.
A roof is a structure carrying a load. An engineer is needed when that load goes up or the timbers show trouble.
When a heavier covering needs a check
Slate, clay and concrete all differ in weight per square metre, and the timbers beneath were sized for a particular covering. Swapping lightweight tiles for natural slate, or slate for thick stone-flag, loads the structure differently.
What does the Planning Portal say about weight?
The Planning Portal requires approval where the new covering makes the roof covering 15 per cent or more heavier, and it advises taking a structural engineer's or surveyor's view before the work starts. Approved Document A, the structure part of the Building Regulations, uses slightly different wording: "A significant change in roof loading is when the loading upon the roof is increased by more than 15%." The two sources differ by the word "more", so when quoting a figure, name the source.
The question for your roof is therefore the weight of the old covering against the new, and the strength of the rafters and walls that carry both. We tell you the covering we propose at the survey, and you can take that to an engineer or your building control body for a view.
Signs that the roof timbers have moved
A structural look is also worth having when the roof shows signs of distress, whatever covering goes on.
What does a sagging roof look like?
From the ground, look along the ridge, the top line, and along the eaves. A ridge that dips between the gable ends, a slope that bellies inward, or tiles that have slid and bunched point to movement in the rafters or purlins, the horizontal timbers that support them. Walls leaning out at the top, cracks that run diagonally from the corners of windows, and doors that suddenly stick can also be linked to roof spread.
From the loft, we look for split or notched rafters, joints pulled apart, and timbers stained and soft from long-term damp. Our guide to rotten roof timbers goes into the damp side. Small local rot is a carpentry repair. Movement that has bowed the whole slope is different, and an engineer's drawing is the sensible next step before the new covering is fixed.
What an engineer actually does for a re-roof
A structural engineer is a chartered professional who calculates loads and specifies timber sizes and fixings. For a roof, that usually means a site visit, a look at the rafters, ceiling joists, walls and any supporting beams, and a short report or calculation.
What do you get from the visit?
A written opinion on whether the structure can carry the proposed covering, and if not, what must be added. That can be larger rafters, extra supports or strapping. The report is the thing your building control body will want to see if the weight change brings the job into approval.
You can ask for one even when the rules do not require it. If a survey shows the covering is going to be heavier, an engineer's note costs little against the risk of an overloaded roof. A survey is a different thing from a structural report, and we are roofers, not engineers: we describe what we see and when a question is beyond what we can judge, we tell you that. Our answer on changing slate to tiles shows how weight comes into a material swap, and the roof replacement page covers the wider job. Photographs from the loft help whoever comes to assess it, so we keep the survey set for you.
Getting approval and what to ask your building control body
Where approval is needed, the application goes to your building control body. For a Taunton house, Somerset Building Control is the council's own service, on 0300 303 7790, and private registered approvers are another option.
Does a plain re-roof need an application?
Not if the covering is like for like, less than 25 per cent of the building envelope is affected, and less than 50 per cent of the roof. Somerset Building Control states no re-roofing percentage, so use the Planning Portal's test. A full re-roof can go over the roof percentage, in which case ask the building control body what it needs from you.
Bring to that conversation the old and new covering types, the roof area, photos from the survey and any engineer's report. When the survey is done we give one written price, and it will say what covering the price is for, so the weight comparison is plain. If the figures say a structural check is wise, we will say so before any covering is ordered.
Weight and timbers
Is slate heavier than concrete tile?
It depends on the product, the size and the thickness, so there is no safe general answer. Check the weight per square metre on each maker's data sheet and compare like with like. If the new covering is 15 per cent or more heavier, bring in a structural engineer.
Who pays for the engineer?
The property owner normally does, as the report is for your own roof and your building control application. It is a separate fee from the roofing price. Ask the engineer for a fixed fee before the visit, and give them the covering details from the survey so they can answer in one trip.
The full picture: Roof replacement and new roofs in Taunton ยท More short answers