Best Practices for Adding Loft or Converting Attic

[TRAK.Structural]

I see attics converted to livable space and/or lofts added to existing homes in my area all the time, but haven't done one myself. I'm curious to know from the other Engineers what they have found to be best practices in terms of verifying the existing structure can handle the additional loads.

Checking size/spacing of wall framing and headers would seem simple enough (although may require selective demo of existing finishes); but establishing the makeup of existing foundations feels to me to be a much more complicated task. Is it common to require digging test pits at various locations to see buried foundations? What about homes built on slabs, I don't know how one could figure out the dimensions of a turndown at the perimeter or thickened areas at interior bearing walls. And what about soil bearing pressure capacity, every set of plans I come across uses a presumptive bearing capacity of 2,000 psf to be verified by the inspector. Is that a reasonable approach for older structures that were built before the development of the codes we use today?

 

[skeletron]

I think digging out and around the perimeter of the building is probably the most accurate method and the most conservative. You would be able to measure the foundation and hire a geotech to review the soil bearing pressure on site. It would be difficult, but not impossible, to determine the reinforcement in the footing or slab. Most homeowners seem to balk at this idea because: (A) they want to spend their money on the interior not exploring the exterior; (B) there's always a possibility that something like perimeter drainage will need replacement.

Depending on the size of the conversion, you could also hire the geotech to provide an updated bearing pressure on site using a simple test pit instead of digging up the footing. This could bump you up from 2000 psf to 2500 psf or 3000 psf. It could also bump you down, of course. You would need to make some guesstimate of the existing footing size. Review the drawings? Become familiar with common construction practice of that era? Grab a baseline bearing pressure by estimating the existing vertical loading on the foundation. Then compare to the new loading. Is the increase acceptable with the existing bearing pressure? What about the new updated bearing pressure? Make a judgement call that you are comfortable with.

 

[RPGs]

Checking size/spacing of wall framing and headers would seem simple enough... may require selective demo of existing finishes

Correct, that's usually the simplest aspect of the existing conditions survey scope.

establishing the makeup of existing foundations feels to me to be a much more complicated task

Yes, it can be.

Is it common to require digging test pits at various locations to see buried foundations?

For me, yes.

What about homes built on slabs, I don't know how one could figure out the dimensions of a turndown at the perimeter or thickened areas at interior bearing walls.

I haven't run into that exact situation with a turn down slab, but I have hired a GPR survey before to determine slab thicknesses, dimension of interior footings below slabs. It's possible the GPR might be able to detect the interior edge of a turn down slab if it's wide enough. Otherwise, potentially cut the slab and verify.

every set of plans I come across uses a presumptive bearing capacity of 2,000 psf to be verified by the inspector. Is that a reasonable approach for older structures that were built before the development of the codes we use today?

Are you asking if it's reasonable to assume that? I don't think I would. I just default to doing the load analyses and let that be my guide. I talk to a geotech (we have one in-house) if I have concerns with the existing soil conditions.

 

[milkshakelake]

Converting an attic to a livable space is easier than adding a loft. I think most attics are designed for small live load, and the joists are not sufficient. I sister them. Problem solved.

Lofts are more tricky, but very doable. In my particular area, houses don't see much settlement because the soils are generally non-cohesive 4000 to 6000 psf. Settlement is usually because of poor drainage rather than a soil issue itself. So houses can easily be 2 stories without too much foundation investigation, and lofts can be added without any hesitation. The code in these cases need a test pit at each foundation location and a visual inspection by a PE. I don't do these inspections myself because I don't know anything about soils. Once in a while, the person doing the inspection will require additional testing of bearing capacity or full borings, but it's relatively rare for small homes.

I don't like these kinds of projects where you add lofts because they need a ton of investigation of the members. Holes/probes need to be opened at several locations for joists/headers/studs, and homeowners cry about it because they're going to have family over for the holidays. They also need convincing to ensure that it's necessary, and they cry more when reinforcement is needed. It drives me insane, and I'm not doing that work anymore.

 

[TRAK.Structural]

Appreciate the input here. All good thoughts and experience.