Loft conversion - new attic trusses vs. modify existing

4 Sep 2023
Reaction score
United Kingdom
Hi all,

We're starting out on a loft conversion project. Have got plans made up to a point and looking into structural side. We'll be commissioning a builder to carry out the bulk of the work. However we've hit on a point where we're not sure how to proceed.

As we understand it traditionally a loft conversion would involve structural strengthening of the property with beams (usually steel) and modification to the existing roof structure. Ours would be fairly straightforward as there are no W shaped trusses, however due to the age of the property the span of the rafters is not uniform so would require a skilled builder / be time consuming to ensure everything is done with the right dimensions.

An alternative we have had suggested is replacing the whole roof with new 'attic trusses' essentially already designed to give the necessary space and structural strength. These can be pre-fabricated by a timber engineering company an erected on site by a builder. It seems this can achieve a weatherproof roof more quickly than if messing around with the original structure and may turn out to be similar in cost.

What we can't work it is the best way forward. I'm guessing a lot of builders that do loft conversions will want to do it 'their way'. That said doing a new roof with brand new trusses should be pretty familiar to most builders who have worked on new builds?

Any advice appreciated especially from any trades people and clients with experience of loft conversions.
Sponsored Links
Rafters of different lengths wont be a problem for a builder, in fact they would expect it. Get a proper design prepared by a professional who will discus the options with you. I doubt a new roof will be structurally required, or feasible as an option cost wise.

It seems extreme doesn't it to replace the whole roof? But I have heard it can work out well. You obviously need several days clear weather and very good planning for it to work. If it all lines up we could have a lot of the structural work done in a week or so. And a roof that's good for a long time. It's a 60s property and some other houses of the street have issues with sagging roofs so there might be some merit in that.

That said like you say I can't see how it isn't going to be more costly and complicated. To be honest I'm finding the whole process quite tricky. We already have an architect on board and structural engineer coming soon but it seems there is no obvious way forward. I don't think we've had enough advice on the structural side yet to choose the approach. My concern is with a truss company involved as well where is the liability for any problems? Then when you add in builders to the mix it just seems too much opportunity for mistakes and disputes to creep in.

I suppose if we went with a larger building firm that would sort the design as well there are potential pitfalls with that too as they're likely to do a 'one-size-fits-all' type of design which concerns me too. We already had one company come who quoted us for the loft without even looking at it or checking any structural aspect of the building - bit of a red flag even in our inexperienced eyes.

Oh well. Hoping we can get more advice from our SE once they've visited as they suggested the truss route initially, then our architect seems to have run with it. At least at this early stage we still have options and haven't run up too many costs yet.
If you've got an architect and an SE on board you're covered.

Sponsored Links
Thanks, yes we're realising now we probably are worrying overly about things that the SE, architect and builder between them will take care of. Now we have had more discussion with the SE it seems like attic trusses are the way forward for our project and with the type of roof and gable/party wall, we have a significant amount of extra work would be needed to go down the cut roof option. The foundations would possibly also need work. With the attic trusses as I understand it the load is better spread across the front and back wall, but this will require us to beef up the lintels (which to be honest probably needs doing in this property anyway). Also the current roof is of a type they did around 1950s-1960s where they had fewer trussed rafters to save on costs. But apparently there should be at least 2 more sets of these diagonal rafters and it seems the builder skimped on them. I did wonder why there was already so much space up there and also why several of the other bungalows in the area had sagging roofs! Basically either way it's going to be quite costly!

Now the fun of finding a builder who is not afraid to take the roof off our house. We ideally don't want to have to have expensive scaffolding with cover so I guess this means moving out, accepting that the ceilings could need taking down (again they're a bit knackered anyway) and getting that part done quickly in good weather. The carpets all need replacing too so not too bothered about them but might risk a few bits of furniture getting ruined rather than put everything in storage. Oh and find the money for all this :oops:Reading on here at least it seems it can be done.
Attic trusses are half the price of an equivalent cut roof conversion.
We did a hip to gable conversion in February 22. The truss designer do all the calculations and design for you based on your planning consultants drawings. The only thing we needed the SE calcs for was the lintel loadings over our existing windows and a new steel for a bifold doorway into proposed extension.
We did a 10mtr by 10mtr roof on attic trusses at 400mm centres, though Pasquals including the full design all the metalwork, gable ladders was about 15k.
Cost me £700 for the hiab long reach crane.
Full tin hat scaffold was 5k, to strip roof and redo roof battens, tyvek, and retile.
Materials about 1.5k say 1.5k labour.
Leadwork was £750.
Chimney rebuild was £750 labour materials were free of last new build.
To have gables build up in concrete blocks and intertal studwork and celotex about 2.5k concrete lintels all round £500
Steels £400.
New windows for gables £600. Velux windows x 4 £1200.
Labour for my dad and builder to build the truss sections. £4500

Smitch over 35k. We did reuse our tiles.
Obviously needed roof vents, dry ridge kits and tile and halves.


  • IMG_20220220_140825.jpg
    444.7 KB · Views: 34
Thanks for sharing that detailed breakdown on your conversion. That seems like a really good price overall for the work you had done. Ours is a slightly smaller area and possibly more straightforward as standard pitched-roof semi bungalow. We have been quoted around 7-8k for trusses. Gives me hope we can get it done on budget even with tin hat scaffolding. One builder we've spoken to reckons it could be a couple of weeks while the roof is not weatherproof so we might not get away without it and could slow down the work without.

I think it's the building work / labour we're probably most likely to get shafted on because (though I'd love to get involved) we don't really know what we're doing and realistically most builders would rather just have us out the way while they can crack on and coordinate the build. I guess the price will come down to how many other builders are keen for the work in the area and how busy they are around here at the moment.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links