Lateral Strength of Mulled and/or Stacked Window Frames

[Forensic74]

Mulling and/or Stacking of residential window frames seems to be a relatively common thing to do. (mulling = tieing multiple window frames together horizontally) (stacking = tieing window frames together vertically). Obviously, this reduces the strength of the assembly to resist wind pressures. However, I never see contractors hiring engineers to make sure these types of window systems meet any sort of code pressures. Is anyone familiar with this issue and how it might be regulated?

 

[dhengr]

I’ve seen Archs. and Contractors do it when there is no engineer involved. And, I’ve seen them leak like a sieve, everyplace except through the glass itself, until that cracks. I’ve investigated water problems and seen instances where you could actually see the grouped units flexing and moving in and out in gusty wind conditions. You really have to get some tall wall framing btwn. stacked groups, or horiz. framing btwn. mulled groups, and they still end up pretty flexible. Archs. never like it, but just show them the ASCE 7 code, and ask them to explain it to you. From the window mfr’s. standpoint, when he mulls them together he is then assuming that his units span from your sill to the header. If he stacks units together he is assuming they span from jamb to jamb, and you will provide. Ask him which way his units and frames span best, and/or which way he wants the structure, and where, for the units he is proposing. You ignore this at your own peril, but many builders have gotten way with it, and been long gone before the problems cropped up.

 

[Ron]

MM10...in my state, such calculations must be on file with the state building commission for each window manufacturer under a product approval program. Then the contractor has to submit evidence of the product approval for that particular window when the permit is applied for. Then the contractor is supposed to follow the installation instructions for that window such as installing mullion clips, using the right fasteners for heads, sills and jambs, etc.

Great in theory, but we test a lot of windows and they flex and leak from the beginning!