Need engineer: Attic convertion

Need advice on options and sign-off on design of attic conversion. Any engineer contacts appreciated.

Any ballpark estimates?

It’s a new-build, woodframe, 3 bed semi, A2/A3.
Inner dimensions of house: 5.8m wide x 9.1 deep. Roof is gable-ended, factory strut system (9 struts excluding ends) with verticals towards sides and a tie across the top. There’s about 4 meters space front to back. A little over 2m floor to horizontal tie.
HRV unit within central space would be moved into side space behind verticals. Water panel on one side of roof. Two water tanks behind verticals. Currently fiber insulation rolled over attic.

Stairs would run from halfway along landing, out to turn at wall and up towards rear, parallel to main stairs. Small hall at top required?
Insulation options? Maybe two options:
Envelope running in on ceiling to verticals and then up around roof. Need airtight doors to side spaces.
Run the roof insulation right out to near the walls and join back to ceiling there. Entire attic in the warm side. Water tanks affected?

Can I put two velux in the spacings between struts without cutting any and does that cause any issues, e.g. cold bridges. Is it better to make single bigger window that cuts through struts?

All advice appreciated.

We went with Hogan and Associates based in Windy Arbour. They have a bit of an Attic ‘niche’

ballpark how much would one expect to pay for this kind of a job? we are going to get our attic done soon though its probably going to be next spring at the earliest.

We were charged 2K plus VAT just to planning stage. In the past we got a structural engineer from Galway who charged about the same (no VAT) for sign off on a 8 week 100K extension

A regular builder guy quoted 16- 20k for basic conversion that required no structural change to roof.
An engineer guy quoted 2k just for his part of job, which I presume would include sign-off on structural adjustments.

A friend mentioned that with factory trusses in new builds, the engineering of the trusses is fine tuned to what is required and there isn’t much scope for messing with them without a good deal of extra support. This includes not doing much that might weaken the floor beams.

I know a lad who’s done these for a living. He ballparks 15k for a conversion with stairs/veluxs/strengthening floor to take the weight. This in an attic ready to go otherwise.

You typically can’t lay a floor on existing ceiling joists - they are too light as they are, designed simply to tie the roof together (for which slender sections are required) and holding the ceiling plasterboard up.

Modern multi-element roof trusses is a much more involved project. You can’t remove those elements without serious reinforcement/design involving steel - a different prospect altogether.

Some attics are ready to go: good access route, nothing in the way of putting up walls, solid ceiling joists. Others lie at the other end of the spectrum. Impossible to generate a typical price to cover all sorts.

@york: Could you pm your friends details?

We’re thinking to wait a while to decide what we’ll do with the attic. Should I assume I can use it as a storage area if I put in perpendicular beams and floor? Beams are to raise floor enough not to compress insulation. Not sure of best way to do that.

He’s left that game. Builder for years but tired of the the dirt, the dust … and the grief people give. But his pricing tends to be sound enough so I’d query someone charging significantly more for a straightforward conversion.

How much higher than the existing joists is the insulation? Is it simply carpeting the attic covering all joists?

I’d roll up insulation (carefully - for reuse) down to joist level. Add new joists on top of old (running in same direction so as to pick up on same mid-span supports that the existing joists are resting on). Screw new to old every 18" or so over length, screwing through the side of the new joist at an angle down into the old joist. Use 80mm long screws or so. Screws alternating each side as you go along the length to tie the new joist to old both sides which stops it tilting to one side.

Install a line of noggins (google noggin timber stud) mid span to provide stability. Lay insulation between new joists stopping 2" shy of the top of the new joist (so use a 6x1 unplaned for the new joist and fill to 2" from top). This allows air to vent over the insulation to prevent moisture build up under the new decking

Lay smart ply (handy size, tongue and grooved sides and ends) on top and screw down to every joist, every foot or so. The t&g means it doesn’t matter whether the sheet joints land on a joist or not. Don’t push the sheeting right into the eaves (there’s no storage there anywhere). This’ll leave plenty of room for venting. And don’t pack the second layer of insulation all the way to the pitched roof (which will block air movement). Stop this insulation short of contacting the roof by perhaps 6"