Enlarging existing Velux / Cutting Attic Truss

Discussion in 'Builders' Talk' started by Marcus Dare, Sep 13, 2017.

  1. Marcus Dare

    Marcus Dare New Member


    Sorry i wasn't sure which forum area to post this on.

    We currently live in a dormer bungalow that has a new loft conversion with new engineered attic trusses, such as this:


    Our roof has a 45 degree pitch and the trusses had a bottom chord of 225 x 47mm and sloped chords of 175mm x 47mm. We have 20+ trusses at 600mm centres. Each truss is supported by the load bearing external walls and a load bearing internal wall (roughly middle of bungalow).

    Our roof was Originally designed for roof tiles of 60+ kg psm and even with these heavy tiles, the frames were over sized / engineered. We then changed our mind on the tiles and we opted to have 20kg psm tiles instead. At this point we could have Easily have gone with thinner attic trusses OR the same trusses spaced further apart, but we kept the current ones because they had already been made.

    Our tiles alone are 4 tonnes lighter than the roof was Originally designed for, which means that each attic truss is Underloaded by approx 200kg.

    We currently have small velux windows (550 x 1200), however we are Not happy at all with these windows and so we wish to enlarge them to something like 900 x 1400.

    However, this will mean that we will have to cut through one sloped 175 x 47mm chord for each window, install new double thickness 175 x 94mm timber cross pieces and purlins, install new vertical 175 x 47mm vertical pieces and then centre the large window across multiple trusses.

    Here is a very Basic diagram of what i mean. The vertical timbers are the 175 x 47m sloped chords of each attic truss. Brown is for original timber and Black is for new timber pieces.

    I have effectively created a strong double thickness box around the window. With the the some of the load of the middle truss will be transfered from top to bottom through the box and then some of the load will be transferred sideways to the adjacent trusses Left and Right.

    The total material dead load of the entire roof section above each window is only 40kg per 600mm section or 1 truss. And even with full snow loading of 300mm deep snow, the total load will be 130kg (above window). Which would be spread partly through the box, partly to the Left truss and partly to the Right truss.

    So without snow loading, only approx 20kg of additional load would be placed onto each adjacent Truss. And with snow loading, 65kg of extra load would be placed. On trusses that are underloaded by 200kg.

    Under Normal circumstances (thinner timbers) i would double up the vertical timbers either side of the window all the wall down to the wall plate and up to the ridge. However, this is not normal circumstances and our current roof frames are thicker and stronger than they need to be for our roof. Also access is VERY limited.

    My Question is: Due to the heavy duty nature of our timbers, will this amount of bracing be enough? Or do we still need to double up all the way down to the wall plate?

    Many Thanks
  2. Jord86

    Jord86 Well-Known Member

    Your pictures haven't uploaded. You really need a structural engineer or the person who originally designed the roof to provide you with an answer unfortunately, no-one on a chat forum can give you 100% assurance that nothing detrimental will happen. It probably would be ok if you trim out, double up and bolt timbers together with timberlok screws, but nothing's a given I'm afraid, don't touch the trusses without manufacturers recommendation first.
    Marcus Dare likes this.
  3. Marcus Dare

    Marcus Dare New Member


    Thanks for your reply. Yes i will try and get some advise from the manufacturer. But they want to charge £360 just for basic calcs and drawings!!

    Here is one of the images again:



    Black is new timber. Brown is existing timber.

  4. Jord86

    Jord86 Well-Known Member

    But it would be £360 well spent, as any issues would then be traceable back to them.
    Marcus Dare and leesparkykent like this.
  5. stevie22

    stevie22 Active Member

    Your proposal would be fine for a conventional roof but NOT for trusses.

    I frankly doubt if an SE would get involved: like it or not I fear you need to go back to the manufacturer.

    I'm intrigued to know why you think the trusses were overdesigned in the first place. Manufacturer's are normally right on the money with their design which is why it is critical to follow their installation instructions.
    Marcus Dare likes this.
  6. Marcus Dare

    Marcus Dare New Member

    OK thank you. The reason why the roof was originally overengineered is a bit of a long story, but it's to do with the roof being designed the span the entire property without the middle internal load bearing wall. But then we got a heavy load bearing concrete wall installed through the middle of the property.

    Bascially, we could have used timbers 2" thinner on both the bottom chord and sloped chords, and this was WITH the heavier 60kg tiles. But once the light 20kg tiles are factored in, our timbers are really overboard.

    I agree that they are normally tight. But we have lot's of variables and so they decided to over egg everything to be safe.
  7. stevie22

    stevie22 Active Member

    It's unusual to rely on 3 supports for a truss as unless everything is spot on dimensionally it will only ever sit on 2 of the 3 until there's some deformation. It's what we call a statically indeterminate structure.

    Did the manufacturers design for this? I assume from your comments that they did but the fact remains that they designed a complete system and they (or a brave SE who will likely charge >£360 anyway) will have to design any changes for any form of design warranty to remain.
    Marcus Dare likes this.
  8. Bob Rathbone

    Bob Rathbone Active Member

    Why not go back to the truss manufacturer and ask them for a solution, ie for them to make and sell you the timber and fixings needed to do the job. I reckon they will do the design free as they are getting a sale.
  9. Marcus Dare

    Marcus Dare New Member


    So i have heard back from the truss manufacturer. Their exact comment is;

    "Cut two single trusses ( 1-No. at each location ) – support back to single either side with new purlins & rafter level. No binders required at floor level.

    Strengthen adjacent single trusses with a scab to joist ( long side from internal bearing to wall"

    What do they mean by Internal Bearing? Is this the Ridge at top of roof? Or is this the vertical 7" x 2" timber that spans between the bottom chord and sloped chord?


    Also, with our tiles, lats, felt and timber, i have calculated the basic material deadload as no more than 35kg per sm. However the manufacturer has set this basic deadload value at 90kg per sm as standard.

    I am willing to consider doubling up from wall plate to ridge. But this will be a massive amount of work and seems really really overkill.

    The manufacturer are willing to supply all timber, but only at an extortionate rate 3x more than it would cost to just buy good quality C24 grade 7"x2" timber + correct fixings from a merchant.

    Last edited: Sep 14, 2017
  10. stevie22

    stevie22 Active Member

    At 3x price of material with no design fee and manufacturer happy I'd say you have a result
  11. Marcus Dare

    Marcus Dare New Member

    Hi, there is a design fee of £360 for drawings and calcs.
  12. Wayners

    Wayners Active Member

    Biggest mistake I made was fitting large velux windows. The heat from the 3 we have on sunny days is incredible.

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