Expert: Bruce E. Johnson - 9/25/2008

Question
QUESTION: Bruce, if I were to extend an existing built-up tar flat roof (rafters are 2x6 at 16" o.c.) such that I would go out another ten feet and use 6x6 posts (10 feet high and placed on existing 4" slab) spaced eight feet apart; what size beams would I use and how would I attach beams to posts and then rafters to beam? I've asked a similar ? before but then I was thinking of attaching rafters on exterior to the header; now think best to place rafters on existing exterior wall aligned with the existing rafters. (how should those be spliced?)  thanx so much for response; worried about cracking the 4" slab also with the weight of the built-up roof; I live in Jacksonville, Florida.

ANSWER: Hi Martin, if your post spacing is 8' then a 4x8 beam or a double 2x8 beam should suffice.  To attach the beam to the post a simpson post cap is needed.  To attach the post to the slab a simpson post base is needed.  To attach the rafters to the beam simpson H-clips are needed.  All of these simpson clips can be found at any Lowes or Home Depot.  I'm not sure if your 4" slab will hold up this roof section or not.  If there is a thickened edge then it is possible.  If the slab is reinforced with wire mesh it should hold together.  You can undermine the slab where each post will go and then repour some concrete under those posts to add more bearing capabilities to that area.  Do I understand you to say that you are keeping the existing bearing wall and then lapping the new rafters on top of that wall and tying them into the old rafters?  If so, just lap the rafters a foot or so and then nail them together...I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: Thanx for such quick reply; are you saying to just use regular, un-reinforced concrete under the posts?  Say maybe a cubic foot pour?  Also, do you think i could use 4x4's for the posts and still have 8 foot span; guess trying to keep weight that's bearing on the slab lower.

Answer
Hi again Martin, yes unreinforced concrete would be fine.  If you are doing it from bagged goods and you can get the fiber reinforced mix that would be even better.  4x4 posts will work fine but the beam stil needs to be 4x8..I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com