Type of roof and roof window replacement / change

Hello everyone,

I want to replace 4 roof windows in my house built in 1986. 3 in the attic and one on the first floor. According to the building documents, the rafter spacing is 75cm. I want to install the Velux sizes: 2x Mk08 in the attic and one Mk06 on the first floor. The width of the windows/frame is 78cm plus another 2-3cm on each side (Velux BDX insulation and connection set). Therefore, the new windows would not fit. My carpenter said that a beam replacement would be necessary.

In my building documents it says the roof is a "purlin roof according to statics". I am not an expert but I think my house has a rafter roof. What do you think, what kind of roof does my house have and what does the above roof description mean? I am attaching photos before the interior insulation.

In the bedroom a beam replacement was already done once (rafter number 2 from the left), if a beam replacement is done in the attic, then rafter number 4 would have to be cut and the beam replacement placed between rafters 3 and 5. Thus, rafter 3 would have to carry the partial load of rafters 2 and 4.

On the opposite side, the same construction would have to be done in the attic and below on the first floor.

Is it allowed to do 2 beam replacements in the attic and 1 on the first floor? I have read that it is not advisable to cut rafters in a rafter roof. Or could one reinforce one rafter from one side and cut the rafter at the required length? Since the rafter is about 7/7.5cm wide, I should be able to manage with the width.

Another question: I got a quote from a carpenter. Now it turns out that my friend (roofer) has time and we can do it together. The price difference between doing it myself and the carpenter is about 3,500-4,000 euros. What would you recommend? I am a bit worried because it concerns a roof construction...

That is finally an excellently illustrated question contribution. I do not recognize any purlins here. Regarding the concerns about multiple replacements, I am too much of a universalist; you should best ask a carpenter who should also carry them out (or a structural engineer). You can certainly do the actual replacement / installation well with the roofer buddy.

According to the construction documents, the rafter spacing is 75 cm.

So the 75 cm is the plan. I would first advise you to measure the exact rafter spacing. At least the ones where the new windows are supposed to be installed. They usually don’t fit exactly to the centimeter, and the tolerances can make a big difference. And maybe the new windows will simply fit in without having to install a header.

Otherwise, if it’s really only about 3 cm, which the new windows are wider, you could mill off 1.5 cm on each side of the existing rafters and then reinforce the other sides with a 5 cm thick board. That would work and would be easier than installing a header.

But as I said, just measure the exact distances of the relevant rafters. Mine were off by about 5 cm here and there as well.