Porch construction questions

[ejb]

Hi guys.

I am about to begin construction of a 28X12 foot screen porch off the back of the house. I live in the colder parts of Massachusetts, where snow loads and ground frost are defintely an issue so I would like to overbuild the porch somewhat (but not wastefully). I'd like to run my "plans" by you guys and elicit some feedback to see if I am missing anything:

First off, the 28 foot span runs against the house and the porch will extend out 12 feet. All lumber will be pressure treated.

Here is what I am thinking so far:

I will dig 4 holes and put in 24x24X12 inch thick cement footer blocks down 5 feet from grade (so the top will be at about 4 feet).

Under each footer I will put some gravel to level the hole.

The footers are boing poured seperately into forms I made and I plan on putting in some re-bar horizontally to increase strength and a hooked vertical piece so I can lower them into the hole.

After footers are placed I was going to but a 12inch diamter 5ft long sono-tube on top of each one and level/align them and then backfill.

After backfilling I was going to fill the tubes with some hand-mixed quikcrete and put a bolt sunk into the top.

On top of each cement post, I would put a length of 4x6 PT lumber and on top of each of those 4x6 posts I was going to use a triple 2x12 beam (nailed and glued) that would run the entire 28 foot of the porch.

BTW: I planned on having the beam hit the porch ten feet out from the foundation so the span from house to beam would be 10 feet and 2 feet would be cantilevered.

The floor joists I plan on using all 2x10 PT lumber, 12 inches on center. I could probably get away with 16in OC but figured the extra would would be stronger.

The real questions I have are:

Do the sizes all sound correct?

Is 4 posts enough?

Is the 10 foot span and then the 2 foot cantilevel OK even for a porch? I know it would work for a deck.

Are the footer sizes about right? the depth?

Any other suggestions appreciated.

Thanks.

-ejb

 

[beenthere]

My only suggestion would be to bolt the beam (3 ply 2x12's) with 1/2" bolts. Nails (unless stainless steel) and glue (as it is treated wood) will not last.

Your construction is very healthy, in my opinion, and should serve you well.

 

[chim]

Is there a reason you want to pour the footer blocks separately and then lower them into the holes? If you poured them in place, you wouldn't need to form them, and you would be sure there are no voids under them. I'd stick a few "L" shaped rebar into the footers so they extend up to be embedded into pour you're doing with the sonotube.................chim

 

[JJT]

"Under each footer I will put some gravel to level the hole." NO, use concrete. How would you compact the gravel?

Pour the footers in the hole.

I'd use 2 girders rather than 1, 4 piers / girder is fine. The first girder at 3' from the house and the second 9' from the house, then the porch is fully supported by it's own foundation. Where you thinking of lagging a ledger board on the house? Don't do it. The double girder will allow you to use 2x8 floor joist, easily paying for the second set of piers and girder. And your porch will have the proper foundation.

Duct tape your sonotubes, it's a real PITA when one blows out.

Use 1/2" threaded rod instead of bolts in your piers. Put a nut, some oversized washers and another nut embedded in the concrete.

How long are 4x6's? If they're more than a few feet, go with 6x6's and make sure you use knee braces, (distributes the load and will prevent racking).

Don't bother to glue your laminated beam, use 16D or 20D galvanized ring nails.

What are you using to deck the floor? If it's 1 layer of 5/4, stick with the 12" OC, if it's anything more substantial, switch to 16" OC, 12" would be a waste.

 

[ejb]

>>Is there a reason you want to pour the footer blocks separately

I guess my feeling was that I could cut down on the amount of time I will need a backhoe on site and having large holes exposed in the backyard as a hazard to small kids...if they are pre-formed, and then set in place, I can have them ready to go, dig the hole, place them in, immediately put in the sono-tubes/backfill and pour the columns.

Do you think it would make a difference to pour the footers directly into the hole? If so, how long do I need to wait for the footers to dry before I can place the sono-tubes and backfill?

I do like the idea of the pouring into the hole to be self-leveling with no voids.../w3tcompact/icons/smile.gif

 

[RobS]

When we poured our porch footers we just dug down to the necessary depth and filled the whole thing back up with concrete. I couldn't get the concrete all the way up to grade in all cases as the backfill wasn't up that high but those can be blocked up higher by the mason. The dig and pour method is about the easiest /w3tcompact/icons/smile.gif

 

[crb478]

sounds very heavy. I think you would be fine on 16 inch centers. The only thing I would rethink is the cantilever. If you have post that support the roof on the edge of the floor I would want the extra strength of the footings under the post with snow loads. Just my opinion. If you connect the ledger board to the main house make sure you either tie into the main footing or support the ledger adequately.

 

[ejb]

>The only thing I would rethink is the cantilever

Thats one of my main concerns..the weight of the roof (and snow) is going to be coming down on the end of a 2 foot cantilever...wish I could figure out what the building code says (but I'd probably have to actually get a permit if I start asking the building inspector questions/w3tcompact/icons/shocked.gif). I was going to avoid the permit route...

The ledger board would be attached directly to the house-sill, which sits atop a very substantialy puroed foundation. I was going to use PT 2x10 for the ledger and bolt it every 18-24 inches, so I figured that would more than adequately support the house side of the porch.

 

[beenthere]

Now that you described how you are going to rely on a ledger at the house end, look very carefully at the design of what you are bolting to, on the house. The rim board of the house is not adequate by itself to hold a ledger board. Often with add-on decks, the ledger attachment is the weakest link, and where failures often occur. Just a caution, not a criticism.

I would agree with the caution on the cantilever, but would think 2' would be okay - just put solid blocking between the joists over the beam and at the ends of the joists under the outside wall. This will keep the joists perpendicular even if they want to twist a bit when drying out. Also, selecting the joists to eliminate any large knots in the tension side (over the beam) will help with the strength requirements of the joists.

 

[ejb]

Re: Porch construction/massive footers OK?

Hi guys again.

Another question...construction is proceeding. I have dug four holes for the footers/posts, the depth is 5 feet on average and the holes are roughly 2 feet wide by 6 feet long (5 feet deep). Because of the soil conditions here (extremly rocky and clay) there was no way that I could have used a post-hole digger to make the hole any smaller (perhaps if I was a bit more skilled with the backhoe I rented - which was a lot of fun, but that is another story...).

Anyway, the original plan was to put in 2x2x1 foot thick forms in the bottom of each hole and then pour the footers, let them set-up and then use 4 foot long, 12 inch sono-tubes on top for to get above grade.

Now that I have these massive holes in the ground however, I am waying the costs and effort of bulding forms for the footers and then mixing up (by my estimates) about 50 bags of concrete mix by hand and setting it in place, versus have a load of mix delivered??

Problem is, if I get a delivery, I'll need to order a lot more concrete than I really need (6 yards) and that will cost about $425 deliverd with tax.

The alternative is 50 bagsX$3.25 = $162.50, plus I figure with my truck Ill need to make three trips to home depot (50 miles round-trip each time, so figure $30 gas) and I would also need to rent a decent size mixer ($90 for weekend) (also 50 miles round-trip, twice, in the other direction - another $20 in gas)...plus add a few bucks for the wood for the forms and I end up with pretty close to $300 in materials to end up with average sized foooters...the alternative, gives me truly massive footers (2 feet wide, 5-6 feet long and almost 3 feet thick).

Once those foooters are set-up I would still need to fill the 12 inch sono-tubes, but they would only need about 2 feet long in each one, so maybe 12-15 bags of concrete, which I could do by hand with no mixer and one-trip to HD.

Clearly the less expensive option is to do it all by hand...but, having it delivered is very tempting when I think about handling almost 2 tons of concrete twice) loading and unloading, mixing and then pouring into the holes...a couple of hard days of work I am guessing.

Finally...the question...if I go the "massive footer" route and decide to spend the extra $150-$200 bucks, will I truly end-up with a better footer? I know it is clearly overkill, but I want to make sure that I am not paying extra money to have an inferior end-result...my prefernce is to have the "best" construction...even if its overkill.

I figure, the foundation is going to be pretty hard to change down the road, and common-sense tells me that bigger is definintely better in this case...opinions?

(PS: The porch is going to be large (28x12) and we sometimes have as much as 4-5 feet of snow on the ground so that is why I don't mid oversizing a bit)